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Digestive System 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 mck78097_ch26_779-816.indd 779 2/25/11 2:20 PM 780 Chapter Twenty-Six Digestive System 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 mck78097_ch26_779-816.indd 780 2/25/11 2:20 PM Chapter Twenty-Six Digestive System 781 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.
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