OUTLINE

26.1 General Structure and Functions of the Digestive System 780 26.1a Digestive System Functions 780 26 26.2 Oral Cavity 781 26.2a Cheeks, Lips, and Palate 781 26.2b Tongue 782 26.2c Salivary Glands 782 26.2d Teeth 784 Digestive 26.3 Pharynx 786 26.4 General Arrangement of Abdominal GI Organs 787 26.4a Peritoneum, Peritoneal Cavity, and Mesentery 787 System 26.4b General Histology of GI Organs (Esophagus to Large Intestine) 788 26.4c Blood Vessels, Lymphatic Structures, and Nerve Supply 790 26.5 Esophagus 790 26.5a Gross Anatomy 791 26.5b Histology 791 26.6 The Swallowing Process 792 26.7 Stomach 793 26.7a Gross Anatomy 793 26.7b Histology 793 26.7c Gastric Secretions 794 26.8 Small Intestine 797 26.8a Gross Anatomy and Regions 797 26.8b Histology 799 26.9 Large Intestine 799 26.9a Gross Anatomy and Regions 799 26.9b Histology 801 26.9c Control of Large Intestine Activity 802 26.10 Accessory Digestive Organs 803 26.10a Liver 804 26.10b Gallbladder 805 26.10c Pancreas 807 26.10d Biliary Apparatus 808 26.11 Aging and the Digestive System 810 26.12 Development of the Digestive System 810 26.12a Stomach, Duodenum, and Omenta Development 810 26.12b Liver, Gallbladder, and Pancreas Development 810 26.12c Intestine Development 810

MODULE 12: DIGESTIVE SYSTEM

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ach time we eat a meal and drink fluids, our bodies take in the ingested material, digest the material into smaller usable components, E nutrients necessary for survival. However, these nutrients must be absorb the necessary digested nutrients into the bloodstream, and expel digested and processed—broken down both mechanically and chemi- the waste products from the body. When you eat, you put food into your cally—into components small enough for our cells to use. Once the mouth and it mixes with saliva as you chew. The chewed food mixed nutrients are broken down sufficiently, they are absorbed from the diges- with saliva is called a bolus (bō′ lŭ s; lump), and it is the bolus that is tive system into the bloodstream and transported to the body tissues. swallowed. The stomach processes the bolus and turns it into a paste- Not everything we eat can be used by the body. After the like substance called chyme. Hereafter in the chapter, we will simplify nutrients from foods are absorbed, some materials remain that can- our discussion by referring to the ingested contents as “material.” not be digested or absorbed, such as cellulose and fiber. These mate- The digestive system is composed of two separate categories rials must be expelled from the body via a process called defecation. of organs: digestive organs and accessory digestive organs (fig- All of these functions are the responsibility of the digestive system. ure 26.1). The digestive organs collectively make up the gastro- intestinal (GI) tract, also called the digestive tract or alimentary (al-i-men′ter-ē; alimentum = nourishment) canal. The GI tract organs 26.1 General Structure and Functions are the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine. These organs form a continuous tube from the of the Digestive System mouth to the anus. The contraction of muscle in the GI tract wall Learning Objectives: propels materials through the tract. Accessory digestive organs are not part of the long GI tube, 1. Identify the GI tract organs and accessory digestive organs. but often develop as outgrowths from and are connected to the 2. Describe the basic functions of the digestive system. GI tract. The accessory digestive organs assist the GI tract in the 3. Compare and contrast mechanical and chemical digestion. digestion of material. Accessory digestive organs include the teeth, 4. Identify the processes of peristalsis and segmentation. tongue, salivary glands, liver, gallbladder, and pancreas. 5. Define the processes of secretion, absorption, and elimination of wastes. 26.1a Digestive System Functions The digestive (di-jes′tiv, dı̄ -; digestus = to force apart, divide, dis- The digestive system performs six main functions: ingestion, diges- solve) system includes the organs that ingest the food, transport the tion, propulsion, secretion, absorption, and elimination of wastes.

Accessory Digestive Organs Gastrointestinal Tract (Digestive Organs)

Parotid salivary gland Teeth Oral cavity Tongue Pharynx

Sublingual salivary gland Submandibular salivary gland

Esophagus Figure 26.1 Digestive System. The digestive system is composed of the gastrointestinal (GI) tract and accessory digestive organs that assist the GI tract in the process of digestion. Liver Stomach

Gallbladder Pancreas

Large intestine

Small intestine

Anus

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secretions serve a protective function. Mucin mixes with water to Wave of contraction Wall of form mucus, and the mucus coats the GI wall to protect and lubri- GI tract cate it against acidic secretions and abrasions by passing materials. Lumen Absorption (ab-sō rp′shŭ n; absorptio = to swallow) involves either passive movement or active transport of electrolytes, diges- Mixing tion products, vitamins, and water across the GI tract epithelium Relaxation and into GI tract blood and lymph vessels. The final function of the digestive system is the elimination of wastes. Our bodies use most, but not all, of the components of what we eat. All undigestible materials as well as the waste Further mixing products secreted by the accessory organs into the GI tract are Bolus compacted into feces (fē ′sē z; faex = dregs), or fecal material, and then eliminated from the GI tract by the process of defecation (def- ĕ -kā ′shŭ n; defaeco = to remove the dregs).

(a) Peristalsis (b) Segmentation WHATW DID YOU LEARN? ●1 What structures compose the GI tract? Figure 26.2 ●2 How do secretion and absorption differ? Peristalsis and Segmentation. A swallowed bolus is propelled through the GI tract by the coordinated contraction and relaxation of the musculature in the GI tract wall. ( a) Peristalsis is a wave of contraction that 26.2 Oral Cavity moves material ahead of the wave through the GI tract toward the anus. (b) Segmentation is a back-and-forth movement in the small intestine Learning Objective: whereby ingested material mixes with secretory products to increase the 1. Identify and describe the structure and function of the efficiency of digestion and absorption. tongue, salivary glands, and teeth. ′ = Ingestion (in-jes chŭ n; ingero to carry in) is the introduc- The oral cavity, or mouth, is the entrance to the GI tract tion of solid and liquid materials into the oral cavity. It is the first (figure 26.3). The mouth is the initial site of mechanical digestion step in the process of digesting and absorbing nutrients. (via mastication) and chemical digestion (via an enzyme in saliva). Digestion is the breakdown of large food items into smaller The epithelial lining of the oral cavity is a nonkeratinized stratified structures and molecules. There are two aspects to digestion: squamous epithelium that protects against the abrasive activities Mechanical digestion physically breaks down ingested materials associated with digestion. This lining is moistened continually by into smaller pieces, and chemical digestion breaks down ingested the secretion of saliva. material into smaller molecules by using enzymes. (Review The oral cavity is bounded anteriorly by the teeth and lips enzymes in chapter 2.) The first part of mechanical digestion is and posteriorly by the oropharynx. The superior boundary of the ′ ′ = mastication (mas ti-kā shŭn; mastico to chew), the chewing of oral cavity is formed by the hard and soft palates. The floor, or ingested material by the teeth in the oral cavity. inferior surface, of the oral cavity is formed by the mylohyoid After the materials are swallowed, they move through the GI muscle covered with a mucous membrane. The tongue attaches to ′ = tract by a process termed propulsion (prō -pŭ l shŭ n; propello to move the floor of the oral cavity. forth). Two types of movement are involved in propulsion: peristalsis The oral cavity has two distinct regions: The vestibule is the ′ = and segmentation. Peristalsis (per-i-stal sis; stalsis constriction) space between the cheeks or lips and the gums. The oral cavity proper is the process of muscular contraction that forms ripples along part lies central to the alveolar processes of the mandible and maxillae. of the GI tract and forces material to move further along the tract (figure 26.2a). Peristalsis is like pushing toothpaste through a 26.2a Cheeks, Lips, and Palate toothpaste tube: As you push on the flat end of the tube, a portion The lateral walls of the oral cavity are formed by the cheeks, which of toothpaste moves toward the opening. As you push the middle of are covered externally by the integument and contain the buccinator the tube, the segment of toothpaste moves even closer to the opening. muscles. The buccinator muscles compress the cheeks against the Churning and mixing movements in the small intestine, called seg- teeth to hold solid materials in place during chewing. The cheeks mentation (seg′men-tā′ shŭ n), help disperse the material being digest- terminate at the fleshy lips (or labia), which form the anterior wall of ed and combine it with digestive organ secretions (figure 26.2b). The the oral cavity. The lips are formed primarily by the orbicularis oris material within the lumen of the small intestine is moved back-and- muscle and covered with keratinized stratified squamous epithe- forth to mix it with the secretory products that are being introduced lium. Lips have a reddish hue because of their abundant supply of into that region of the GI tract. Thus, contraction and relaxation of the superficial blood vessels and the reduced amount of keratin within digestive wall musculature creates a “blender” effect, mixing ingested their outer epithelial layer. The gingivae (jin′ji-vē), or gums, are com- materials with digestive enzymes and mechanically breaking down posed of dense irregular connective tissue, with an overlying non- larger digested particles into smaller ones. keratinized stratified squamous epithelium that covers the alveolar Secretion (se-krē ′shŭ n; secerno = to separate) is the process processes of the upper and lower jaws and surrounds the necks of of producing and releasing mucin or fluids such as acid, bile, the teeth. The internal surfaces of the superior and inferior lips each and digestive enzymes. When these products are secreted into are attached to the gingivae by a thin mucosa fold in the midline, the lumen of the GI tract, they facilitate chemical digestion and called the labial (lā′ bē -ă l) frenulum (fren′ū -lŭ m; frenum = bridle). the passage of material through the GI tract. Some of these prod- The palate (pal′ă t) forms the roof of the oral cavity and acts as ucts (e.g., acid, bile, digestive enzymes) help digest food. Mucin a barrier to separate it from the nasal cavity. The anterior two-thirds

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Superior lip Superior labial frenulum

Soft palate Transverse palatine folds Nasopharynx Hard palate Hard palate Palatoglossal Oral cavity arch Uvula Soft palate Tongue Palatine tonsil Palatoglossal arch Uvula Oropharynx Palatopharyngeal arch Fauces Vestibule Lingual tonsil Palatine tonsil Tongue Epiglottis Laryngopharynx Salivary duct orifices Lingual frenulum Sublingual Teeth Submandibular Esophagus Gingivae Larynx Inferior labial frenulum Trachea Inferior lip (a) Oral cavity, anterior view (b) Sagittal section Figure 26.3 Oral Cavity. (a) Ingested food and drink enter the GI tract through the oral cavity, shown here in anterior view. (b) A diagrammatic sagittal section shows the structures of the oral cavity and the pharynx.

of the palate is hard and bony (called the hard palate), while the the floor of the oral cavity by a thin, vertical mucous membrane, posterior one-third is soft and muscular (called the soft palate). The the lingual frenulum (figure 26.3a). In addition, the posteroinferior hard palate is formed by the palatine processes of the maxillae and surface of the tongue contains lingual tonsils. Two categories of the horizontal plates of the palatine bones. It is covered with dense skeletal muscles move the tongue (see chapter 11). connective tissue and nonkeratinized stratified squamous epithe- lium and exhibits prominent transverse palatine folds, or friction 26.2c Salivary Glands ridges, that assist the tongue in manipulating ingested materials The salivary glands collectively produce and secrete saliva (s ă-l ı̄′ vă), prior to swallowing. The arching soft palate is primarily composed a fluid that assists in the initial activities of digestion. The volume of of skeletal muscle and covered with nonkeratinized stratified squa- saliva secreted daily ranges between 1.0 and 1.5 liters. Most saliva mous epithelium. Extending inferiorly from the posterior part of the is produced during mealtime, but smaller amounts are produced soft palate is a conical median projection called the uvula (ū′ vū -lă ; continuously to ensure that the oral cavity mucous membrane uva = grape). When you swallow, the soft palate and the uvula remains moist. Water makes up 99.5% of the volume of saliva and elevate to close off the posterior entrance into the nasopharynx and is its primary ingredient. Saliva also contains a mixture of other prevent ingested materials from entering the nasal region. components (table 26.1). The fauces represent the opening between the oral cavity Saliva has various functions. It moistens ingested food and and the oropharynx. The fauces are bounded by paired muscular helps it become a slick, semisolid bolus that is more easily swal- folds: the palatoglossal arch (anterior fold containing the palato- lowed. Saliva also moistens, cleanses, and lubricates the oral cavity glossus muscle) and the palatopharyngeal arch (posterior fold structures. The first step in chemical digestion occurs when amy- containing the palatopharyngeal muscle). The palatine tonsils are lase in saliva begins to break down carbohydrates. Saliva contains housed between the arches (see chapter 24). These tonsils serve as antibodies and an antibacterial substance called lysozyme that help an “early line of defense” as they monitor ingested food and drink inhibit bacterial growth in the oral cavity. Finally, saliva is the for antigens, and initiate an immune response when necessary. watery medium into which food molecules are dissolved so that taste receptors on the tongue can be stimulated. 26.2b Tongue Three pairs of multicellular salivary glands are located exter- The tongue (tŭ ng) is an accessory digestive organ that is formed nal to the oral cavity: the parotid, submandibular, and sublingual primarily from skeletal muscle and covered with stratified squa- glands (figure 26.4a). mous epithelium. As described in chapter 19, numerous small pro- The parotid (pă-rot ′id; para = beside, ot = ear) salivary glands jections, termed papillae (pă-pil ′ē; sing., papilla; papula = pimple), are the largest salivary glands. Each parotid gland is located anterior cover the superior (dorsal) surface of the tongue. The tongue’s and slightly inferior to the ear, partially overlying the masseter mus- stratified squamous epithelium is keratinized over the filiform cle. The parotid salivary glands produce about 25–30% of the saliva, papillae but nonkeratinized over the rest of the tongue. Chapter 25 which is conducted through the parotid duct to the oral cavity. The described the tongue as a participant in sound production. In addi- parotid duct extends from the gland, parallel to the zygomatic arch, tion, the tongue manipulates and mixes ingested materials during before penetrating the buccinator muscle and opening into the ves- chewing and helps compress the materials against the palate to tibule of the oral cavity near the second upper molar. turn them into a bolus. The tongue also performs important func- As their name suggests, the submandibular (sŭ b-man-dib′ū - tions in swallowing. The inferior surface of the tongue attaches to lă r) salivary glands are inferior to the body of the mandible. They

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Table 26.1 Saliva Characteristics Neural Control of Saliva Production Rate pH Range Composition of Saliva Solute Components Secretion 1–1.5 L/day Slightly acidic (pH 6.4 to 6.8) 99.5% water; 0.5% solutes Ions (e.g., Na+, K+, chloride, Parasympathetic axons in CN IX bicarbonate) stimulate parotid salivary gland Immunoglobulin A (helps secretions decrease bacterial infections) Parasympathetic axons in CN Lysozyme (antibacterial VII stimulate submandibular enzyme) and sublingual salivary gland secretions Mucin Sympathetic activation from Salivary amylase (enzyme that cervical ganglia stimulates mucin breaks down carbohydrates) secretion

Parotid salivary gland Figure 26.4 Parotid duct Salivary Glands. Saliva is produced by three pairs of salivary glands. (a) The relative locations of the parotid, submandibular, and sublingual salivary glands are shown in a diagrammatic sagittal section. (b) Serous and mucous alveoli are shown in a diagrammatic representation of salivary gland histology. (c) A photomicrograph reveals the histologic detail Masseter muscle of the submandibular salivary gland.

Mucosa (cut) Sublingual ducts Submandibular duct Sublingual salivary gland Mylohyoid muscle (cut) Submandibular salivary gland

(a) Salivary glands

Salivary duct Mucous cells Mucous cell Salivary duct

Mucous alveolus

Duct epithelium

Mixed alveoli

Serous alveolus

LM 200x Serous cell (b) Salivary gland histology (c) Submandibular salivary gland Serous cells

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Table 26.2 Salivary Gland Characteristics Salivary Gland Structure and Location Types of Secretion Percentage of Saliva Produced Parotid Largest of the salivary glands; Only serous secretions 25–30% located anterior and slightly inferior to ears; parotid duct conducts secretions into the vestibule near upper 2nd molar Submandibular Located inferior to mandibular body; Both mucous and serous secretions 60–70% submandibular duct opens lateral to lingual frenulum Sublingual Smallest of the salivary glands; Both mucous and serous secretions 3–5% located inferior to tongue; tiny sublingual ducts open into fl oor of oral cavity

produce most of the saliva (about 60–70%). A submandibular duct transports saliva from each gland through a papilla in the floor of the mouth on the lateral sides of the lingual frenulum. Crown Enamel The sublingual (sŭ b-ling′gwă l) salivary glands are inferior Gingiva to the tongue and internal to the oral cavity mucosa. Each sublin- gual salivary gland extends multiple tiny sublingual ducts that Neck Dentin open onto the inferior surface of the oral cavity, posterior to the submandibular duct papilla. These tiny glands contribute only Pulp cavity about 3–5% of the total saliva. Two types of secretory cells are housed in the salivary glands: mucous cells and serous cells (figure 26.4b, c). Mucous cells secrete mucin, which forms mucus upon hydration, while serous cells Root canal Root secrete a watery fluid containing ions, lysozyme, and salivary amy- lase. The proportion of mucous cells to serous cells varies among the Cementum three types of salivary glands. The submandibular and sublingual glands produce both serous and mucous secretions, whereas the Periodontal ligaments parotid glands produce only serous secretions. Table 26.2 sum- marizes the structure of the salivary glands and their secretions. Dental alveolus The salivary glands are primarily innervated by the parasym- Blood vessels and nerves in pathetic division of the autonomic nervous system. In particular, the apical foramen facial nerve (CN VII) innervates the submandibular and sublingual Figure 26.5 glands, while the glossopharyngeal nerve (CN IX) innervates the parotid gland. Parasympathetic innervation stimulates salivary gland Anatomy of a Molar. Ingested material is chewed by the teeth in the oral cavity. secretion, which is why your mouth “waters” when you see a deli- cious dinner in front of you. Your salivary glands are preparing the durable layer of enamel (ē -nam′ĕl) forms the crown of the tooth. body for the start of the digestion process. In contrast, sympathetic Enamel, the hardest substance in the body, is composed primarily of stimulation inhibits normal secretion from these glands, which is calcium phosphate crystals. Dentin (den′tin) forms the primary mass why you may experience a dry mouth after a fight-or-flight response. of a tooth. Dentin is comparable to bone but harder, and is deep to the cementum and the enamel. The center of the tooth is a pulp cavity WHAT DO YOU THINK? that contains a connective tissue called pulp. A root canal opens into ●1 Research suggests that a “dry mouth” (inadequate production of the connective tissue through an opening called the apical foramen saliva) is correlated with an increase in dental problems, such as and is continuous with the pulp cavity. Blood vessels and nerves pass cavities. What are the possible reasons for this correlation? through the apical foramen and are housed in the pulp. The mesial (mē′ zē -ă l; mesos = middle) surface of the tooth 26.2d Teeth is the surface closest to the midline of the mouth, while the distal The teeth are collectively known as the dentition (den-tish′ŭ n; den- surface of the tooth is farthest from the mouth midline. Other tition = teething). Teeth are responsible for mastication, the first tooth surfaces include: the buccal surface, adjacent to the internal part of the mechanical digestion process. A tooth has an exposed surface of the cheek; the labial surface, adjacent to the internal crown, a constricted neck, and one or more roots that anchor it to surface of the lip; the lingual surface, facing the tongue; and the the jaw (figure 26.5). The roots of the teeth fit tightly into dental occlusal (ŏ -kloo′ză l) surface, where the teeth from the opposing alveoli, which are sockets within the alveolar processes of both superior and inferior arches meet. the maxillae and the mandible. Collectively, the roots, the dental Two sets of teeth develop and erupt during a normal lifetime. alveoli, and the periodontal ligaments that bind the roots to the In an infant, 20 deciduous (dē -sid′ū -ŭs; deciduus = falling off) teeth, alveolar processes form a gomphosis joint (described in chapter 9). also called milk teeth, erupt between 6 months and 30 months after Each root of a tooth is ensheathed within hardened mate- birth. These teeth are eventually lost and replaced by 32 permanent rial called cementum (se-men′tū m; rough quarry stone). A tough, teeth. As figure 26.6 shows, the more anteriorly placed permanent

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Central incisor (7–9 mos.) Lateral incisor (9–11 mos.) Canine (18–20 mos.)

1st molar (14–16 mos.) Upper 2nd molar (24–30 mos.) Permanent teeth teeth

2nd molar (20–22 mos.) Lower teeth Deciduous teeth 1st molar (12–14 mos.)

Canine (16–18 mos.) Lateral incisor (7–9 mos.) (a) Child’s skull Central incisor (6–8 mos.)

(b) Deciduous teeth Right Upper (Maxillary) Quadrant Left Upper (Maxillary) Quadrant

Central incisor (7–8 yrs.) Lateral incisor (8–9 yrs.) Canine (11–12 yrs.) 1st premolar (10–11 yrs.)

2nd premolar (10–12 yrs.) 89 Upper teeth 7 10 1st molar (6–7 yrs.) 6 11 5 12 2nd molar (12–13 yrs.) 4 13 3 14 3rd molar (17–25 yrs.) Hard palate 2 15 1 16

32 17 3rd molar (17–25 yrs.) 31 18 30 19 20 2nd molar (11–13 yrs.) 29 28 21 1st molar (6–7 yrs.) 22 Lower teeth 27 23 26 2425 2nd premolar (11–12 yrs.) 1st premolar (10–12 yrs.) Canine (9–10 yrs.) (d) Teeth numbering Lateral incisor (7–8 yrs.) Central incisor (6–7 yrs.)

Right Lower (Mandibular) Quadrant Left Lower (Mandibular) Quadrant

(c) Permanent teeth Figure 26.6 Teeth. (a) Both deciduous teeth and unerupted permanent teeth are visible in this cutaway section of a child’s skull. (b, c) Comparison of the dentition of deciduous and permanent teeth, including the approximate age at eruption for each tooth. Also shown in (c) are the dental quadrants, which are formed by a sagittal plane that passes between the central incisors in the maxilla and mandible. (d) Teeth may be precisely identified using the Universal Numbering System.

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Table 26.3 Oral Cavity Structures and Their Functions Structure Description Function Gingivae Composed of dense irregular connective tissue and Surround necks of teeth and cover alveolar processes nonkeratinized stratifi ed squamous epithelium Hard palate Anterior roof of mouth; bony shelf covered by dense connective Forms anterior two-thirds of roof of mouth; separates oral tissue and nonkeratinized stratifi ed squamous epithelium cavity from nasal cavity Lips Form part of anterior walls of oral cavity; covered with Close oral cavity during chewing keratinized stratifi ed squamous epithelium Salivary glands Three pairs of large multicellular glands: parotid glands, Produce saliva sublingual glands, and submandibular glands Soft palate Posterior roof of mouth formed from skeletal muscle and Forms posterior one-third of roof of mouth; helps close off covered with nonkeratinized stratifi ed squamous epithelium; entryway to nasopharynx when swallowing the uvula hangs from it Teeth Hard structures projecting from the alveolar processes of the Mastication (chewing food) maxillae and mandible: incisors, canines, premolars, and molars Tongue Composed primarily of skeletal muscle and covered by stratifi ed Manipulates ingested material during chewing; pushes material squamous epithelium; surface covered by papillae against palate to turn it into a bolus; detects tastes (via taste buds) Tonsils Aggregates of partially encapsulated lymphatic tissue Detect antigens in swallowed food and drink and initiate immune response if necessary Uvula Conical, median, muscular projection extending from the soft Assists soft palate in closing off entryway to nasopharynx when palate swallowing Vestibule Space between cheek and gums Space between lips/cheeks and gums where ingested materials are mixed with saliva and mechanically digested

teeth tend to appear first, followed by the posteriorly placed teeth. left side of the maxilla (number 16). Number 17 is the most poste- (The major exception to this rule are the first molars, which appear rior tooth on the left side of the mandible, and it continues along at about age 6 and are sometimes referred to as the “6-year molars.”) the mandibular arch to the most posterior tooth on the right side of The last teeth to erupt are the third molars, often called wisdom the mandible (number 32). This system is based on 32 teeth so—if teeth, in the late teens or early 20s. Often the jaw lacks space to someone is missing tooth number 1 (wisdom tooth)—the first num- accommodate these final molars, and they may either emerge only ber will be 2 instead of 1, acknowledging the missing tooth. partially or grow at an angle and become impacted (wedged against Table 26.3 summarizes the structures of the oral cavity and another structure). Impacted teeth cannot erupt properly because of their functions. the angle of their growth. WHATW DID YOU LEARN?

WHAT DO YOU THINK? ●3 What are the main components of saliva, and what functions do they serve? ●2 If the deciduous teeth are eventually replaced by permanent teeth, why do humans have deciduous teeth in the first place? ●4 What are the types of permanent teeth, and what is each tooth’s main function in mastication? The most anteriorly placed permanent teeth are called inci- sors (in-sı̄′ zŏ r; incido = to cut into). They are shaped like a chisel and have a single root. They are designed for slicing or cutting into 26.3 Pharynx food. Immediately posterolateral to the incisors are the canines Learning Objectives: (kā′ nı̄ n; canis = dog), which have a pointed tip for puncturing and 1. Describe the structure of the pharynx. tearing food. Premolars are located posterolateral to the canines 2. Explain the action of the pharyngeal constrictors. and anterior to the molars. They have flat crowns with prominent ridges called cusps that are used to crush and grind ingested mate- The common space used by both the respiratory and diges- rials. Premolars may have one or two roots. The molars (mō′ lă r; tive systems is the pharynx (see figure 26.1 and chapter 25). The molaris = millstone) are the thickest and most posteriorly placed nonkeratinized stratified squamous epithelial lining of the oro- teeth. They have large, broad, flat crowns with distinctive cusps, pharynx and the laryngopharynx provides protection against the and three or more roots. Molars are also adapted for grinding and abrasive activities associated with swallowing ingested materials. crushing ingested materials. If the mouth is divided into quadrants, Three skeletal muscle pairs, called the superior, middle, and each quadrant contains the following number of permanent teeth: inferior pharyngeal constrictors, form the wall of the pharynx. two incisors, one canine, two premolars, and three molars (figure Sequential contraction of the pharyngeal constrictors decreases the 26.6c). In the United States, general dentists have adopted the diameter of the pharynx, beginning at its superior end and moving Universal Numbering System (recognized by the American Dental toward its inferior end, thus pushing swallowed material toward Association) to identify teeth (figure 26.6d). Tooth number 1 is the the esophagus. As the pharyngeal constrictors constrict, the epi- most posterior tooth on the right side of the maxilla. Numbering glottis of the larynx closes over the laryngeal opening to prevent continues anteriorly and around to the most posterior tooth on the ingested materials from entering the larynx and trachea.

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The vagus nerves (CN X) innervate most pharyngeal mus- cles. The principal arteries supplying the pharynx are branches of the external carotid arteries. The pharynx is drained by the inter- Liver nal jugular veins. Diaphragm

WHATW DID YOU LEARN? Lesser omentum

●5 How do pharyngeal constrictors move swallowed material to the Pancreas Stomach esophagus? Duodenum Mesocolon Transverse colon 26.4 General Arrangement Jejunum of Abdominal GI Organs Greater omentum Parietal peritoneum Learning Objectives: 1. Identify and describe the peritoneum location and function. Mesentery proper 2. Explain the derivation of specific mesenteries. 3. Compare and contrast the four tunics in the GI tract wall. Visceral peritoneum 4. Describe the blood vessels, lymphatic structures, and nerves that supply the GI tract. The abdominal organs of the GI tract are supported by serous membranes, and the walls of these organs have specific layers, called tunics. Ileum Peritoneal cavity 26.4a Peritoneum, Peritoneal Cavity, and Rectum Urinary bladder Mesentery The abdominopelvic cavity is lined with moist serous membranes (figure 26.7). The portion of the serous membrane that lines the inside surface of the body wall is called the parietal peritoneum (per′i-tō -nē ′ŭ m; periteino = to stretch over). The portion of the Figure 26.7 serous membrane that folds back (reflects) to cover the surface of Peritoneum and Mesenteries. Many abdominal organs are held in internal organs is called the visceral peritoneum. Between these place by double-layered serous membrane folds called mesenteries. two layers is the peritoneal cavity, a potential space where the The peritoneum is the serous membrane lining the internal abdominal peritoneal layers that face each other secrete a lubricating serous wall (parietal layer) and covering the outer surface of the abdominal fluid. The thin layer of fluid in the peritoneal cavity lubricates organs (visceral layer). This sagittal view through the abdominopelvic both the body wall and the internal organ surfaces, allowing the cavity shows the relationship between the peritoneal membranes and abdominal organs to move freely, and reducing any friction result- the abdominal organs they ensheathe. ing from this movement. The mesenteries (mes′en-ter-ē; mesos = middle, enteron = intestine) are folds of peritoneum that support and stabilize the Study Tip! intraperitoneal GI tract organs. Blood vessels, lymph vessels, and The serous peritoneum is very similar to the serous pleura and nerves are sandwiched between the two folds and supply the diges- the serous pericardium. These membranes have an outer parietal layer tive organs. There are several different types of mesenteries. The ′ that lines the body wall and a visceral layer that covers the organ. The greater omentum (ō -men tŭ m) extends inferiorly like an apron from space between the parietal and visceral layers is where serous fluid is the greater curvature of the stomach and covers most of the abdomi- secreted, and this fluid acts as a lubricant to prevent friction as the nal organs (figure 26.8a). It often accumulates large amounts of organ moves. So, if you remember the basics about the pleura and the adipose connective tissue. The lesser omentum connects the lesser pericardium, you will know the basics about the peritoneum too. curvature of the stomach and the proximal end of the duodenum to the liver. The lesser omentum may be subdivided into a hepatogas- tric ligament, which runs from the liver to the stomach, and a hepa- Within the abdomen, organs that are completely surrounded toduodenal ligament, which runs from the liver to the duodenum. by visceral peritoneum are called intraperitoneal (in′tră-per ′i-tō - The mesentery proper is a fan-shaped fold of peritoneum nē′ ă l) organs. They include the stomach, part of the duodenum that suspends most of the small intestine from the internal surface of the small intestine, the jejunum and the ileum of the small of the posterior abdominal wall (figure 26.8b). The peritoneal fold intestine, the cecum, the appendix, and the transverse and sigmoid that attaches parts of the large intestine to the internal surface colon of the large intestine. Retroperitoneal (re-trō -per′i-tō -nē′ ă l) of the posterior abdominal wall is called the mesocolon (mez′ō - organs typically lie directly against the posterior abdominal wall, kō′ lon). Essentially, a mesocolon is a mesentery for parts of the so only their anterolateral portions are covered with peritoneum. large intestine. There are several distinct sections of the mesocolon, Retroperitoneal organs include most of the duodenum, the pan- each named for the portion of the colon it suspends. For example, creas, the ascending and descending colon of the large intestine, transverse mesocolon is associated with the transverse colon, while and the rectum. sigmoid mesocolon is associated with the sigmoid colon.

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Greater omentum (reflected) Liver Falciform ligament

Round ligament of the liver Transverse colon Lesser omentum Stomach Transverse mesocolon

Greater omentum Mesentery proper

Small intestine

(a) Omenta (b) Mesentery proper and mesocolon Figure 26.8 Omenta and Mesentery. Cadaver photos show anterior views of (a) the greater and lesser omenta, and (b) the mesentery proper and some of the mesocolon.

A peritoneal ligament is a peritoneal fold that attaches one For most of the abdominal GI tract organs, the lining epithe- organ to another organ, or attaches an organ to the anterior or lium is a simple columnar epithelium. The exception to this rule lateral abdominal wall. Some examples of peritoneal ligaments is the esophagus, which is lined with nonkeratinized stratified include: the coronary ligament, a peritoneal fold attaching the squamous epithelium. superior surface of the liver to the diaphragm at the margins of the bare area of the liver; the falciform (fal′si-fō rm; falx = sickle) Submucosa ligament, a peritoneal fold that attaches the liver to the anterior The submucosa is composed of either areolar or dense irregu- internal abdominal wall; and the lienorenal ligament, a fold of lar connective tissue and has far fewer cells than the lamina peritoneum between the spleen and the kidney. propria. Submucosa components include: accumulations of lymphatic tissue in some submucosal regions; mucin-secreting 26.4b General Histology of GI Organs (Esophagus glands that project ducts across the mucosa and open into the to Large Intestine) lumen of the tract in the esophagus and duodenum; many large The GI tract from the esophagus through the large intestine is blood vessels and lymph vessels; and nerves that extend fine a tube composed of four concentric layers, called tunics. From branches into both the mucosa and the muscularis. These nerve deep (the lining of the lumen) to superficial (the external covering), fibers and their associated ganglia are collectively referred to as these tunics are the mucosa, the submucosa, the muscularis, and the submucosal nerve plexus (or Meissner plexus). It contains the adventitia or serosa (figure 26.9). The general pattern of the sensory neurons, sympathetic postganglionic axons, and para- tunics is described next. There are variations in the general pat- sympathetic ganglia. tern, which will be described in detail when we discuss the specific organs in which they appear. Muscularis The muscularis (mŭ s-kū′ lā ′ris) typically contains two lay- Mucosa ers of smooth muscle. Exceptions to this pattern include the The mucosa (mū -kō′ să ), or mucous membrane, has three compo- esophagus (which contains a mixture of skeletal and smooth nents: (1) an inner (superficial) epithelium lining the lumen of the muscle) and the stomach (which contains three layers of GI tract; (2) an underlying areolar connective tissue, called the smooth muscle). The fibers of the inner layer of smooth mus- lamina propria; and (3) a relatively thin layer of smooth muscle, cle are oriented circumferentially around the GI tract, and are termed the muscularis mucosae. The muscularis mucosae is very called the inner circular layer. The fibers of the outer layer thin (or absent) at the level of the laryngopharynx and thickens are oriented lengthwise along the GI tract, and are called the progressively as it approaches the stomach. outer longitudinal layer.

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Mucosa Epithelium Lamina propria Muscularis mucosae Mesentery

Vein Artery Lymph vessel Submucosa Submucosal gland Blood vessel Lumen

Submucosal nerve plexus

Muscularis Inner circular layer

Myenteric nerve plexus

Outer longitudinal layer

Serosa

Figure 26.9 Tunics of the Abdominal GI Tract. The wall of the abdominal GI tract has four tunics. From the lining of its lumen to the external covering, the tunics are the mucosa, submucosa, muscularis, and adventitia or serosa.

If you think of the GI tract as a “tube,” then contractions Adventitia or Serosa of the circular layer constrict the diameter of the tube lumen, The outermost tunic may be either an adventitia or a serosa. while contractions of the longitudinal layer shorten the tube. At An adventitia (ad-ven-tish′ă ) is composed of areolar connective specific locations along the GI tract, the inner circular muscle tissue with dispersed collagen and elastic fibers. A serosa layer is greatly thickened to form a sphincter. A sphincter (se-rō′ să ) has the same components as the adventitia, but is closes off the lumen opening at some point along the GI tract, covered by a visceral peritoneum. Intraperitoneal organs have and in so doing it can help control the movement of materials a serosa, because they are completely surrounded by visceral through the GI tract. The nerve fibers and the associated gan- peritoneum. Retroperitoneal organs primarily have an adventitia, glia located between the two layers of smooth muscle control its since these organs are only partially covered by visceral peritone- contractions and are collectively referred to as the myenteric um. For example, the ascending colon (which is retroperitoneal) (mı̄ -en-ter′ik; mys = muscle, enteron = intestine) nerve plexus has an adventitia, while the stomach (which is intraperitoneal) (or Auerbach plexus). has a serosa.

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and smooth muscle in the middle region, and smooth muscle in its Study Tip! inferior region.

You have just learned that the “default” pattern of the tunics is ■ The stomach has these tunics, but it deviates from the pattern as follows: in that its muscularis has three layers of smooth muscle, not two. 1. Mucosa (typically lined with simple columnar epithelium) 2. Submucosa ■ The small intestine follows the basic “default” pattern of tunics. 3. Muscularis (typically formed from two layers of smooth muscle) ■ The large intestine has these tunics, but in its muscularis, the 4. Adventitia or serosa outer longitudinal layer of muscle forms three distinct bands called teniae coli (described later in this chapter). As you learn the basic GI organs, determine how these organs follow or deviate from this default pattern. For example: Knowing the basic tunic pattern, and then figuring out how an organ may deviate from this pattern, will help you better distinguish ■ The esophagus has these tunics, but it deviates from the pattern in two ways: Its mucosa has a stratified squamous epithelium, and the histology of the esophagus, stomach, small intestine, and large its muscularis has a skeletal muscle in its superior region, skeletal intestine.

26.4c Blood Vessels, Lymphatic Structures, Nerves and Nerve Supply The nerves associated with the GI tract consist of both autonomic The GI tract has a rich blood and nerve supply. In addition, exten- motor and sensory axons. There are three main groups of auto- sive lymphatic structures along the length of the GI tract act as nomic plexuses: sentinels to monitor for antigens that may have been ingested. ■ The celiac plexus contains sympathetic axons (from the T5–T9 The blood vessels, lymphatic structures, and nerves enter the GI segments of the spinal cord) and parasympathetic axons (from tract from either the surrounding structures (e.g., nearby organs, the vagus nerve). This plexus supplies structures that receive abdominal wall) or via the supporting mesentery. their blood supply from branches of the celiac trunk. ■ Blood Vessels The superior mesenteric plexus contains sympathetic axons (from the T8–T12 segments of the spinal cord) and Branches of the celiac trunk, superior mesenteric artery, and parasympathetic axons (from the vagus nerve). This plexus inferior mesenteric artery supply the abdominal GI tract (see transmits autonomic innervation to structures that receive chapter 23). These artery branches split into smaller branches their blood supply from branches of the superior mesenteric that extend throughout the walls of the GI organs. Branches travel artery. within the tunics, and the mucosa contains capillaries that have ■ The inferior mesenteric plexus contains sympathetic fenestrated endothelial cells to promote absorption. The veins axons (from the L1–L2 segments of the spinal cord) and arising in the mucosa form anastomoses in the submucosa before parasympathetic axons (from the pelvic splanchnic nerves). exiting the wall of the GI tract adjacent to their companion arteries. This plexus supplies structures that receive blood from Eventually, the veins merge to form the hepatic portal system of branches of the inferior mesenteric artery. veins, to be discussed later in this chapter. In general, parasympathetic innervation promotes digestive Lymph Vessels and Tissues system activity by stimulating GI gland secretions and peristalsis, Lymphatic capillaries arise as blind tubes in the mucosa of the GI and by relaxing GI sphincters. These changes in activity induce tract. In the small intestine, each villus usually contains a single, vasodilation and an increase in GI blood flow. Sympathetic innerva- blind-ended, central lymphatic capillary called a lacteal. Recall tion inhibits digestive system activity, and it tends to do the oppo- from chapter 24 that lacteals are responsible for absorbing dietary site of parasympathetic innervation. So sympathetic innervation lipids and lipid-soluble vitamins (vitamins that can be absorbed inhibits some GI gland secretions, inhibits peristalsis, closes the GI only if they are dissolved in lipids first). Outside the organ walls, sphincters, and vasoconstricts the blood vessels to the GI tract. lymphatic capillaries merge to form lymphatic vessels. These ves- sels enter and exit the many lymph nodes scattered near the organs WHATW DID YOU LEARN? and within the mesentery. Eventually, this lymph will be trans- ●6 Compare the terms intraperitoneal and retroperitoneal, and give ported to the cisterna chyli, which drains into the thoracic duct. examples of each type of organ. The lymphatic structures within the GI tract lie primar- ●7 What are the four main tunics of the abdominal GI tract, and what ily in the lamina propria of the mucosa. Lymphatic structures is the “default” pattern in each tunic? called MALT (mucosa-associated lymphatic tissue) are found in the small intestine and appendix (see chapter 24). In the small intestine, these aggregate nodules are called Peyer patches. They appear to the naked eye as oval bodies about the size of a pea, 26.5 Esophagus but are often much larger structures. Less commonly, lymphatic Learning Objective: structures are found external to the simple epithelium through- 1. Describe the structure and function of the esophagus. out the stomach and intestines where they are known as diffuse lymphatic tissue. Also, solitary lymphatic nodules may occur in The esophagus (ē -sof′ă -gŭ s; gullet) is a tubular passageway the esophagus, the pylorus of the stomach, and along the entire for swallowed materials being conducted from the pharynx to the length of the small and large intestines. stomach (see figure 26.1). The inferior region of the esophagus

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Mucosa Stratified Muscularis squamous mucosae epithelium Submucosa

Muscularis Mucosa

Lamina propria Adventitia

LM 11x

(a) Esophagus, transverse section Muscularis mucosae

LM 65x

(b) Esophageal mucosa Figure 26.10 Histology of the Esophagus. The esophagus extends inferiorly from the pharynx and conducts swallowed materials to the stomach. (a) A photomicrograph of a transverse section through the esophagus identifies the tunics in its wall. (b) A photomicrograph shows the esophageal mucosa.

connects to the stomach by passing through an opening in the of the muscularis layer are skeletal, rather than smooth, to ensure diaphragm called the esophageal hiatus (hı̄ -ā ′tŭ s; to yawn). that the swallowed material moves rapidly out of the pharynx and into the esophagus before the next respiratory cycle begins. 26.5a Gross Anatomy (Remember that skeletal muscle contracts more rapidly than does The esophageal wall is thick and composed of concentric tunics smooth muscle.) Striated and smooth muscle fibers intermingle in that are continuous superiorly with those of the pharynx and the middle one-third of the esophagus, and only smooth muscle inferiorly with those of the stomach. In the living adult human, is found within the wall of the inferior one-third. This transition the esophagus is about 25 centimeters (about 10 inches) long, and marks the beginning of a continuous smooth muscle muscularis most of its length is within the thorax, directly anterior to the that extends throughout the stomach and the small and large vertebral bodies. Only the last 1.5 centimeters of the esophagus intestines to the anus. The outermost layer of the esophagus is an are located in the abdomen. The empty esophagus is flattened; adventitia. The esophagus adheres to the posterior body wall via thus, there is no continuously open lumen. Only a passing bolus its adventitia. of food slightly expands the esophagus. At the superior end of the esophagus, the superior esopha- geal sphincter (or pharyngoesophageal sphincter) is a thickened 26.5b Histology ring of circular skeletal muscle marking the area where the The esophageal mucosa is different from that of the abdominal esophagus and the pharynx meet. This sphincter is closed during GI tract organs in that it is composed of thick, nonkeratinized inhalation of air, so that air doesn’t enter the esophagus and enters stratified squamous epithelium (figure 26.10). The stratified the larynx and trachea instead. The orifice between the esophagus squamous epithelium is better suited to withstand the abrasions of and the stomach is bounded by a thin band of circular smooth the bolus as it moves through the esophagus. Since the esophagus muscle, the inferior esophageal sphincter (esophagealgastric or does not absorb any nutrients, this thicker, protective epithelium cardiac sphincter). This sphincter isn’t strong enough alone to supports its function. prevent materials from refluxing back into the esophagus; instead, The submucosa is thick and composed of abundant elastic the esophageal opening of the diaphragm assumes the duty of the fibers that permit distension during swallowing. It houses numer- “stronger sphincter” to prevent materials from regurgitating from ous mucous glands that provide a thick, lubricating mucus for the the stomach into the esophagus. epithelium. The muscularis is composed of an inner circular layer and an outer longitudinal layer. The muscularis of the esophagus is WHATW DID YOU LEARN? unique in that it contains a blend of both skeletal and smooth ●8 How do the esophageal tunics differ from the “default” tunic muscle fibers. The two layers of muscle in the superior one-third pattern?

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CLINICAL VIEW Reflux Esophagitis and Sleeping with the head of the bed elevated 4 to 6 inches, so that the Gastroesophageal Reflux Disease body lies at an angle rather than flat, appears to alleviate symptoms. Chronic reflux esophagitis can lead to gastroesophageal reflux disease The inferior esophageal sphincter and the diaphragm usually prevent (GERD). In this condition, frequent gastric reflux erodes the esophageal acidic stomach contents from regurgitating into the esophagus. tissue, so over a period of time, scar tissue builds up in the esophagus, However, sometimes acidic chyme refluxes into the esophagus, caus- leading to narrowing of the esophageal lumen. An endoscope (an opti- ing the burning pain and irritation of reflux esophagitis. Because cal tubular instrument used to visualize the lumina of internal organs) the pain is felt posterior to the sternum and can be so intense that it may be inserted into the laryngopharynx and esophagus to visualize is mistaken for a heart attack, this condition is commonly known as the damage. In advanced cases of GERD, the esophageal epithelium “heartburn.” Unlike the stomach epithelium, the esophageal epithelium may change from stratif ied squamous to columnar secretor y epithelium, is poorly protected against acidic contents and easily becomes inflamed a condition known as Barrett esophagus. The specific reasons why and irritated. Other symptoms include abdominal pain, difficulty in Barrett esophagus develops are not known. Barrett esophagus is asso- swallowing, increased belching, and sometimes bleeding. ciated with an increased risk of cancerous growths in the esophagus, Reflux esophagitis can occur in anyone, but is seen most frequently as these new secretory cells can develop into cancer. The continual in overweight individuals, smokers, those who have eaten a very reflux and injury may also lead to esophageal ulcers. large meal (especially just before bedtime), and/or people with hiatal GERD can be treated with a series of medications. Proton pump inhibitors hernias (hı¯-a¯′ta˘l her′ne¯-a˘ ; rupture), in which a portion of the stomach (e.g., omeprazole [Prilosec], esomeprazole [Nexium]) limit acid secretion protrudes through the diaphragm into the thoracic cavity. Eating spicy in the stomach by acting on the proton (hydrogen ion) “pumps” that help foods or ingesting too much caffeine may exacerbate the symptoms in produce acid. Histamine (H ) blockers (e.g., famotidine [Pepcid], nizatidine people affected by reflux esophagitis. Preventive treatment includes 2 [Axid], ranitidine [Zantac]) also help limit acid secretion in the stomach. lifestyle changes such as limiting meal size, not lying down until Antacids (e.g., Tums, Rolaids) help neutralize stomach acid. at least 2 hours after eating, quitting smoking, and losing weight.

Columnar secretory epithelium

(a) Endoscopic view of a normal (b) An endoscopic view of the esophagus. esophagus shows the signs LM 400x of reflux esophagitis. An endoscopic view of the esophagus shows the signs of reflux esophagitis. The condition known as Barrett esophagus.

The appearance of the bolus at the entryway to the oropharynx 26.6 The Swallowing Process initiates the pharyngeal phase, when tactile sensory receptors trigger Learning Objective: the swallowing reflex, which is controlled by the swallowing center in the medulla oblongata. The bolus passes quickly and involuntarily 1. List and explain the three phases of swallowing. through the pharynx to the esophagus. During this phase, (1) the Swallowing, also called deglutition (dē -gloo-tish′ŭ n; degluto = soft palate and uvula elevate to block the passageway between the to swallow), is the process of moving ingested materials from the nasopharynx and oropharynx; (2) the bolus enters the oropharynx; oral cavity to the stomach. There are three phases of swallowing: and (3) the larynx and laryngeal opening elevate toward the epiglot- the voluntary phase, the pharyngeal phase, and the esophageal tis, ultimately covering and sealing the glottis to prevent swallowed phase (figure 26.11). materials from entering the trachea. Pharyngeal constrictors are The voluntary phase occurs after ingestion. Food and saliva skeletal muscle groups that contract involuntarily and sequentially to mix in the oral cavity. Chewing forms a bolus that is mixed and ensure that the swallowed bolus moves quickly (1 second elapses in manipulated by the tongue and then pushed superiorly against the this phase) through the pharynx and into the esophagus. hard palate. Transverse palatine folds in the hard palate help direct The esophageal phase is involuntary. It is the time (about the bolus posteriorly toward the oropharynx. 5 to 8 seconds) during which the bolus passes through the

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Hard palate Bolus Soft palate and uvula elevate Soft palate and uvula return and cover nasopharynx to pre-swallowing position

Direction of bolus movement

Uvula Oropharynx Tongue Bolus Epiglottis

Larynx Epiglottis closes over Esophagus Esophagus laryngeal Bolus Trachea opening

123Voluntary phase: Bolus of food is pushed Pharyngeal phase (involuntary): As Esophageal phase (involuntary): by tongue against hard palate and then bolus moves into oropharynx, the soft Peristaltic contractions of the esophageal moves toward oropharynx. palate and uvula close off nasopharynx, muscle push bolus toward stomach; soft and the larynx elevates so the epiglottis palate, uvula, and larynx return to their closes over laryngeal opening. pre-swallowing positions. Figure 26.11 Phases of Swallowing. Swallowing, or deglutition, occurs as a result of coordinated muscular activities that force the bolus (1) into the pharynx from the oral cavity, (2) through the pharynx, and (3) into the esophagus on the way to the stomach.

esophagus and into the stomach. This phase begins when the ■ The fundus (fŭ n′dŭ s; bottom) is the dome-shaped region superior esophageal sphincter relaxes to allow ingested materials lateral and superior to the esophageal connection with the into the esophagus. The presence of the bolus within the lumen of stomach. Its superior surface contacts the diaphragm. the esophagus stimulates peristaltic waves of muscular contraction ■ The body (corpus) is the largest region of the stomach; it is that assist in propelling the bolus toward the stomach. The soft inferior to the cardiac orifice and the fundus. palate, uvula, and larynx return to the pre-swallowing positions. ■ The pylorus (pı̄ -lō r′ŭ s; gatekeeper) is a narrow, medially directed, funnel-shaped pouch that forms the terminal WHATW DID YOU LEARN? region of the stomach. The pyloris is divided into two parts: the pyloric antrum (more expanded region near the body) ●9 Briefly describe the three phases of swallowing. and the pyloric canal (more narrow region that attaches to the duodenum). Its opening with the duodenum of the small intestine is called the pyloric orifice. Surrounding 26.7 Stomach this pyloric orifice is a thick ring of circular smooth Learning Objectives: muscle called the pyloric sphincter. The pyloric sphincter regulates the entrance of chyme into the small intestine by 1. Describe the gross anatomy of the stomach. closing upon sympathetic innervation and opening upon 2. Explain the histology of the stomach wall. parasympathetic innervation. 3. Compare and contrast the secretions of the stomach. The inferior convex border of the stomach is the greater cur- The stomach (stŭ m′ŭ k; stomachus = belly) is a muscular, vature, while the superior concave border is the lesser curvature. J-shaped sac that occupies the left upper quadrant of the abdomen, The greater omentum attaches to the greater curvature edge of the immediately inferior to the diaphragm (figure 26.12). It contin- stomach, and the lesser omentum extends between the lesser cur- ues the mechanical and chemical digestion of the bolus. After the vature and the liver. bolus has been completely processed in the stomach, the product is Internally, the stomach lining is composed of numerous gas- called chyme (kı̄ m; chymos = juice). Chyme has the consistency of a tric folds, or rugae (roo′gē ; ruga = wrinkle). These gastric folds, pastelike soup. The stomach facilitates mechanical digestion by the which are observed only when the stomach is empty, allow the contractions of its thick muscularis layer, which churns and mixes stomach to expand greatly when it fills and then return to its nor- the bolus and the gastric secretions. The stomach facilitates chemi- mal J-shape when it empties. cal digestion through its gastric secretions of acid and enzymes. 26.7a Gross Anatomy 26.7b Histology The stomach is lined by a simple columnar epithelium, although The stomach is composed of four regions: little absorption occurs in the stomach. This epithelium does not ■ The cardia (kar′dē -ă ; heart) is the small, narrow, superior contain goblet cells; instead, surface mucous cells (described later) entryway into the stomach lumen from the esophagus. The secrete mucin onto the epithelial lining. Another feature that dis- internal opening where the cardia meets the esophagus is tinguishes the stomach mucosa from the default pattern is that the called the cardiac orifice. stomach lining is indented by numerous depressions called gastric

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Fundus Esophagus

Cardia

Longitudinal layer (outer) Pyloric Circular orifice Pyloric Lesser layer Three layers Duodenum sphincter curvature (middle) of smooth Oblique muscle layer (inner) Body

Pyloric Greater curvature canal Pylorus

Pyloric antrum Gastric folds

(a) Stomach regions, anterior view

Figure 26.12 Liver (cut) Lesser curvature Diaphragm Gross Anatomy of the Stomach. The stomach is a muscular sac where mechanical and chemical digestion of the bolus occurs. (a) The major regions of the stomach are the cardia, the fundus, the body, and the pylorus. Three layers of Esophagus smooth muscle make up the muscularis. Cardiac orifice (b) A cadaver photo shows an anterior section of the stomach, illustrating the gastric folds and the cardiac orifice.

Gastric folds

Body of stomach Pylorus of stomach

Greater curvature

(b) Gross anatomy of stomach (cut open)

pits. At the base of each pit are the openings of several branched WHAT DO YOU THINK? tubular glands, called gastric glands, which extend through the ●3 The stomach secretes gastric juices, which are highly acidic and length of the mucosa to its base (figure 26.13). The mucosa of can break down and chemically digest food. What prevents the the stomach is only 1.5 millimeters at its thickest point (about the gastric juices from eating away at the stomach itself? thickness of a nickel). The muscularis mucosae lies between the base of the gastric glands and the submucosa, and it helps expel gastric gland secretory products when it contracts. 26.7c Gastric Secretions The muscularis of the stomach varies from the default pattern Gastric juices are produced by cells in the gastric glands, and their in that it is composed of three smooth muscle layers instead of two: secretions are released into gastric pits, which funnel them to the an inner oblique layer, a middle circular layer, and an outer longitu- lumen of the stomach. Five types of secretory cells form the gastric dinal layer. The oblique layer is best-developed at the cardia and the epithelium (figure 26.13b, c). body of the stomach. The presence of a third layer of smooth muscle Surface mucous cells line the stomach lumen and extend assists the continued churning and blending of the bolus. The stom- into the gastric pits. They continuously secrete mucin onto the ach is intraperitoneal, so its outermost tunic is a serosa. gastric luminal surface to prevent ulceration of the lining upon

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Opening to gastric pit

Gastric pit

Simple columnar epithelium

Lamina propria

Mucosa Lymph vessel

Muscularis mucosae

Submucosa Submucosal nerve plexus

Oblique layer

Muscularis Circular layer

Longitudinal layer

Serosa

Myenteric nerve plexus Artery Vein (a) Stomach wall, sectional view

Opening to gastric pit Surface mucous cell (secretes mucin) Simple Gastric pit columnar epithelium Mucous neck cell Gastric pit (secretes acidic mucin)

Parietal cell (secretes hydrochloric acid and intrinsic factor)

Gastric gland Chief cell (secretes pepsinogen)

Gastric glands Enteroendocrine cell (secretes gastrin) LM 60x

(b) Stomach mucosa (c) Gastric pit and gland Figure 26.13 Histology of the Stomach Wall. (a) The stomach lumen contains invaginations within the mucosa called gastric pits that lead into gastric glands. (b) A photomicrograph shows gastric glands and the cells lining the gastric pit. (c) A diagrammatic section of a gastric pit and gland shows their structure and the distribution of different types of secretory cells.

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CLINICAL VIEW Peptic Ulcers Irritation of the gastric mucosa (gastritis) has been linked to many cases of peptic ulcer. Nonsteroidal anti-inflammatory drugs (NSAIDs), such as Normally there is a balance in the stomach between the acidic gastric ibuprofen and aspirin, are a common cause of gastritis, and these drugs juices and the protective regenerative nature of the mucosa lining. also impair healing of the gastric lining. However, the major player in When this balance is thrown off, the stage is set for the development peptic ulcer formation is a bacterium called Helicobacter pylori, which is of a peptic ulcer, which is a chronic, solitary erosion of a portion present in over 70% of gastric ulcer cases and well over 90% of duodenal of the lining of either the stomach or the duodenum. Annually, over ulcer cases. H. pylori resides in the stomach and produces enzymes that 4 million people in the United States are diagnosed with an ulcer. break down the components in the gastric mucus, weakening its protective Gastric ulcers are peptic ulcers that occur in the stomach, whereas effects. As leukocytes enter the stomach to destroy the bacteria, they also duodenal ulcers are peptic ulcers that occur in the superior part of destroy the mucous neck cells. This further irritates the stomach lining the duodenum, which is the first segment of the small intestine. and creates an ideal environment for continued H. pylori colonization. Duodenal ulcers are common because the first part of the duode- Thus, the bacteria initiate a cascade of events that lead to erosion of the num receives the chyme from the stomach but has yet to receive gastric lining and eventual perforation of the stomach wall if not treated. the alkaline pancreatic juice that can neutralize chyme’s acidic Treatment for a gastric ulcer involves eliminating the bacteria as well as content. Symptoms of an ulcer include a gnawing, burning pain in reducing gastric acidity to promote healing. Categories of medications that the epigastric region, which may be worse after eating, as well as help include an antibiotic taken for 2 weeks to eradicate H. pylori, an antacid nausea, vomiting, and extreme belching. Bleeding may also occur, (Tums, Rolaids) to help neutralize stomach acid, a proton-pump inhibitor and the partially digested blood results in dark and tarry stools. If (e.g., omeprazole, esomeprazole), and/or a histamine (H ) blocker (e.g., left untreated, an ulcer may erode the entire organ wall and cause 2 famotidine, nizatidine, ranitidine) to help limit acid secretion in the stomach. perforation, which is a medical emergency.

Stomach

Duodenal ulcer Gastric ulcers Mucosa Submucosa Muscularis Serosa

Duodenum

(a) Common locations of gastric and duodenal ulcers

(a) A sectioned view of the stomach and duodenum illustrates some common locations for gastric and duodenal ulcers. (b) A gross anatomic specimen shows a perforated gastric ulcer.

(b) Perforated gastric ulcer

exposure to the high acidity of the gastric fluid and protect the The acidic mucin helps maintain the acidic conditions resulting epithelium from gastric enzymes. from the secretion of hydrochloric acid by parietal cells. Mucous neck cells are located immediately deep to the base Parietal cells (also called oxyntic cells) are located primarily of the gastric pit and are interspersed among the parietal cells. in the proximal and middle parts of the gastric gland. Their dis- These cells produce an acidic mucin that differs structurally and tinctive features are small intracellular channels called canaliculi, functionally from the mucin secreted by the surface mucous cells. which are lined by microvilli. Hydrochloric acid secreted across

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the parietal cells’ vast internalized surface helps denature proteins to facilitate chemical digestion. Parietal cells also produce intrin- Duodenum sic factor, a molecule that binds vitamin B12 in the stomach lumen and assists in B absorption in the ileum of the small intestine. 12 Duodenojejunal Chief cells (also called zymogenic cells or peptic cells) are flexure housed primarily in the distal part of the gastric gland. These cells Large intestine synthesize and secrete enzymes, primarily inactive pepsinogen, into the lumen of the stomach. The acid content of the stomach Jejunum then converts inactive pepsinogen into the active enzyme pepsin, which chemically digests denatured proteins in the stomach into smaller fragments. Enteroendocrine (en′ter-ō-en′dō-krin; enteron = gut, intestine) Ileocecal valve cells are endocrine cells widely distributed in the gastric glands of the stomach. These cells secrete gastrin, a hormone that enters the blood and stimulates the secretory activities of the chief and parietal Ileum cells and the contractile activity of gastric muscle. Enteroendocrine cells also produce other hormones, such as somatostatin, that modu- late the function of nearby enteroendocrine and exocrine cells. There are numerous gastric pits in the mucosa of the stomach, but the types of secretory cells differ in the various regions of the Figure 26.14 stomach. The glands of the fundus and body contain all of the cell Gross Anatomy of the Small Intestine. The three regions of the types previously discussed; however, the glands of the cardia and small intestine—duodenum, jejunum, and ileum—are continuous and pylorus contain mostly mucous neck cells. Parietal cells are abundant “framed” within the abdominal cavity by the large intestine. in the glands of the fundus and body, but they are virtually absent in pyloric glands. Thus, the glands of the fundus and body produce pylorus of the stomach to the cecum of the large intestine, and thus highly acidic secretions, whereas those in the cardia and pylorus pro- occupies a significant portion of the abdominal cavity. The small duce mainly protective, alkaline, mucin. This makes functional sense, intestine receives its blood supply primarily from branches of the since alkaline mucus can help protect the adjacent regions of the superior mesenteric artery, and it is innervated by the superior digestive tract (esophagus and duodenum) from damage by stomach mesenteric plexus. The small intestine consists of three specific seg- acid and digestive enzymes. Surface mucous cells protect the stomach ments: the duodenum, the jejunum, and the ileum (figure 26.14). lining, but the esophagus and duodenum lack such protection. The duodenum (doo-ō -dē′ nŭ m, doo-od′ĕ -nŭ m; breadth of Saliva, mucin, and gastric secretions contribute substantially twelve fingers) forms the initial or first segment of the small intestine. to the volume of ingested material. In fact, if a person eats 1 liter It is approximately 25 centimeters (10 inches) long and originates at of food, the amount of chyme that enters the small intestine is 3 to the pyloric sphincter. The duodenum is arched into a C-shape around 4 liters, due to the volume of the secretions. the head of the pancreas and becomes continuous with the jejunum ′ = WHATW DID YOU LEARN? at the duodenojejunal flexure (flek sher; fleksura bend). Most of the duodenum is retroperitoneal, although the very proximal part ●10 What are the four regions of the stomach, and where is each located? is intraperitoneal and connects to the liver by the lesser omentum. ●11 What are the five types of secretory cells in the stomach, and Within the wall of the duodenum is the major duodenal papilla, what does each secrete? through which bile and pancreatic juice enter the duodenum. A minor duodenal papilla also receives an additional small amount of pancreatic juice via an accessory pancreatic duct. 26.8 Small Intestine The jejunum (jĕ -joo′nŭ m; jejunus = empty) is the middle Learning Objectives: region of the small intestine. Extending approximately 2.5 meters (7.5 feet in an unembalmed cadaver), it makes up approximately 1. Describe the gross anatomy of the small intestine. two-fifths of the small intestine’s total length. The jejunum is the 2. Compare and contrast the three regions of the small primary region within the small intestine for chemical digestion intestine. and nutrient absorption. It is intraperitoneal and suspended in the 3. Explain the microscopic structure of the small intestine. abdomen by the mesentery proper. The small intestine finishes the chemical digestion process The ileum (il′ē -ŭ m; eiles = twisted) is the final or last region and is responsible for absorbing up to 90% of the nutrients and of the small intestine. At about 3.6 meters (10.8 feet) in length in an water. Ingested nutrients spend at least 12 hours in the small intes- unembalmed cadaver, the ileum forms approximately three-fifths tine as chemical digestion and absorption are completed. of the small intestine. Its distal end terminates at the ileocecal (il′ē -ō -sē ′kă l) valve, a sphincter that controls the entry of materials 26.8a Gross Anatomy and Regions into the large intestine. The ileum is intraperitoneal and suspended The small intestine, also called the small bowel, is a coiled, thin- in the abdomen by the mesentery proper. walled tube about 6 meters (20 feet) in length in the unembalmed Internally, the mucosal and submucosal tunics of the small cadaver. (It is much shorter in a living individual due to muscle intestine are thrown into circular folds (also called plicae [plı̄′ kē ; tone. We do not know the exact length in a living human because fold] circulares) (figure 26.15a, b). Circular folds, which can be it would be physically difficult to measure the intestine, and would seen with the naked eye, help increase the surface area through cause severe discomfort to a living individual.) It extends from the which nutrients can be absorbed. In addition, the circular folds

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Simple columnar epithelium with microvilli (absorbs nutrients)

Circular folds Capillary network

Mucosa Goblet cells Submucosa Muscularis Inner circular layer Lacteal Outer longitudinal layer Enteroendocrine cells Circular fold (secrete hormones) Serosa

(a) Intestinal gland

Lymphatic nodule

Intestinal villi Muscularis mucosae Venule Lymph vessel Submucosa Arteriole

(c) Intestinal villus

Inner circular layer Muscularis Outer longitudinal layer Serosa

(b) Section of small intestine

Simple columnar cell Intestinal lumen Microvilli Villi

Simple columnar epithelium

Lamina propria Intestinal lumen

TEM 18,000x

(e) Microvilli Goblet cells Figure 26.15 Histology of the Small Intestine. The surface area of the small intestine wall is vastly increased by specific structural modifications. (a) The wall is composed of four concentric tunics. (b) Circular folds formed from the mucosa and submucosa LM 70x are lined by a dense covering of fingerlike projections called intestinal villi, which are (d) Intestinal villi formed from mucosa only. (c) The structure of a single villus. (d) A photomicrograph shows the internal structure of villi projecting into the intestinal lumen. (e) The plasma membrane along the apical surface of the epithelium contains microvilli.

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Table 26.4 Small Intestine Structures Involved in Digestion and Absorption Structure Anatomy Function Circular folds Circular folds of mucosa and submucosa Slow down the passage of materials undergoing digestion; increase surface area for both absorption and chemical digestion Villi Fingerlike projections of mucosa Increase surface area for both absorption and chemical digestion Microvilli Folded, fi ngerlike projections of plasma membrane on apical Increase surface area for both absorption and chemical surface of columnar epithelial cells digestion Intestinal glands Invaginations into mucosa between villi Increase surface area for both absorption and chemical digestion; enteroendocrine cells lining intestinal glands secrete digestive hormones Submucosal glands Coiled tubular glands within submucosa with ducts opening Secrete alkaline mucin to protect and lubricate lining of small into intestinal lumen intestine

act like “speed bumps” to slow down the movement of chyme jejunum. The number of goblet cells in the mucosa increases from and ensure that it remains within the small intestine for maximal the duodenum to the ileum, due to the increased need for lubrica- nutrient absorption. Circular folds are more numerous in the duo- tion as digested materials are absorbed and undigested materials denum and jejunum, and least numerous in the ileum. are left behind. Peyer patches are abundant primarily in the ileum. Table 26.4 summarizes all of the small intestine structures WHAT DO YOU THINK? that aid in digestion and absorption.

●4 Why are the circular folds much more numerous in the duodenum and least numerous in the ileum? How does the abundance of WHATW DID YOU LEARN? circular folds relate to the main functions of the duodenum and ●12 Compare and contrast the gross anatomic and histologic ileum? characteristics that distinguish the duodenum, jejunum, and ileum. 26.8b Histology When circular folds are viewed at the microscopic level, smaller, fingerlike projections of mucosa only, called villi, can be seen 26.9 Large Intestine along their surface. These villi further increase the surface area for Learning Objectives: absorption and secretion. Increasing the absorptive surface area 1. Describe the gross anatomy of the large intestine. even further are microvilli (mı̄ -krō -vil′ı̄ ; mikros = small) along the 2. Compare and contrast the large intestine regions. free surface of the simple columnar cells (figure 26.15e). Individual 3. Explain the microscopic structure of the large intestine. microvilli are not clearly visible in light micrographs of the small 4. Trace the movement of material through the large intestine; instead, they collectively appear as a brush border that intestine. resembles a brightly staining surface on the apical end of the simple columnar cells (figure 26.15d). Each villus contains an arte- The large intestine, also called the large bowel, forms a riole and a venule, with a rich capillary network between them. three-sided perimeter in the abdominal cavity around the centrally The capillaries absorb most nutrients. In the center of the villus is located small intestine (figure 26.16). From its origin at the ileo- a lacteal, which is responsible for absorbing lipids and lipid-soluble cecal junction to its termination at the anus, the large intestine vitamins, which are too large to be absorbed by the capillaries. has an approximate length of 1.5 meters (5 feet) and a diameter of Between some of the intestinal villi are invaginations of mucosa 6.5 centimeters (2.5 inches). It is called the “large” intestine because called intestinal glands (also known as intestinal crypts or crypts its diameter is greater than that of the small intestine. Recall that of Lieberkuhn). These glands extend to the base of the mucosa and the small intestine absorbs much, but not all, of the digested mate- slightly resemble the gastric glands of the stomach. Lining them rial. On average, about 1 liter of remaining material passes from the are simple columnar epithelial cells (with goblet cells) and entero- small intestine to the large intestine daily. endocrine cells. These enteroendocrine cells release hormones The large intestine absorbs most of the water and ions from such as secretin, cholecystokinin, and gastric inhibitory peptide. the remaining digested material. In so doing, the watery material Some of these hormones temporarily slow down digestive activities that first enters the large intestine soon solidifies and becomes feces. as material from the stomach begins to enter the small intestine, The large intestine stores the feces until the body is ready to defecate thereby prolonging the time for stomach emptying into the small (expel the feces). The large intestine also absorbs a very small per- intestine. The goblet cells produce mucin to lubricate and protect centage of nutrients still remaining in the digested material. the intestinal lining as materials being digested pass through. Distinctive histologic features characterize the three small 26.9a Gross Anatomy and Regions intestine regions. The proximal duodenum contains submucosal The initial or first region of the large intestine is a blind sac called glands (or Brunner glands), which produce a viscous, alkaline the cecum (sē ′kŭ m; caecus = blind), which is located in the right mucus that protects the duodenum from the acidic chyme. Circular lower abdominal quadrant. This pouch extends inferiorly from folds are best-developed in the jejunum and nearly absent in the the ileocecal valve, which represents the attachment of the dis- ileum. Additionally, villi are larger and more numerous in the tal end of the small intestine to the proximal region of the large

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Transverse colon

Ascending Descending colon colon

Cecum Vermiform Sigmoid colon appendix Rectum Anal canal Left colic flexure

Transverse mesocolon Right colic flexure Omental appendices Haustrum Tenia coli

Superior Descending abdominal aorta mesenteric artery Inferior mesenteric artery

Ileocecal Rectum valve Rectal valve Cecum Ileum Levator ani muscle Vermiform appendix Anal canal

Rectum Veins Sigmoid Internal mesocolon anal sphincter Anus External anal sphincter Anal canal Anal columns Anal sinuses (a) Large intestine, anterior view (b) Anal canal Figure 26.16 Gross Anatomy of the Large Intestine. (a) Anterior view of the large intestine, which forms the distal end of the GI tract. (b) Details of the anal canal.

intestine. Projecting inferiorly from the posteromedial region of with peritoneum. As it approaches the inferior surface of the liver, the cecum is the vermiform (ver′mi-fō rm; vermis = worm) appen- the ascending colon makes a 90-degree turn toward the left side dix (ă -pen′diks; appendage), a thin, hollow, fingerlike sac lined of the abdominal cavity. This bend in the colon is called the right by lymphocyte-filled lymphatic nodules. Both the cecum and the colic (kol′ik) flexure, or the hepatic flexure (figure 26.16a). vermiform appendix are intraperitoneal organs. The transverse colon originates at the right colic flexure At the level of the ileocecal valve, the colon begins and forms and curves slightly anteriorly as it projects horizontally across an inverted U-shaped arch. The colon is partitioned into four seg- the anterior region of the abdominal cavity. A type of mesen- ments: the ascending colon, transverse colon, descending colon, tery called the transverse mesocolon connects the transverse and sigmoid colon. portion of the large intestine to the posterior abdominal wall. The ascending colon originates at the ileocecal valve and Hence, the transverse colon is intraperitoneal. As the transverse extends superiorly from the superior edge of the cecum along the colon approaches the spleen in the left upper quadrant of the right lateral border of the abdominal cavity. The ascending colon abdomen, it makes a 90-degree turn inferiorly. The resulting is retroperitoneal, since its posterior wall directly adheres to the bend in the colon is called the left colic flexure, or the splenic posterior abdominal wall, and only its anterior surface is covered (splen′ik) flexure.

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The descending colon is retroperitoneal and found along retained during the passing of gas. The rectum then terminates the left side of the abdominal cavity. It originates at the left colic at the anal canal. flexure and descends vertically until it terminates at the sigmoid The terminal few centimeters of the large intestine are colon. The sigmoid (sig′moyd; resembling letter S) colon originates called the anal (ā′ nă l) canal (figure 26.16b). The anal canal passes at the sigmoid flexure, where the descending colon curves and through an opening in the levator ani muscles of the pelvic floor turns inferomedially into the pelvic cavity. The sigmoid colon is and terminates at the anus. The internal lining of the anal canal intraperitoneal and has a mesentery called the sigmoid mesoco- contains relatively thin longitudinal ridges, called anal columns, lon. The sigmoid colon terminates at the rectum. between which are small depressions termed anal sinuses. As The rectum (rek′tŭ m; rectus = straight) is a retroperito- fecal material passes through the anal canal during defecation, neal structure that connects to the sigmoid colon. The rectum pressure exerted on the anal sinuses causes their cells to release is a muscular tube that readily expands to store accumulated excess mucin. As a result, this extra mucus lubricates the anal fecal material prior to defecation. Three thick, transverse folds canal during defecation. At the base of the anal canal are the of the rectum, called rectal valves, ensure that fecal material is internal and external anal sphincters, which close off the opening to the anal canal and relax (open) during defecation. The internal anal sphincter is involuntary, whereas the external anal sphincter is voluntary. Study Tip! Many of the segments of the colon are named for the direction 26.9b Histology in which materials travel. So material ascends (travels superiorly) in The mucosa of the large intestine is lined with simple colum- the ascending colon, material travels across in the transverse colon, and nar epithelium and goblet cells (figure 26.17). Unlike the material descends (travels inferiorly) in the descending colon. small intestine, the large intestine mucosa lacks intestinal villi; however, it contains numerous intestinal glands that extend to

Goblet cells Opening to Opening to intestinal gland intestinal gland

Simple columnar epithelium Goblet cells

Mucosa Simple columnar epithelium Intestinal gland

Lamina propria Intestinal gland

Lymphatic nodule Submucosa Muscularis mucosae Muscularis

Muscularis mucosae Inner circular Outer layer of longitudinal muscularis layer of LM 80x Serosa muscularis (or adventitia) Nerves Arteriole Venule (tenia coli) (a) Large intestine tunics (b) Large intestine mucosa and submucosa Figure 26.17 Histology of the Large Intestine. (a) The luminal wall of the large intestine is composed of several layers. (b) A photomicrograph shows the histology of the wall of the large intestine.

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CLINICAL VIEW: In Depth Colorectal Cancer should see their doctor if they experience rectal bleeding or any persistent change in bowel habits. By age 50 (or earlier, if you have Colorectal cancer is the second most common type of cancer in the symptoms or a family history of colorectal cancer), individuals should United States, with over 140,000 cases occurring annually and 60,000 take advantage of the following screening methods: of those resulting in death. In recent years, high-profile individuals 1. Take a yearly fecal occult (o˘-ku˘ lt′, ok′u˘ lt) blood test, which such as baseball players Eric Davis and Darryl Strawberry and Supreme checks for the presence of blood in the stools. Court Justice Ruth Bader Ginsburg have been treated for colorectal 2. Every 5 years, have a sigmoidoscopy (sig′moy-dos′ko˘-pe¯). cancer. These well-publicized cases have helped increase awareness In this procedure, which is done in the doctor’s office, an of this deadly disease. endoscope is inserted into the anus, rectum, and sigmoid colon The term colorectal cancer refers to a malignant growth anywhere to check for polyps or cancer. along the colon or rectum. The majority of colorectal cancers appear 3. Every 10 years, undergo a colonoscopy (ko¯-lon-os′ko¯-pe¯; skopeo = in the rectum, sigmoid colon, and distal descending colon, which are to view). A colonoscopy is more extensive than a sigmoidoscopy. the segments of the large intestine that have the longest contact with The endoscope is inserted through the anus and into the large fecal matter before it is expelled from the body. Most colorectal cancers intestine at least up to the right colic flexure of the colon, and arise from polyps (pol′ip; polys = many), which are outgrowths from the sometimes as far proximally as the ileocecal valve. colon mucosa. Note, however, that colon polyps are very common, and In addition, people are advised to eat a high-fiber diet to reduce the most of them never become cancerous. Low-fiber diets have also been risk of developing colorectal cancer. implicated in increasing the risk of colon cancer, because decreased dietary fiber leads to decreased stool (section of fecal matter) bulk and Polyps longer time for stools to remain in the large intestine, thus theoretically exposing the large intestine mucosa to toxins in the stools for longer periods of time. Other risk factors include a family history of colorectal cancer, personal history of ulcerative colitis, and older age (since most Colon patients are over the age of 40). cancer Initially, most patients are asymptomatic. Later, they may notice rectal bleeding (often evidenced as blood in the stool or on the toilet paper) and a persistent change in bowel habits (typically constipation). For some, the bleeding may not be noticeable, but can be detected in a stool sample. Eventually, the person may experience abdominal pain, fatigue, unexplained weight loss, and anemia. A cancerous growth in the colon must be removed surgically, and sometimes radiation and/or chemotherapy are used as well. Colorectal cancers that are limited to the mucosa have a 5-year survival rate, but the prognosis is poor for cancers that have spread into deeper colon wall tunics or metastasized to the lymph nodes. The key to an increased survival rate for colorectal cancer is early detection. If caught early, colorectal cancer is very treatable. People Polyps in the large intestine sometimes lead to colorectal cancer.

the muscularis mucosae. The glands’ goblet cells secrete mucin The teniae coli act like elastic in a waistband—they help bunch to lubricate the undigested material as it passes through, and the up the large intestine into many sacs, collectively called haustra simple columnar epithelial cells continue to absorb nutrients that (haw′stră ; sing., haustrum; haustus = to drink up) (see figure were not absorbed during passage through the small intestine. 26.16a). Hanging off the external surface of the haustra are lob- Many lymphatic nodules and lymphatic cells occupy the lamina ules of fat called omental appendices, or epiploic (ep′i-plō′ ik; propria of the large intestine. membrane-covered) appendages. The muscularis of the colon and cecum has two layers of smooth muscle, but the outer longitudinal layer does not 26.9c Control of Large Intestine Activity completely surround the colon and cecum. Instead, these longi- Large intestine movements are regulated by local reflexes in the tudinal smooth muscle fibers form three thin, distinct, longitu- autonomic nervous system. With the ingestion of more food, peri- dinal bundles called teniae (tē′ nē -ē; ribbons, band) coli (kō′ lı̄ ). staltic movements in the ileum increase, as does the frequency

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CLINICAL VIEW CLINICAL VIEW

Appendicitis Diverticulosis and Diverticulitis

Inflammation of the appendix is called appendicitis (a˘-pen- Diverticulosis is the presence of out-pocketings of the di-sı¯′tis). Most cases of appendicitis occur because fecal mat- intestinal wall known as diverticula. Diverticula are acquired ter obstructs the appendix, although sometimes an appendix protrusions of the mucosa through the colonic wall that have becomes inflamed without any obstruction. As the tissue in evaginated through the submucosa and a weakened and/or its wall becomes inflamed, the appendix swells, the blood sup- diminished muscularis mucosa. Diverticula occur in weakened ply is compromised, and bacteria may proliferate in the wall. areas in the bowel wall where the nutrient vessels pierce the Untreated, the appendix may burst and spew its contents into muscularis near the teniae coli and the omental appendices. the peritoneum, causing a massive infection called peritonitis, These defects can become weaker with age. Diverticula are and possibly leading to death. most common in the sigmoid colon (95%) but may be con- centrated along the course of the teniae coli for the entire During the early stages of acute appendicitis, the smooth muscle length of the colon. wall contracts and goes into spasms. Because this smooth muscle is innervated by the autonomic nervous system, pain is referred The exact etiology of diverticulosis is poorly understood; to the T10 dermatome around the umbilicus. however, high intraluminal pressures caused by straining in people with motility problems or constipation may be a sig- As the inflammation worsens and the parietal peritoneum becomes nificant factor. Most scientists agree that a low-fiber diet is inflamed as well, the pain becomes sharp and localized to the right an underlying cause of diverticulosis. Most individuals with lower quadrant of the abdomen. Individuals with appendicitis diverticulosis are asymptomatic, without evidence of com- typically experience nausea or vomiting, abdominal tenderness in plications. Complications of diverticulosis include bleeding, the inferior right quadrant, a low fever, and an elevated leukocyte peridiverticular abscess, perforation, stricture, and fistula count. An inflamed appendix is surgically removed in a procedure formation. If some of the diverticula become infected or called an appendectomy. inflamed, the condition is known as diverticulitis. Diverticulitis occurs in only about 10-20% of patients with diverticulosis. Treatment for diverticulitis requires the use of a special diet, antibiotics, and occasionally surgery.

of the opening of the ileocecal valve. This so-called gastroileal reflex results in the accumulation of more chyme in the cecum and ascending colon. Segmentation movements are infrequent in the colon. Instead, three types of movements are typically asso- ciated with the passage of digested material through the large intestine, the absorption of fluid and ions, and the packaging of waste materials for defecation: peristaltic movements, haustral churning, and mass movement. Peristaltic movements of the large intestine are usually (a) (b) weak and sluggish, but otherwise they resemble those that occur Diverticulosis (a) An external view of the sigmoid colon showing in the wall of the small intestine. Haustral churning occurs after diverticula. (b) An endoscopic view of diverticula. a relaxed haustrum fills with digested or fecal material until its distension stimulates reflex contractions in the muscularis, causing churning and movement of the material to more distal haustra. Mass movements are powerful, peristaltic-like contrac- tions involving the teniae coli that propel fecal material toward the rectum. Generally, mass movements occur two or three times 26.10 Accessory Digestive Organs a day, often during or immediately after a meal. This is the gas- trocolic (gas′trō -kol′ik) reflex. Learning Objectives: 1. Identify liver anatomy and blood supply. 2. Describe bile secretion. WHATW DID YOU LEARN? 3. Explain the gross anatomy and microanatomy of the ●13 Identify the retroperitoneal and intraperitoneal large intestine pancreas. structures. 4. Compare and contrast pancreatic acinar cell function. ●14 What specific movements and reflexes propel material through the 5. Trace and describe how secretory products travel through large intestine? the biliary apparatus.

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Inferior Anterior Right lobe vena cava Left lobe Quadrate lobe

Round ligament of liver Gallbladder

Porta hepatis Cystic duct Hepatic artery proper Common hepatic duct Hepatic portal vein Right lobe Left lobe Falciform ligament Inferior vena cava Ligamentum Round ligament venosum of liver Coronary ligament Gallbladder Caudate lobe Bare area Posterior (a) Anterior view (b) Posteroinferior view Figure 26.18 Gross Anatomy of the Liver. The liver is in the upper right quadrant of the abdomen. (a) Anterior and (b) posteroinferior views show the four lobes of the liver, as well as the gallbladder and the porta hepatis.

The accessory digestive organs produce secretions that facilitate embryo. This vessel shunted blood from the umbilical vein to the the chemical digestive activities of GI tract organs. Important acces- inferior vena cava.) Finally, the porta (pō r′tă ; gate) hepatis (hep′ă - sory digestive organs are the liver, the gallbladder, and the pancreas. tis) represents the horizontal crossbar of the H and is where blood and lymph vessels, bile ducts, and nerves enter and leave the liver. 26.10a Liver In particular, the hepatic portal vein and branches of the hepatic The liver (liv′er) lies in the right upper quadrant of the abdomen, artery proper enter at the porta hepatis. immediately inferior to the diaphragm. Weighing 1 to 2 kilograms (2.25 to 4.5 pounds), it constitutes approximately 2% of an adult’s Histology body weight. The liver is covered by a connective tissue capsule A connective tissue capsule branches through the liver and forms and a layer of visceral peritoneum, except for a small region on its septa that partition the liver into thousands of small, polyhedral diaphragmatic surface called the bare area. hepatic lobules, which are the classic structural and functional units of the liver (figure 26.19). Within hepatic lobules are liver Gross Anatomy cells called hepatocytes (hep-a′tō -sı̄ t). At the periphery of each lob- The liver is composed of four incompletely separated lobes and ule are several portal triads, composed of branches of the hepatic supported by two ligaments (figure 26.18). The major lobes portal vein, the hepatic artery, and the bile duct. are the right lobe and the left lobe. The right lobe is separated A dual blood supply serves the liver. The hepatic portal vein from the smaller left lobe by the falciform ligament, a peritoneal carries blood from the capillary beds of the GI tract, spleen, and pan- fold that secures the liver to the anterior abdominal wall. In the creas. It brings approximately 75% of the blood volume to the liver. inferior free edge of the falciform ligament lies the round ligament This blood is rich in nutrients and other absorbed substances but of the liver (or ligamentum teres), which represents the remnant of relatively poor in oxygen. The hepatic artery proper, a branch of the the fetal umbilical vein. Subdivisions of the right lobe include the celiac trunk, splits into left and right hepatic arteries. These arter- caudate (kaw′dā t; cauda = tail) lobe and the quadrate (kwah′drā t; ies carry well-oxygenated blood to the liver. Blood from branches quadratus = square) lobe. The caudate lobe is adjacent to the infe- of the hepatic arteries and hepatic portal vein mixes as it passes rior vena cava, and the quadrate lobe is adjacent to the gallbladder to and through the hepatic lobules. At the center of each lobule (figure 26.18b). is a central vein that drains the blood from the lobule. Central Along the inferior surface of the liver are several structures veins collect venous blood and merge throughout the liver to form that collectively resemble the letter H. The gallbladder and the numerous hepatic veins that eventually empty into the inferior round ligament of the liver form the vertical superior parts of vena cava. the H; the inferior vena cava and the ligamentum venosum In cross section, a hepatic lobule looks like a side view of a form the vertical inferior parts. (Recall from chapter 23 that the bicycle wheel. The hub of the wheel is represented by the central ligamentum venosum was a remnant of the ductus venosus in the vein. At the circumference of the wheel where the tire would be are

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Hepatic sinusoid

Central vein Hepatocytes Hepatic Bile canaliculi lobule Reticuloendothelial Central vein cell Hepatic sinusoid Bile canaliculi Hepatocyte

Portal triad Branch of (a) Hepatic lobules bile duct Branch of hepatic portal vein Branch of hepatic artery

Portal triad Branch of bile duct Branch of Hepatic hepatic sinusoid portal vein Branch of (b) Hepatocytes and sinusoids Hepatocytes hepatic artery

LM 220x

(c) Portal triad Figure 26.19 Histology of the Liver. (a) The functional units of the liver are called hepatic lobules. (b) A central vein projects through the center of a hepatic lobule, and several portal triads define its periphery. (c) A photomicrograph depicts the portal triad and hepatocytes.

several portal triads that are usually equidistant apart. The numer- also store excess nutrients and vitamins and release them when ous spokes of the wheel are the hepatic sinusoids (si′nū -soyd; they are needed. Finally, hepatocytes synthesize blood plasma sinus = cavity), which are bordered by cords of hepatocytes. Hepatic proteins such as albumins, globulins, and proteins required for sinusoids are thin-walled, porous or “leaky” capillaries where blood clotting. Reticuloendothelial cells in the liver sinusoids venous and arterial blood are mixed and then flow slowly through phagocytize debris in the blood as well as help break down and the hepatic lobule toward the central vein. The sinusoids are lined recycle components of aged erythrocytes and damaged or worn- with stellate cells called reticuloendothelial cells (or Kupffer cells), out formed elements. which are phagocytic cells that have an immune function. Hepatocytes absorb nutrients from the sinusoids, and they 26.10b Gallbladder also produce bile, a greenish fluid that breaks down fats to assist Attached to the inferior surface of the liver, a saclike organ called in their chemical digestion. Between each cord of hepatocytes is a the gallbladder (see figure 26.18) concentrates bile produced by small bile canaliculus. Bile canaliculi conduct bile from the hepa- the liver and stores this concentrate until it is needed for digestion. tocytes to the bile duct in the portal triad. The cystic (sis′tik; cysto = bladder) duct connects the gallbladder to the common bile duct. The gallbladder can hold approximately Liver Function 40 to 60 milliliters of concentrated bile. The gallbladder has three The liver has numerous functions. Besides producing bile, hepa- regions: the neck, body, and fundus (see figure 26.21). At the neck tocytes detoxify drugs, metabolites, and poisons. Hepatocytes of the gallbladder, a sphincter valve controls the flow of bile into

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CLINICAL VIEW Cirrhosis of the Liver The fibrosis and scarring of liver cirrhosis are irreversible. However, further scarring can be slowed or prevented by treating the cause of Chronic injury to the liver inevitably leads to liver cirrhosis (sir-ro¯′sis; the cirrhosis (hepatitis, alcoholism, etc.). Advanced liver cirrhosis may kirrhos = yellow). Liver cirrhosis results when hepatocytes have been have a variety of complications: destroyed and are replaced by fibrous scar tissue. This scar tissue often surrounds isolated nodules of regenerating hepatocytes. The fibrous ■ Jaundice (yellowing of the skin and sclerae of the eyes) occurs scar tissue also compresses the blood vessels and bile ducts in the when the liver can’t eliminate enough bilirubin. (Bilirubin liver, leading to hepatic portal hypertension (high blood pressure in is a yellowish product formed when aged erythrocytes are the hepatic portal venous system) and bile flow impediments. broken down. See chapter 21 for further information.) It is accompanied by darkening of the urine. Liver cirrhosis is caused by chronic injury to the hepatocytes, as ■ Edema (accumulation of fluid in body tissues) and ascites may result from chronic alcoholism, liver disease, or certain drugs or (a¯-sı¯′te¯z) (fluid accumulation in the abdomen) develop because toxins. Chronic hepatitis (hep-aˇ-tı¯′tis) is a long-term inflammation of of decreased albumin production. the liver that leads to necrosis of liver tissue. Most frequently, viral ■ Intense itching occurs when bile products are deposited in the infections from either hepatitis B or hepatitis C produce chronic skin. hepatitis. Other disorders that result in liver cirrhosis include some ■ Toxins in the blood and brain accumulate because the liver inherited diseases, chronic biliary obstruction, and biliary cirrhosis. cannot effectively process them. Early stages of liver cirrhosis may be asymptomatic. However, once ■ Hepatic portal hypertension can lead to dilated veins of the liver function begins to falter, the patient complains of fatigue, weight inferior esophagus (esophageal varices). loss, and nausea, and may have pain in the right hypochondriac region. End-stage liver cirrhosis can be treated only with a liver transplant. During a physical, the doctor may palpate an abnormally small and Otherwise, death results either from progressive liver failure or from hard liver. To confirm the diagnosis, a liver biopsy is done by obtaining the complications. In the United States, over 25,000 people die each a small portion of liver tissue through a needle passed into the liver year of liver cirrhosis. and then examining the cells.

Fibrous scar tissue

LM 100x

(a) Nodular cirrhosis of the liver (b) Histology of liver cirrhosis Normal hepatocytes

(a) This gross specimen depicts a type of nodular cirrhosis of the liver. (b) A photomicrograph shows how fibrous scar tissue infiltrates and replaces hepatocytes.

and out of the gallbladder. The gallbladder has three tunics: an WHAT DO YOU THINK? inner mucosa, a middle muscularis, and an external serosa. Folds in the mucosa permit distension of the wall as the gallbladder fills ●5 If your gallbladder were surgically removed, how would this affect your digestion of fatty meals? What diet alterations might you with bile. have to make after this surgery?

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Body of pancreas Main pancreatic duct Common bile duct Tail of pancreas

Duodenum

Accessory pancreatic duct Duodenojejunal flexure Hepatopancreatic ampulla Pancreatic acini Major duodenal Pancreatic papilla islet

Jejunum

Head of pancreas (a) Duodenum and pancreas, anterior view LM 75x

Acinar cell

LM 200x

Pancreatic acinus (b) Histology of pancreas Figure 26.20 Anatomy and Histology of the Pancreas. (a) The components of the pancreas are shown in relationship to the pancreatic duct and the duodenum. (b) Photomicrographs depict the histology of acinar cells within the pancreas.

26.10c Pancreas the mucin and digestive enzymes of the pancreatic juice. The The pancreas is referred to as a mixed gland because it exhibits both simple cuboidal epithelial cells lining the pancreatic ducts secrete endocrine and exocrine functions. The endocrine functions are per- bicarbonate (alkaline fluid) to help neutralize the acidic chyme formed by the cells of the pancreatic islets (see chapter 20). Exocrine arriving in the duodenum from the stomach. Most of the pancreatic activity results in the secretion of digestive enzymes and bicarbon- juice travels through ducts that merge to form the main pancreatic ate, collectively called pancreatic juice, into the duodenum. duct, which drains into the major duodenal papilla in the duode- The pancreas is a retroperitoneal organ that extends horizon- num. A smaller accessory pancreatic duct drains a small amount tally from the medial edge of the duodenum toward the left side of pancreatic juice into a minor duodenal papilla in the duodenum. of the abdominal cavity, where it touches the spleen. It exhibits a Both hormonal and neural mechanisms control pancreatic wide head adjacent to the curvature of the duodenum, a central, juice secretions. Enteroendocrine cells in intestinal glands release elongated body projecting toward the left lateral abdominal wall, cholecystokinin to promote the secretion of pancreatic juices from and a tail that tapers as it approaches the spleen (figure 26.20). acinar cells, and secretin to stimulate the release of alkaline fluid The pancreas contains modified simple cuboidal epithelial from pancreatic duct cells. Pancreatic juice secretion is also stimu- cells called acinar cells. These cells, which are organized into lated by parasympathetic (vagus nerve) activity, while sympathetic large clusters termed acini (sing., acinus), or lobules, secrete activity inhibits pancreatic juice secretion.

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CLINICAL VIEW Gallstones (Cholelithiasis) Following surgery, the liver continues to produce bile, even in the absence of the gallbladder, but there is no means of concentrating High concentration of certain materials in the bile can lead to the the bile, so further gallstones are unlikely. eventual formation of gallstones. Gallstones occur twice as frequently in women as in men, and are more prevalent in developed countries. Obesity, increasing age, female sex hormones, Caucasian ethnicity, and lack of physical activity are all risk factors for developing gallstones. The term cholelithiasis (ko¯′le¯-li-thı¯′a˘-sis; chole = bile, lithos = stone, iasis = condition) refers to the presence of gallstones in either the gallbladder or the biliary apparatus. Gallstones are typically formed from condensations of either cholesterol or calcium and bile salts. These stones can vary from the tiniest grains to structures almost the size of golf balls. The majority of gallstones are asymptomatic until a gallstone becomes lodged in the neck of the cystic duct, causing the gallbladder to become inflamed (cholecystitis) and dilated. The most common symptom is severe pain (called biliary colic) perceived in the right hypochondriac region or sometimes in the area of the right shoulder. Nausea and vomiting may occur, along with indigestion and bloating. Symptoms are typically worse after eating a fatty meal. Treatment consists of surgical removal of the gallbladder, called cho- lecystectomy (ko¯′le¯-sis-tek′to¯-me˘ ; kystis = bladder, ektome = excision). Photo of gallstones in a gallbladder.

26.10d Biliary Apparatus Left and right The biliary (bil′ē -ā r-ē ; bilis = bile) apparatus is a network of thin hepatic ducts ducts that carry bile from the liver and gallbladder to the duodenum (figure 26.21). The left and right lobes of the liver drain bile into Cystic duct 1 Common hepatic duct the left and right hepatic ducts, respectively. The left and right hepatic ducts merge to form a single common hepatic duct. The cys- Neck 2 tic duct is attached to the common hepatic duct and carries bile to Common bile duct and from the gallbladder. The cystic duct allows bile to enter and to leave the gallbladder; the direction of flow is controlled by hormonal Gallbladder Body influences. Bile travels from the common hepatic duct through the cystic duct to be stored in the gallbladder; stored bile travels back through the cystic duct for conduction to the small intestine. The Fundus union of the cystic duct and the common hepatic duct forms the common bile duct that extends inferiorly to the duodenum. The hepatopancreatic ampulla is a posteriorly placed swelling Main pancreatic on the duodenal wall where the common bile duct and main pan- duct Hepatopancreatic creatic duct merge and pierce the duodenal wall. Bile and pancreatic ampulla juice mix in the hepatopancreatic ampulla prior to emptying into the Major duodenal 3 duodenum via the major duodenal papilla. papilla 4

WHATW DID YOU LEARN? Duodenum

●15 Identify the structural components of a portal triad. 1 Left and right hepatic ducts merge to form a common hepatic duct. ●16 What is the function of the gallbladder? ●17 What is the function of pancreatic acini? 2 Common hepatic and cystic ducts merge to form a common bile duct.

3 Pancreatic duct merges with common bile duct at the hepatopancreatic ampulla. Figure 26.21 4 Bile and pancreatic juices enter duodenum at the major duodenal Biliary Apparatus. Bile flows through the biliary apparatus (green), papilla. and pancreatic juice flows through the main pancreatic duct until the two vessels merge at the hepatopancreatic ampulla. Numbers indicate the pathway sequence for bile and pancreatic juice.

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CLINICAL VIEW Intestinal Disorders tissue can lead to bowel obstruction. Other patients develop selective malabsorption of certain vitamins. Celiac disease (also known as celiac sprue or gluten-sensitive enteropa- The age distribution and symptoms of ulcerative colitis are similar to thy) is an autoimmune disorder of the small intestine. This disease those of Crohn disease, but ulcerative colitis involves only the large runs in families and may be triggered after surgery, pregnancy, or a intestine. The rectum and descending colon are the first to show signs viral infection. Affected individuals cannot tolerate a protein called of inflammation and are generally the most severely affected. Also, in gluten, which is found in all forms of wheat, rye, and barley. When ulcerative colitis the inflammation is confined to the mucosa, instead an individual with celiac disease ingests food containing gluten, the of the full thickness of the intestinal wall. Finally, unlike Crohn dis- body’s immune cells attack the small intestine mucosa and damage ease, ulcerative colitis is associated with a profoundly increased risk the villi, severely impairing nutrient absorption. Symptoms of celiac of colon cancer. Historically, patients who have had ulcerative colitis disease vary, but may include gas and bloating, diarrhea, fatigue, and for more than 10 years develop colon cancer at a frequency 20 or weight loss (due to malnutrition). Because these symptoms are similar 30 times that seen in the rest of the population. to those of other bowel disorders, diagnosis may be difficult. The only treatment for celiac disease is to follow a gluten-free diet, but this is challenging because many commercially prepared foods use gluten as a preservative or stabilizer. Thus, the patient with celiac disease must carefully read all ingredient lists on prepared foods.

A section of large intestine showing the signs of ulcerative colitis.

Treatment of either Crohn disease or ulcerative colitis is complex. A section of small intestine showing the signs of Crohn disease. Anti-inflammatory drugs, as well as stress reduction and possibly nutritional supplementation, help control symptoms. Surgery may be The term inflammatory bowel disease (IBD) applies to two autoim- necessary in both forms of inflammatory bowel disease. mune disorders, Crohn disease and ulcerative colitis. In both of these Crohn disease, ulcerative colitis, and celiac disease are distinctly disorders, selective regions of the intestine become inflamed. different from a much more common disorder called irritable bowel Crohn disease is a condition of young adults characterized by intermit- syndrome. Irritable bowel syndrome is characterized by abnormal tent and relapsing episodes of intense abdominal cramping and diar- function of the colon with symptoms of crampy abdominal pain, bloat- rhea. Although any region of the gastrointestinal tract, from esophagus ing, constipation, and/or diarrhea. Irritable bowel syndrome occurs to anus, may be involved, the distal ileum is the most frequently and in approximately one in every five people in the United States, and severely affected site. Inflammation involves the entire thickness is more common in women than men. Irritable bowel syndrome may of the intestinal wall, extending from the mucosa to the serosa. For be diagnosed if a medical evaluation has ruled out Crohn disease and reasons that are not clear, lengthy regions of the intestine having no ulcerative colitis. Although neither a cause nor a cure for irritable trace of injury or inflammation may be followed abruptly by several bowel syndrome is known, most people can control their symptoms inches of markedly diseased intestine. In some patients scarring of the by reducing stress, changing their diet, and using certain medicines.

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or caudal hindgut (figure 26.22a). Table 26.5 lists the struc- 26.11 Aging and the Digestive System tures derived from the foregut, midgut, and hindgut. Portions of the gut tube expand to form some of the abdominal organs. Some Learning Objective: accessory digestive system organs develop as buds, or outgrowths, 1. Explain how the digestive system changes as we age. from the gut tube. Many of these structures rotate or shift before assuming their final positions in the body. Age-related changes in digestion system function usu- ally progress slowly and gradually. Overall, reduced secretions 26.12a Stomach, Duodenum, and Omenta accompany normal aging. Mucin secretion decreases, reducing the Development thickness and amount of mucus in the layer that protects the GI tract and making internal damage to digestive organs more likely. By the fourth week of development, a portion of the foregut expands Decreased secretion of enzymes and acid results in diminished and forms a spindle-shaped dilation that will become the stomach. effectiveness of chemical digestion, which in turn can diminish Between the fifth and seventh weeks, the posterior (dorsal) part nutrient absorption. The reduction of glandular secretion is com- of this dilation grows faster than the anterior (ventral) part, so pounded by decreased replacement of epithelial cells. the anterior wall becomes the concave lesser curvature, and the Another change that occurs with age is a reduction in posterior wall becomes the convex greater curvature (figure 26.22b). the thickness of the smooth muscle layers in the muscularis. The dorsal mesentery that attaches to the stomach becomes thin- Consequently, both muscular tone and GI tract motility may be ner and stretches out from the greater curvature. This mesentery lowered significantly. Additionally, smooth muscle ensheathing will form the greater omentum, while the ventral mesentery that the GI tract may adversely affect the function of sphincters or attaches to the lesser curvature will form the lesser omentum. valves that help move materials through the GI tract. As a result, During the seventh week, the developing stomach rotates significant changes in nutrient absorption, as well as gastroesoph- 90 degrees clockwise about a longitudinal axis, as viewed from ageal reflux disease (GERD), may occur. the superior aspect of the organ (figure 26.22c). Thus, the poste- Improper dental care or sometimes simply aging can result rior side of the stomach (the greater curvature) rotates so that it in either the loss of teeth or periodontal disease. Both conditions faces the left side of the body, while the anterior side (the lesser adversely affect normal eating habits and nutrition. As food intake curvature) rotates and faces the right side. One week later, the dwindles, changes occur in GI tract motility, secretion, and absorp- stomach and duodenum rotate about an anterior-posterior axis. tion. Marked dietary alterations may also increase the chance of This rotation pulls the pyloric region of the stomach and the duo- developing cancer in either the stomach or the intestines. denum superiorly while moving the fundus and the cardia of the Finally, a direct result of aging is a reduction in olfactory and stomach slightly inferiorly (figure 26.22d). gustatory sensations. When a person loses the ability to smell or 26.12b Liver, Gallbladder, and Pancreas Development taste food, his or her dietary intake usually suffers. The liver parenchyma, gallbladder, pancreas, and biliary apparatus WHATW DID YOU LEARN? develop from buds or outgrowths from the endoderm of the duode- num. (The liver stroma or connective tissue framework is formed ●18 How might the decrease in mucin secretion due to aging affect GI from nearby mesoderm.) The liver bud appears first, during the tract organs? third week, develops within the ventral mesentery, and grows superiorly toward the developing diaphragm. By the fourth week, the liver continues to grow, and its attachment to the duodenum 26.12 Development of the Digestive becomes a thinned tube that forms the common bile duct. An out- growth forms off the bile duct that becomes the gallbladder. The System connection to the gallbladder and bile duct thins and becomes the Learning Objective: cystic duct. As the liver enlarges, the ventral mesentery thins and becomes the falciform ligament. 1. Identify and describe the major events in digestive system The pancreas starts to form at week 4 from two separate development. outgrowths of the bile duct, called the ventral pancreatic bud and At the end of the third week of development, the disc-shaped the dorsal pancreatic bud. By week 6, the ventral pancreatic bud embryo undergoes lateral folding, which transforms the endoderm and the biliary apparatus rotate behind the duodenum. The ventral and some lateral plate mesoderm into a cylindrical primitive gut and dorsal pancreatic buds fuse to form the pancreas. Due to this tube. The gut tube eventually connects to the primitive mouth and rotation, much of the biliary apparatus now lies posterior to the the developing anus, while still maintaining a connection to the duodenum and drains into the major duodenal papilla. yolk sac via a thin stalk called the vitelline (vı̄ -tel′in, -ē n) duct. A double-layered membrane called dorsal mesentery wraps around 26.12c Intestine Development the gut tube and helps anchor it to the posterior body wall. A The small and large intestines are formed from the midgut and portion of the gut tube called the abdominal foregut has ventral hindgut, as shown in figure 26.23. During the fifth week, the mesentery that attaches it to the anterior body wall. midgut rapidly elongates and forms a primary intestinal loop. The gut tube may be divided into three components: a supe- The cranial portion of the loop forms the jejunum and most of the rior or cephalic foregut, a centrally located midgut, and an inferior ileum, while the caudal portion of the loop forms the very distal

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Ventral mesentery Esophagus Dorsal Gallbladder mesentery (cystic bud)

Liver

Falciform Lesser curvature Liver buds ligament of stomach Foregut Greater curvature Cystic bud of stomach Dorsal pancreatic bud Vitelline duct Dorsal Ventral pancreatic bud Ventral pancreatic bud pancreatic bud Vitelline duct Primary intestinal Midgut loop

Hindgut

(a) Week 4: Liver, gallbladder, and pancreatic buds develop (b) Week 5: Greater and lesser curvatures of stomach form

Direction Gallbladder of stomach rotation Falciform ligament Esophagus Falciform ligament

Liver Liver Greater omentum Stomach Lesser omentum

Gallbladder Direction of duodenum Common bile duct Ventral pancreatic bud rotation

Direction of pancreas, Pancreas bile duct rotation Dorsal pancreatic bud Duodenum

(c) Weeks 6Ð7: Rotation of stomach, pancreatic buds (d) Week 8: Postnatal position of organs attained

Figure 26.22 Development of the Digestive System Foregut. (a) The foregut of the digestive system begins to develop from the primitive gut during week 4 of development. Stages of foregut development continue at (b) week 5, (c) weeks 6–7, and (d) week 8.

Table 26.5 Anatomic Derivatives of the Primitive Gut Tube Organs Formed Foregut Midgut Hindgut Digestive organs Pharynx, esophagus, stomach, Distal half of duodenum; jejunum, Distal (left) one-third of transverse proximal half of duodenum ileum, cecum, appendix, ascending colon; descending colon, sigmoid colon, proximal (right) two-thirds of colon, rectum, superior portion of transverse colon anal canal Accessory digestive organs Most of liver; gallbladder, biliary apparatus, pancreas Other organs Lungs Urinary bladder epithelium, urethra epithelium

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Ventral mesentery Liver Dorsal Vitelline mesentery duct Stomach Ventral mesentery Descending abdominal Caudal loop aorta Dorsal Cranial loop mesentery Primary intestinal Caudal loop loop (midgut) Cranial loop Hindgut 90 counterclockwise Superior rotation mesenteric artery (a) Week 5: Primary intestinal loop forms (b) Week 6: Herniation of loop; 90 counterclockwise rotation

Caudal loop (forms much of large intestine) Duodenum

Direction of rotation 180 Transverse colon counterclockwise Colon Cranial loop Small intestine Vitelline duct (forms most of small intestine) Ascending Descending colon colon

(c) Weeks 10Ð11: Retraction of intestines back into abdominal cavity; 180 counterclockwise rotation Cecum Sigmoid colon

Appendix Rectum (d) Postnatal position Figure 26.23 Development of the Digestive System Midgut. The midgut of the digestive system forms almost all of the small intestine and the proximal region of the large intestine. Development of these structures is shown at (a) week 5, (b) week 6, and (c) weeks 10–11. (d) The postnatal position of the digestive organs.

part of the ileum and much of the large intestine. The loop apex counterclockwise rotation, as viewed from the front of the body. connects to the yolk sac via the vitelline duct. Now the cranial loop is on the left side of the body, and the caudal Due to rapid growth of the liver and the resulting limited loop is on the right. Thus, the net intestinal rotation is 270 degrees space in the developing abdominal cavity, the primary intestinal counterclockwise. The cranial loop (forming the jejunum and most loop herniates into the umbilicus during the sixth week. As this of the ileum) retracts and positions itself in the left side of the loop herniates, it undergoes a 90-degree counterclockwise rotation, abdominal cavity. The caudal loop (forming the distal ileum and as viewed from the anterior surface of the body. The cranial loop the proximal part of the large intestine) is pulled to the right side rotates to the right of the body, while the caudal loop rotates to of the abdominal cavity. The vitelline duct begins to regress and the left of the body. For the next several weeks, the intestinal loop disappears before the baby is born. grows and expands in the umbilicus since there is no available space inside the abdominal cavity. By weeks 10 and 11, the midgut retracts into the abdomen WHATW DID YOU LEARN? as the abdominal cavity becomes spacious enough to house all the ●19 What structures develop from endoderm outgrowths of the intestines. As the midgut retracts, it undergoes another 180-degree duodenum?

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CLINICAL TERMS

bariatric surgery (gastric bypass) Surgically narrowing or blocking intussusception (in-tŭ s-sŭ -sep′shŭ n; intus = within, suscipio = to off a large portion of the stomach; typically performed in take up) Type of intestinal obstruction in which one part of severely obese people who have had trouble losing weight by the intestine constricts and gets pulled into the immediately more traditional methods. distal segment of intestine; most commonly seen near the caries (kā r ′ez; dry rot) Cavities in the tooth enamel caused by ileocecal junction. Symptoms include bloody stools and plaque and bacterial buildup on the tooth. severe abdominal pain. colostomy A surgical procedure that moves and attaches a portion mumps (a lump) Viral infection of the parotid glands, resulting in of the colon so it exits through the abdominal wall. Materials painful swelling of the glands. moving through the large intestine drain into a bag (ostomy pancreatitis (pan′krē -ă -tı̄ ′tis) Inflammation of the pancreas; most pouch) attached to the abdomen. often caused by alcoholism or biliary apparatus disease diarrhea (dı̄ -ă -rē ′ă ; dia = through, rhoia = a flow) Frequent, (including gallstones). watery stools. volvulus Twisting or torsion of an intestinal segment around dysentery (dis-en-tē r-ē ; dys = bad, entera = bowels) Painful, itself. If left untreated, obstruction of digestive materials bloody diarrhea due to an infectious agent. results, and the segment necroses due to poor blood supply. gastroenteritis (gas′trō -en-ter-ı̄ ′tis) Inflammation and irritation of Symptoms include intense abdominal pain and vomiting. the GI tract, often associated with vomiting and diarrhea. hemorrhoids (hem′ŏ -roydz; rhoia = flow) Dilated, tortuous veins around the rectum and/or anus.

Chapter Summary

■ The digestive system breaks down ingested food into usable nutrients.

26.1 General ■ The digestive organs are the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine. The accessory Structure and digestive organs are the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. Functions of the Digestive 26.1a Digestive System Functions 780 System 780 ■ Mechanical digestion is the physical breakdown of ingested materials, and chemical digestion is the enzymatic breakdown of molecules. ■ Propulsion moves materials through the GI tract. Peristalsis moves materials from the mouth toward the anus; segmentation mixes ingested materials. ■ Secretion is the production and release of fluids; absorption is the movement or transport of materials across the wall of the GI tract.

26.2 Oral ■ The oral cavity is the entryway into the GI tract. Cavity 781 26.2a Cheeks, Lips, and Palate 781 ■ The cheeks form the lateral walls of the oral cavity, and the lips frame the anterior opening formed by the orbicularis oris muscle. ■ The palate is the oral cavity roof: The hard palate is the anterior, bony portion, and the soft palate is the posterior, muscular portion. 26.2b Tongue 782 ■ The tongue moves and mixes ingested materials; its dorsal surface has papillae. 26.2c Salivary Glands 782 ■ Saliva is a fluid secreted by three pairs of multicellular salivary glands. It moistens food and provides lubrication. 26.2d Teeth 784 ■ Adults have 32 permanent teeth of four types: incisors, canines, premolars, and molars.

26.3 Pharynx 786 ■ Pharyngeal constrictors contract sequentially during swallowing. 26.4 General 26.4a Peritoneum, Peritoneal Cavity, and Mesentery 787 Arrangement of ■ Parietal peritoneum lines the internal body wall; visceral peritoneum covers abdominal organs. Abdominal GI ■ Organs 787 Intraperitoneal organs are completely surrounded by visceral peritoneum; retroperitoneal organs are only partially ensheathed. ■ Mesenteries are double layers of peritoneum. 26.4b General Histology of GI Organs (Esophagus to Large Intestine) 788 ■ The mucosa is composed of epithelium, an underlying lamina propria, and the muscularis mucosae. ■ The submucosa is a dense irregular connective tissue layer containing blood vessels, lymph vessels, and nerves. ■ The muscularis usually has two smooth muscle layers: an inner circular layer and an outer longitudinal layer. ■ The adventitia is the outermost covering; when covered by visceral peritoneum, it is called a serosa.

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Chapter Summary (continued) 26.4c Blood Vessels, Lymphatic Structures, and Nerve Supply 790 ■ GI tract organs have extensive blood vessels, lymphatic structures, and nerves.

26.5 26.5a Gross Anatomy 791 Esophagus 790 ■ The esophagus conducts swallowed materials from the pharynx to the stomach. 26.5b Histology 791 ■ The esophagus is lined by nonkeratinized stratified squamous epithelium. ■ The muscularis has all skeletal fibers superiorly, changes to intermingled skeletal and smooth fibers in the middle, and contains all smooth fibers inferiorly.

26.6 The Swallowing ■ Swallowing has three phases: The voluntary phase moves a bolus into the pharynx; the pharyngeal phase moves the Process 792 bolus through the pharynx into the esophagus; and the esophageal phase conducts the bolus to the stomach. 26.7 Stomach 793 26.7a Gross Anatomy 793 ■ Mechanical digestion and chemical digestion occur in the stomach. ■ The four stomach regions are the cardia, fundus, body, and pylorus. 26.7b Histology 793 ■ The stomach mucosa is composed of a simple columnar epithelium containing gastric pits and gastric glands. ■ The muscularis has three smooth muscle layers: inner oblique, middle circular, and outer longitudinal. 26.7c Gastric Secretions 794 ■ Five types of secretory cells form the gastric glands: surface mucous cells (secrete mucin), mucous neck cells (secrete acidic mucin), parietal cells (secrete hydrochloric acid and intrinsic factor), chief cells (secrete pepsinogen), and enteroendocrine cells (secrete gastrin and other hormones). 26.8 Small 26.8a Gross Anatomy and Regions 797 Intestine 797 ■ The small intestine finishes chemical digestion and absorbs most of the nutrients. ■ The small intestine is divided into the duodenum, jejunum, and ileum. 26.8b Histology 799 ■ The small intestine has circular folds with fingerlike villi projecting from them. The villi are lined with cells with microvilli on their apical surface. The circular folds, villi, and microvilli increase the surface area of the small intestine.

26.9 Large 26.9a Gross Anatomy and Regions 799 Intestine 799 ■ The large intestine absorbs fluids and ions, and compacts undigestible wastes. ■ The large intestine is composed of the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal. 26.9b Histology 801 ■ The large intestine mucosa lacks villi, but contains intestinal glands. ■ The outer longitudinal layer of the muscularis forms three discrete bands called teniae coli. The teniae coli cause the wall of the colon to bunch into sacs called haustra. 26.9c Control of Large Intestine Activity 802 ■ Local autonomic reflexes regulate large intestine movements. 26.10 Accessory 26.10a Liver 804 Digestive ■ The liver receives venous blood from the hepatic portal vein and oxygenated blood from the hepatic artery proper. Organs 803 ■ Liver functions are bile production, drug detoxification, storage of nutrients and glycogen, and synthesis of plasma proteins. 26.10b Gallbladder 805 ■ The gallbladder stores and concentrates bile produced by the liver. 26.10c Pancreas 807 ■ Pancreatic acini produce pancreatic juice that neutralizes acidic chyme. 26.10d Biliary Apparatus 808 ■ The hepatic ducts drain bile into a single common hepatic duct, and the cystic duct merges with the common hepatic duct to form the common bile duct. ■ The common bile duct and the pancreatic duct merge to empty their contents into the duodenum via the major duodenal papilla.

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26.11 Aging and ■ Age-related changes in the digestive system lead to reduced secretions and diminished absorption of nutrients. the Digestive System 810

26.12 Development ■ The gut tube has a superior foregut, a central midgut, and an inferior hindgut. of the Digestive System 810 26.12a Stomach, Duodenum, and Omenta Development 810 ■ Stomach formation begins by week 4 of development. Differential growth rates cause the greater and lesser curvatures of the stomach to form. 26.12b Liver, Gallbladder, and Pancreas Development 810 ■ The liver parenchyma, gallbladder, and pancreas develop from endoderm outgrowths of the duodenum. 26.12c Intestine Development 810 ■ The small and large intestines form from the midgut and hindgut beginning in week 5.

Challenge Yourself

Matching ______2. The ______cells of the stomach secrete hydrochloric Match each numbered item with the most closely related lettered acid (HCl). item. a. chief b. parietal ______1. circular folds a. typically contains two layers c. mucous of smooth muscle ______2. falciform ligament d. enteroendocrine b. epithelium lining small ______3. pyloric sphincter ______3. Material leaving the ascending colon next enters the intestine a. cecum. ______4. haustra c. contains dense irregular b. descending colon. ______5. segmentation connective tissue and blood c. sigmoid colon. vessels d. transverse colon. ______6. simple columnar ______4. Which of these organs is retroperitoneal? d. lymphatic nodules in wall of ______7. muscularis a. stomach ileum b. transverse colon ______8. stratified squamous e. attaches liver to anterior c. descending colon ______9. Peyer patches abdominal wall d. ileum ______10. submucosa f. restricts chyme entry into ______5. Sympathetic innervation of the GI tract is small intestine responsible for a. closing the pyloric sphincter. g. epithelium lining the b. stimulating peristalsis. esophagus c. stimulating secretion of the pancreatic acinar cells. h. sacs of large intestine wall d. vasodilating the major digestive system blood vessels. i. increase surface area of ______6. The ______is derived from the cranial part of the small intestine primary intestinal loop. j. process to mix digested a. jejunum materials in small intestine b. cecum c. ascending colon Multiple Choice d. appendix Select the best answer from the four choices provided. ______7. The main pancreatic duct merges with the ______, and their contents empty into the duodenum ______1. Which organ is located in the right upper quadrant through the major duodenal papilla. of the abdomen? a. left hepatic duct a. liver b. common bile duct b. spleen c. cystic duct c. descending colon d. common hepatic duct d. appendix

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______8. Which statement is false about pancreatic juice? 7. What is the function of the gallbladder, and what role does it a. It is secreted through the main pancreatic duct play in digestion? into the duodenum. 8. List in order the organs of the GI tract through which b. It is responsible for emulsifying (breaking down) ingested material travels, and describe the type(s) of digestion fats. (mechanical/chemical) that takes place in each organ. At c. It is produced by the acinar cells of the pancreas. what point is the material called a bolus, chyme, and feces? d. The juice has an alkaline pH. 9. Why are there so many mucin-producing glands along the ______9. The “living” part of a tooth is the length of the GI tract? a. dentin. 10. Describe how the small and large intestine form. b. cementum. c. pulp. 11. Take a fantastic voyage. Imagine that you are a molecule d. enamel. of fat that’s just about to be eaten. Trace your path as you travel from the mouth to the skin (dermis) of the foot. ______10. Most of the chemical digestion of our food occurs Include all major structures, vessels, tubes, and so on within the that you will pass by during your journey. Pay attention a. large intestine. to any specific enzymes and any modifications to your b. pancreas. structure that may play a role in your successful travels. c. small intestine. (Remember that you will be traveling through parts of the d. esophagus. digestive system and parts of the circulatory system.) Content Review Developing Critical Reasoning 1. What initial stages of digestion occur in the oral cavity? 1. Alexandra experienced vomiting and diarrhea, and was 2. The GI tract from the esophagus to the anal canal is diagnosed with gastroenteritis (stomach flu). What specific composed of four tunics. Describe the general histology of digestive system organs were affected by the illness, and the tunics and the specific features of the stomach tunics. how did the illness interfere with each organ’s function? 3. Compare and contrast gastric juice and pancreatic juice with 2. Most cases of colorectal cancer occur in the most distal respect to their composition and function. part of the large intestine (the rectum, sigmoid colon, and 4. Compare the anatomy and functions of the circular folds, descending colon). Why do fewer instances of colon cancer villi, and microvilli in the small intestine. tend to occur in the proximal part of the large intestine? 5. What are the teniae coli and haustra, and how are they Include the anatomy and function of the colon components associated? in your explanation. 6. What is the function of each structure in the portal triad of the hepatic lobule, and how do they work together to contribute to the overall functioning of the liver?

Answers to “What Do You Think?”

1. Saliva cleanses the mouth in a variety of ways. As 4. The duodenum has many circular folds so that the digested saliva washes over the tongue and teeth, it helps remove materials can be slowed down and adequately mixed with foreign materials and buildup. Lysozyme in saliva is an pancreatic juice and bile. The jejunum also has many antibacterial enzyme. A person who has a dry mouth is not circular folds because the slowing down of the materials aids able to cleanse the mouth well and is more likely to develop in absorbing the maximum amount of nutrients. By the time dental problems as a result of built-up bacterial and foreign chyme reaches the distal ileum, most nutrient absorption has material. occurred, and so the circular folds are not as essential. 2. A young child’s mouth is too small to support adult-sized 5. When the gallbladder is removed, bile is still produced by the teeth. Deciduous teeth are much smaller and fit a child’s liver, but it is secreted in a “slow drip.” A fatty meal requires mouth better. As the mouth increases in size by about much more bile for digestion than the amount supplied by age 6, it is able to support the first adult-sized teeth. a “slow drip,” causing the patient to experience gas, pain, 3. The stomach is lined with specialized surface mucous cells bloating, and diarrhea. Thus, surgical patients are initially that secrete mucus onto the lining. This mucus prevents told to limit their fat intake. The body gradually adjusts so the acid from eating away the stomach wall. In addition, that more bile is secreted, and thus a person may eventually the stomach epithelial lining is constantly regenerating to be able to introduce more fat back into his or her diet. replace any cells that are damaged.

www.mhhe.com/mckinley3 Enhance your study with practice tests and activities to assess your understanding. Your instructor may also recommend the interactive eBook, individualized learning tools, and more.

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