Chapter 24 Lecture Presentation

Chapter 24

The Digestive System

. Digestive system – Acquires nutrients from environment • Used to synthesize essential compounds (anabolism) • Broken down to provide energy to cells (catabolism)

. Digestive system – Digestive tract • Gastrointestinal (GI) tract or alimentary canal • Muscular tube • Extends from oral cavity to anus – Accessory organs • Teeth, tongue, and various glandular organs

Major Organs of the Digestive Tract Oral Cavity (Mouth) Ingestion, mechanical digestion with accessory organs (teeth and tongue), moistening, mixing with salivary secretions Pharynx Muscular propulsion of materials into the esophagus

Esophagus Transport of materials to the stomach

Stomach Chemical digestion of materials by acid and enzymes; mechanical digestion through muscular contractions

Small Intestine Enzymatic digestion and absorption of water, organic substrates, vitamins, and ions

Large Intestine Dehydration and compaction of indigestible materials in preparation for elimination Anus 4 Figure 24–1 Organs of the Digestive System (Part 2 of 2).

Accessory Organs of the Digestive System Teeth Mechanical digestion by chewing (mastication)

Tongue Assists mechanical digestion with teeth, sensory analysis

Salivary Glands Secretion of lubricating ﬂuid containing enzymes that break down carbohydrates

Liver Secretion of bile (important for lipid digestion), storage of nutrients, many other vital functions

Gallbladder

Storage and concentration of bile

Pancreas Exocrine cells secrete buffers and digestive enzymes; endocrine cells secrete hormones

5 24-1 The Digestive System

. Integrated processes of digestive system – Ingestion – Mechanical digestion and propulsion – Chemical digestion – Secretion – Absorption – Defecation

. Ingestion – Occurs when food enters oral cavity . Mechanical digestion and propulsion – Crushing and shearing of food – Propelling food along digestive tract

. Chemical digestion – Chemical breakdown of food into small organic fragments for absorption by digestive epithelium . Secretion – Release of water, acids, enzymes, buffers, and salts – By epithelium of digestive tract, glandular organs, and gallbladder

. Absorption – Movement of organic molecules, electrolytes, vitamins, minerals, and water – Across digestive epithelium – Into interstitial fluid of digestive tract . Defecation – Elimination of wastes from body – Compacted, dehydrated wastes are called feces

. Lining of digestive tract – Safeguards surrounding tissues against • Corrosive effects of digestive acids and enzymes • Mechanical stresses, such as abrasion • Bacteria either ingested with food or that reside in digestive tract

. Peritoneum – Serous membrane lining peritoneal cavity – Superficial mesothelium covering a layer of areolar tissue – Visceral peritoneum (serosa) • Covers organs within peritoneal cavity – Parietal peritoneum • Lines inner surfaces of body wall

. Peritoneal fluid – Produced by serous membrane lining – Allows sliding of parietal and visceral surfaces without friction or irritation – About 7 liters produced and absorbed daily, but very little in peritoneal cavity at one time • Ascites—abdominal swelling due to buildup of peritoneal fluid

. Mesenteries – Double sheets of peritoneal membrane – Suspend portions of digestive tract within peritoneal cavity – Connect parietal peritoneum with visceral peritoneum – Provide a route to and from digestive tract for blood vessels, nerves, and lymphatic vessels – Stabilize positions of attached organs – Prevent intestines from becoming entangled

. During embryonic development – Digestive tract and accessory organs are suspended in peritoneal cavity by • Dorsal mesentery • Ventral mesentery – Ventral mesentery later disappears along most of digestive tract • Persists in adults only as lesser omentum and falciform ligament

Neural tube

Notochord

Mesoderm Parietal Dorsal peritoneum Coelomic mesentery Peritoneal cavity cavity Digestive tract Developing Visceral liver peritoneum Digestive tract 4 weeks Ventral mesentery 5 weeks

a During embryonic development, the digestive tube is initially suspended by dorsal and ventral mesenteries. In adults, the ventral mesentery is lost except at the lesser omentum between the stomach and liver and the falciform ligament between the liver and diaphragm (see part d). 15 24-1 The Digestive System

. Lesser omentum – Stabilizes position of stomach – Provides access route for blood vessels and other structures entering or leaving liver . Falciform ligament – Helps stabilize position of liver relative to diaphragm and abdominal wall

. Dorsal mesentery – Enlarges to form an enormous pouch, the greater omentum • Extends inferiorly between body wall and anterior surface of small intestine • Hangs like an apron from lateral and inferior borders of stomach

. Adipose tissue in greater omentum – Conforms to shapes of surrounding organs – Pads and protects surfaces of abdomen – Provides insulation to reduce heat loss – Stores lipid energy reserves – Contributes to “beer belly”

. Mesentery proper – Thick mesenterial sheet – Provides stability – Permits some independent movement – Suspends all but first 25 cm of small intestine . Mesentery associated with duodenum and pancreas – Fuses with abdominal wall, locking organs in place – Posterior to peritoneal cavity—retroperitoneal

. Mesocolon – Mesentery associated with part of large intestine – During development, mesocolon of ascending colon, descending colon, and rectum • Fuse to posterior body wall • Lock regions in place

Stomach

Mesocolon of ascending Transverse and descending colons colon fused to posterior portion of the parietal peritoneum Transverse mesocolon Ascending colon Descending Mesentery colon proper (mesenterial Small sheet) intestine Sigmoid colon

b A diagrammatic view of the organization of mesenteries in an adult. As the digestive tract enlarges, mesenteries associated with the proximal portion of the small intestine, the pancreas, and the ascending and descending portions of the colon fuse to the body wall. 21 Figure 24–2c The Mesenteries. Falciform ligament Diaphragm

Inferior vena cava

Esophagus Coronary ligament Pancreas of liver Right kidney Left kidney Duodenum Attachment of transverse mesocolon

Superior mesenteric Position of artery and vein ascending Position of colon Root of descending colon mesentery Attachment of proper sigmoid mesocolon Rectum Parietal Urinary peritoneum bladder c An anterior view of the empty peritoneal cavity, showing the attachment of mesenteries to the posterior body wall. Some visceral organs that were originally suspended within the peritoneal cavity are now retroperitoneal due to fusion of the serosa with the parietal peritoneum. 22 Figure 24–2d The Mesenteries.

Falciform Diaphragm Visceral ligament peritoneum

Liver Lesser omentum Stomach Pancreas Transverse Duodenum mesocolon Transverse Mesentery colon proper Greater omentum Sigmoid mesocolon Parietal peritoneum Rectum Small Urinary intestine bladder Uterus

d A sagittal section showing the mesenteries of an adult. Notice that the pancreas, duodenum, and rectum are retroperitoneal. 23 24-1 The Digestive System

. Histology of digestive tract – Major layers of digestive tract • Mucosa • Submucosa • Muscular layer • Serosa . Lining of digestive tract varies by region – Longitudinal folds in empty stomach – Permanent transverse folds in small intestine

Circular Mesentery folds

Mucosa

Submucosa

Muscular layer

Serosa

25 Figure 24–3 Histological Organization of the Digestive Tract (Part 2 of 2). Circular fold Mucosa

Mucosal epithelium Lamina propria Muscularis mucosae

Villi

Intestinal glands

Submucosal gland

Lymphatic vessel Mucosa Artery and vein Submucosa Submucosal neural plexus Muscular Circular muscle layer layer Myenteric plexus Serosa Longitudinal muscle layer 26 24-1 The Digestive System

. Mucosa – Inner lining of digestive tract – Mucous membrane consisting of • Epithelium, moistened by glandular secretions • Lamina propria of areolar tissue • Muscular muscularis mucosae

. Digestive epithelium – Mucosal epithelium is simple or stratified • Depending on location, function, and stresses – Oral cavity, pharynx, esophagus, anal canal • Stratified squamous epithelium – Stomach, small intestine, most of large intestine • Simple columnar epithelium • Enteroendocrine cells – Secrete hormones that coordinate activities of digestive tract and accessory glands

. Lamina propria – A layer of areolar tissue that contains • Blood vessels • Sensory nerve endings • Lymphatic vessels • Smooth muscle cells • Scattered lymphatic tissue

. Muscularis mucosae – In most areas of digestive tract, deep to lamina propria – Narrow sheet of smooth muscle and elastic fibers – Smooth muscle cells are arranged in two concentric layers • Inner layer encircles lumen (circular muscle) • Outer layer contains cells arranged parallel to long axis of tract (longitudinal layer)

. Submucosa – Layer of dense irregular connective tissue – Binds mucosa to muscular layer – Numerous blood vessels and lymphatic vessels – May contain exocrine glands • Secrete buffers and enzymes into digestive tract – Submucosal neural plexus • Innervates mucosa and submucosa • Sensory neurons, parasympathetic ganglionic neurons, and sympathetic postganglionic fibers

. Muscular layer – Dominated by smooth muscle cells • Inner circular layer and outer longitudinal layer – Involved in mechanical digestion and moving materials along digestive tract – Movements coordinated by enteric nervous system (ENS) • Innervated primarily by parasympathetic division • Also by sympathetic postganglionic fibers

. Muscular layer – Myenteric plexus • Network of parasympathetic ganglia, sensory neurons, interneurons, and sympathetic postganglionic fibers • Between circular and longitudinal muscle layers

. Serosa – Serous membrane covering muscular layer • Along most portions of digestive tract enclosed by peritoneal cavity – In areas where serosa is lacking • Adventitia (dense network of collagen fibers) firmly attaches digestive tract to adjacent structures

. Motility of digestive tract – Visceral smooth muscle tissue • Rhythmic cycles of activity • Controlled by pacesetter cells that undergo spontaneous depolarization • Wave of contraction spreads throughout entire muscular sheet

. Peristalsis – Waves of muscular contractions that move a bolus along length of digestive tract 1. Circular muscles behind bolus contract • While circular muscles ahead of bolus relax 2. Longitudinal muscles ahead of bolus contract • Shortening adjacent segments 3. Wave of contraction in circular muscle layer • Forces bolus forward

37 Figure 24–4 Peristalsis (Part 2 of 4).

38 Figure 24–4 Peristalsis (Part 3 of 4).

39 Figure 24–4 Peristalsis (Part 4 of 4).

40 24-1 The Digestive System

. Segmentation – Cycles of contraction that churn and fragment the bolus • Mixing contents with intestinal secretions – Does not follow a set pattern • Does not push materials in any one direction

. Regulation of digestive functions – Local factors – Neural mechanisms – Hormonal mechanisms

. Local factors – pH, volume, or chemical composition of intestinal contents • Can have direct, localized effects on digestive activity – Stretching of intestinal wall • Can stimulate localized contractions – Local factors may stimulate release of chemicals • Prostaglandins, histamine, and other chemicals may affect adjacent cells

. Neural mechanisms – Visceral motor neurons • Control smooth muscle contraction and glandular secretion • Located in myenteric plexus

. Neural mechanisms – Short reflexes (local reflexes) • Control small segments of digestive tract • Operate entirely outside of CNS control – Long reflexes • Involve interneurons and motor neurons in CNS • Provide higher level control • Stimulate large-scale peristalsis • Parasympathetic motor fibers synapse in myenteric plexus

. Hormonal mechanisms – Enteroendocrine cells in digestive tract produce many peptide hormones • Affect almost every aspect of digestion • Some also affect other systems – Travel through bloodstream to reach target organs

The movement of materials along the digestive tract, as well as many secretory functions, is primarily controlled by local factors. Short reflexes are triggered by CNS chemoreceptors or stretch receptors in the walls of the digestive tract; the Long controlling neurons are located in the myenteric plexus. These reflexes are reflex often called myenteric reflexes. Long reflexes involving interneurons and motor neurons in the CNS provide a Myenteric higher level of control over digestive and glandular activi- plexus ties, generally controlling large-scale Short peristalsis that moves materials from one reflex region of the digestive tract to another. Peristalsis and Long reflexes may involve parasympa- Stretch receptors, segmentation thetic motor fibers in the glossopharyn- chemoreceptors movements geal (IX), vagus (X), or pelvic nerves that synapse in the myenteric plexus.

Buffers, acids, Secretory cells 3 enzymes released Hormonal Control Mechanisms

The digestive tract produces 1 Local Factors numerous hormones that affect almost every aspect of diges- Local factors are the primary stimulus tion, and some of them also for digestion. They coordinate the affect the activities of other responses to changes in the pH of the Carried by systems. These hormones are Enteroendocrine bloodstream peptides produced by entero- contents of the lumen, physical distor- cells tion of the wall of the digestive tract, or endocrine cells, endocrine the presence of chemicals—either cells in the epithelium of the digestive tract. We consider specific nutrients or chemical messen- Hormones these hormones as we proceed gers released by cells of the mucosa. released down the digestive tract.

47 24-2 The Oral Cavity

. Functions of oral cavity – Sensory analysis • Of food before swallowing – Mechanical digestion • Through actions of teeth, tongue, and palatal surfaces – Lubrication • By mixing with mucus and saliva – Limited chemical digestion • Of carbohydrates and lipids

. Oral mucosa – Lining of oral cavity – Stratified squamous epithelium – Relatively thin and nonkeratinized on cheeks, lips, and inferior surface of tongue – Thin, vascular mucosa inferior to tongue can rapidly absorb lipid-soluble drugs – Mucosae of cheeks • Supported by pads of fat and buccinator muscles • Continuous with those of lips

. Oral vestibule – Space between cheeks (or lips) and teeth . Gingivae (gums) – Ridges of oral mucosa – Surround base of each tooth on alveolar processes of maxillae and mandible

. Uvula – Dangling process at posterior margin of soft palate – Prevents food from entering pharynx too soon . Palatoglossal arch – Extends between soft palate and base of tongue . Fauces – Space between oral cavity and pharynx – Bounded by soft palate and base of tongue . Palatopharyngeal arch – Extends from soft palate to pharyngeal wall

Palatoglossal Hard palate Soft palate arch Nasal cavity

Pharyngeal tonsil Opening of parotid duct Entrance to auditory tube Upper lip Nasopharynx Cheek Dorsum of Uvula tongue Palatine tonsil Lower lip Fauces Gingiva Palatopharyngeal arch Oral vestibule Oropharynx Lingual tonsil Body of tongue Epiglottis

Root of tongue Hyoid bone Laryngopharynx Geniohyoid Mylohyoid a A sagittal section of the oral cavity 52 Figure 24–6b Anatomy of the Oral Cavity.

Frenulum of upper lip Hard palate

Soft palate Fauces Uvula Palatoglossal arch Palatopharyngeal arch Palatine tonsil Tongue Frenulum of tongue Gingiva Oral vestibule

Frenulum of lower lip

Openings of submandibular ducts b An anterior view of the oral cavity 53 53 24-2 The Oral Cavity

. Tongue – Four primary functions • Mechanical digestion by compression, abrasion, and distortion • Manipulation to assist in chewing and to prepare food for swallowing • Sensory analysis by touch, temperature, and taste receptors • Secretion of mucins and lingual lipase

. Tongue – Anterior body – Posterior root – Frenulum of tongue • Along inferior midline – Extrinsic tongue muscles • Perform all gross movements – Intrinsic tongue muscles • Smaller • Perform precise movements

. Teeth – Assisted by tongue while chewing . Dentin – A mineralized matrix in teeth similar to that of bone – Does not contain cells . Pulp cavity – Interior chamber of tooth that receives blood vessels and nerves through the root canal – Apical foramen—opening through which blood vessels and nerves enter root canal

. Root of tooth – Sits in a bony socket (tooth alveolus) – A layer of cement covers dentin of root • Protects and anchors periodontal ligament – Periodontal ligament extends from dentin of root to alveolar bone • Creating a gomphosis (strong articulation)

. Crown – Exposed portion of tooth – Projects beyond soft tissue of gingiva – Separated from root by neck of tooth • Gingival sulcus surrounds the neck – Enamel • Covers dentin • Forms occlusal surface (biting surface) – Cusps—elevations or projections of occlusal surface

Enamel

Crown Dentin Pulp cavity Gingiva Gingival Neck sulcus

Cement

Periodontal ligament Root Root canal

Alveolar process

Apical foramen

Branches of alveolar a A diagrammatic section vessels and nerve through a typical adult tooth. 59 24-2 The Oral Cavity

. Alveolar processes of maxillae and alveolar part of mandible – Form maxillary dental arcade and mandibular dental arcade . Types of teeth – Incisor teeth – Canine teeth – Premolar teeth – Molar teeth

Incisors Canines Premolars Molars (cuspids) (bicuspids)

Upper jaw

Lower jaw

b The adult teeth from the right side of the upper and lower jaws. Figure 24–8a,b shows a view of the occlusal surfaces.

61 24-2 The Oral Cavity

. Incisor teeth – Blade-shaped – Located at front of mouth – Used for clipping or cutting – Have a single root . Canine teeth (cuspids) – Conical with single, pointed cusp – Used for tearing or slashing – Have a single root

. Premolar teeth (bicuspids) – Flattened crowns – Two prominent, rounded cusps – Used to crush, mash, and grind – Have one or two roots . Molar teeth – Very large, flattened crowns – Four to five prominent, rounded cusps – Used for crushing and grinding – Have two to three roots

. Sets of teeth – During development, two sets of teeth form • Deciduous teeth • Permanent teeth . Deciduous teeth – Also called primary teeth, milk teeth, or baby teeth – 20 temporary teeth – Five on each side of upper and lower jaws • 2 incisors, 1 canine, and 2 deciduous molars

Canine (18 mo)

Deciduous 1st molar (14 mo) Deciduous 2nd molar (24 mo)

Deciduous 2nd molar (20 mo) Deciduous 1st molar (12 mo) Canine (16 mo) Lateral incisor (7 mo) Central incisors (6 mo) a The deciduous teeth, with the age at eruption given in months 65 24-2 The Oral Cavity

. Permanent teeth – Replace deciduous teeth by eruption – 32 permanent teeth – 8 on each side of upper and lower jaws • 2 incisors, 1 canine, 2 premolars, and 3 molars

Maxilla exposed to show developing permanent teeth

Erupted deciduous teeth

First and second molars Mandible exposed to show developing permanent teeth b Maxilla and mandible with unerupted teeth exposed

67 Figure 24–8c Deciduous and Permanent Dentitions (Part 1 of 2).

Central incisors (7–8 yr) Lateral incisor (8–9 yr) Canine (11–12 yr) 1st Premolar Maxillary (10–11 yr) dental 2nd Premolar arcade (10–12 yr) 1st Molar (6–7 yr) 2nd Molar Hard palate (12–13 yr) 3rd Molar (17–21 yr)

68 Figure 24–8c Deciduous and Permanent Dentitions (Part 2 of 2).

3rd Molar (17–21 yr)

2nd Molar (11–13 yr) 1st Molar (6–7 yr) 2nd Premolar Mandibular (11–12 yr) dental arcade 1st Premolar (10–12 yr) Canine (9–10 yr) Lateral incisor (7–8 yr) Central incisors (6–7 yr) c The permanent teeth, with the age at eruption given in years

69 24-2 The Oral Cavity

. Salivary glands – Three major pairs secrete into oral cavity • Parotid glands • Sublingual glands • Submandibular glands – Each pair has distinctive cellular organization • And produces saliva with slightly different properties

. Parotid glands – Inferior to zygomatic arch – Produce serous secretion • Containing salivary amylase to break down starches – Each is drained by parotid duct • Empties into vestibule at second upper molar

. Sublingual glands – Covered by mucous membrane of floor of mouth – Produce mucus • Acts as a buffer and lubricant – Numerous sublingual ducts • Open along either side of lingual frenulum

. Submandibular glands – Lie within mandibular groove – Secrete buffers, glycoproteins (mucins), and salivary amylase – Submandibular ducts • Open on each side of frenulum of tongue immediately posterior to teeth

Parotid duct

Openings of Major Salivary sublingual Glands ducts Parotid gland Frenulum of tongue Sublingual gland

Opening of left Submandibular submandibular gland duct Submandibular duct

a A lateral view, showing the relative positions of the major salivary glands and ducts on the left side of the head. For clarity, the left ramus and body of the mandible have been removed. For the positions of the parotid and submandibular ducts in the oral cavity, see Figure 24–6.

74 Figure 24–9b Anatomy of the Salivary Glands.

Mucous Serous Duct cells cells

Submandibular gland LM × 300

a The submandibular gland secretes a mixture of mucins, produced by mucous cells, and enzymes, produced by serous cells.

75 24-2 The Oral Cavity

. Saliva – Salivary glands produce 1.0–1.5 liters each day • 70 percent from submandibular glands • 25 percent from parotids • 5 percent from sublingual glands – 99.4 percent water – Remaining 0.6 percent • Electrolytes, buffers, glycoproteins, antibodies, enzymes, and wastes

. Functions of saliva – Cleaning oral surfaces – Moistening and lubricating food – Keeping pH of mouth near 7.0 – Controlling populations of bacteria and limiting acids that they produce – Dissolving chemicals that stimulate taste buds – Initiating digestion of complex carbohydrates with salivary amylase

. Regulation of salivary secretions – Salivary glands have parasympathetic and sympathetic innervation – Parasympathetic efferents • Originate in superior salivatory nucleus and inferior salivatory nucleus of medulla oblongata • Stimulated by any object in mouth, other brainstem nuclei, and activities of higher centers – Parasympathetic stimulation accelerates secretion by all salivary glands

. Mastication (chewing) – Food is forced from oral cavity to vestibule and back across occlusal surfaces of teeth – Muscles of mastication • Close jaws • Slide lower jaw from side to side – Tongue compacts chewed food into a bolus • Moist, rounded ball • Fairly easy to swallow

. Pharynx (throat) – Common passageway for food, liquid, and air – Regions of the pharynx • Nasopharynx • Oropharynx • Laryngopharynx – Food passes through parts of pharynx on its way to esophagus

. Esophagus – A hollow muscular tube – Conveys food and liquids to stomach – About 25 cm long and 2 cm wide – Begins posterior to cricoid cartilage – Enters abdominopelvic cavity through the esophageal hiatus – Innervated by parasympathetic and sympathetic fibers from esophageal plexus

. Resting muscle tone in circular muscle layer – Prevents air from entering esophagus – Prevents backflow of materials from stomach . Histology of esophagus – Wall of esophagus has three layers • Mucosa • Submucosa • Muscular layer

. Histology of esophagus – Mucosa contains nonkeratinized stratified squamous epithelium – Mucosa and submucosa form large folds – Muscularis mucosae consists of smooth muscle – Submucosa contains esophageal glands that produce mucus – Muscular layer has inner circular and outer longitudinal layers – Adventitia anchors esophagus to body wall

Muscularis mucosae Mucosa

Submucosa

Muscular layer Adventitia

a A transverse section through an empty esophagus (LM ×845). Figure 24–10b Anatomy of the Esophagus.

Stratified squamous epithelium

Lamina propria

Muscularis mucosae

Esophageal mucosa LM × 275

b This light micrograph illustrates the extreme thickness of the epithelial portion of the esophageal mucosal layer. 85 24-3 The Pharynx and Esophagus

. Deglutition (swallowing) – Can be initiated voluntarily, but proceeds automatically – Swallowing reflex • Begins when tactile receptors on palatal arches and uvula are stimulated by bolus • Information is relayed to swallowing center of medulla oblongata – Buccal, pharyngeal, and esophageal phases

1 Buccal Phase The buccal phase begins with the compression of Hard palate the bolus against the hard palate. Retraction of the Soft palate tongue then forces the Bolus bolus into the oropharynx Tongue and assists in elevating Oropharynx the soft palate, thereby sealing off the Epiglottis nasopharynx. Once the Trachea bolus enters the oropharynx, reflex responses begin and the bolus is moved toward the stomach.

87 Figure 24–11 The Process of Deglutition (Swallowing) (Part 2 of 4).

2 Pharyngeal Phase The pharyngeal phase begins as the bolus comes into contact with the palatal arches and Uvula the posterior pharyngeal Tongue wall. Elevation of the larynx and folding of the epiglottis direct the Bolus bolus past the closed glottis. At the same time, Epiglottis the uvula and soft palate Larynx block passage back to the nasopharynx.

88 Figure 24–11 The Process of Deglutition (Swallowing) (Part 3 of 4).

3 Esophageal Phase The esophogeal phase begins as the contraction of pharyngeal muscles forces the bolus through the entrance to the esophagus. Peristalsis Once in the Trachea esophagus, the bolus is pushed toward the stomach by a peristaltic wave. Esophagus

89 Figure 24–11 The Process of Deglutition (Swallowing) (Part 4 of 4).

4 Bolus Enters Stomach The approach of the bolus triggers the opening of the lower esophageal Thoracic sphincter. The bolus cavity then continues into the stomach. Lower esophageal sphincter Stomach

90 24-4 The Stomach

. Major functions of stomach – Temporary storage of ingested food – Mechanical digestion with muscular contractions – Chemical digestion of food with acid and enzymes . Chyme – Partially digested food mixed with acidic secretions of stomach

. Gross anatomy of stomach – Shaped like an expanded J • Short lesser curvature forms medial surface • Long greater curvature forms lateral surface – Anterior and posterior surfaces are rounded – Shape and size vary from person to person and from one meal to the next

– Typically extends between levels of vertebrae T7 and L3

. Regions of stomach – Cardia – Fundus – Body – Pyloric part

. Cardia – Superior, medial portion of stomach – Abundant mucous glands . Fundus – Superior to junction of stomach and esophagus – Contacts diaphragm . Body – Between fundus and curve of the J – Largest region of stomach – Mixing tank for ingested food and secretions

. Pyloric part – Between body and duodenum – Shape changes often during digestion – Pyloric antrum connects to body – Pyloric canal empties into duodenum – Pylorus • Muscular tissue surrounding pyloric orifice (stomach outlet) – Pyloric sphincter • Thick circular layer of muscle within pylorus

Esophagus

Diaphragm Left gastric Liver, Liver, artery right lobe left lobe Vagus Lesser curvature nerve (X) Common Fundus hepatic artery Gallbladder Cardia

Spleen Body of stomach Bile duct Greater Pyloric sphincter curvature with greater omentum Pyloric part attached Greater omentum

a The position and external appearance of the stomach, showing superficial landmarks

96 96 Figure 24–12b Gross Anatomy of the Stomach.

Esophagus Fundus

Anterior surface Cardia

Longitudinal muscle layer Left gastro-epiploic vessels Circular muscle layer

Body Lesser curvature Pyloric sphincter (medial surface) Oblique muscle layer overlying mucosa

Duodenum Rugae

Pyloric orifice Greater curvature Pyloric Pylorus (lateral surface) part Pyloric canal Pyloric antrum b The structure of the stomach wall

97 24-4 The Stomach

. Rugae – Prominent folds in mucosa of empty stomach – Flatten out as stomach fills – Allow for expansion of gastric lumen • Up to 50 times its empty size . Muscularis mucosae and muscular layer – Contain extra layers of smooth muscle cells – Oblique layer in addition to circular and longitudinal layers

. Histology of stomach – Simple columnar epithelium lines all portions – Epithelium is a secretory sheet • Produces mucus that covers interior surface – Gastric pits • Shallow depressions that open onto gastric surface – Mucous cells at base (neck) of each gastric pit • Actively divide, replacing superficial cells

. Gastric glands – In fundus and body of stomach • Extend deep into underlying lamina propria – Each gastric pit communicates with several gastric glands • Parietal cells • Chief cells – Secrete about 1500 mL of gastric juice each day

Diaphragm

Stomach

Greater omentum

Layers of the Stomach Wall

Mucosa Gastric pit (opening to gastric gland) Mucous epithelium

Lamina propria Muscularis mucosae

Submucosa Artery and Muscular layer vein Oblique muscle

Circular muscle Lymphatic Longitudinal muscle vessel Myenteric Serosa plexus a Stomach wall 101 Figure 24–13b Histology of the Stomach Lining.

Lamina propria Gastric Mucous pit cells Neck

Cells of Gastric Glands Parietal cells Gastric gland G cell

Chief cells

Smooth muscle cell b Gastric gland 102 24-4 The Stomach

. Parietal cells – Common along proximal portions of gastric glands – Secrete intrinsic factor

• Glycoprotein that helps absorb vitamin B12 – Also indirectly secrete hydrochloric acid (HCl) . Chief cells – Most abundant near base of gastric glands – Secrete pepsinogen (an inactive proenzyme) • Pepsinogen is converted to pepsin (an active proteolytic enzyme) by HCl in gastric lumen

Hydrogen ions (H+) are generated KEY inside a parietal cell as the enzyme Parietal cell carbonic anhydrase converts CO 2 Diffusion and H2O to carbonic acid (H2CO3), CO2 + H2O which then dissociates. Carbonic Carrier-mediated anhydrase transport 2 4 An anion countertransport The hydrogen ions are Active transport H CO mechanism ejects the bicarbonate 2 3 actively transported into the Countertransport ions into the interstitial fluid and lumen of the gastric gland. imports chloride ions into the cell. Dissociation

– – + HCO3 HCO3 + H H+ Interstitial fluid Cl– Cl– Cl– 3 Alkaline tide The chloride ions then diffuse across the cell and exit through Lumen of open chloride channels into the gastric Enters lumen of the gastric gland. gland bloodstream

104 24-4 The Stomach

. Stomachs of newborn infants – Produce enzymes important for digestion of milk • Rennin (chymosin) • Gastric lipase

. Pyloric glands – Located in pyloric part of stomach – Produce mucous secretions . Enteroendocrine cells – Produce at least seven hormones – G cells produce gastrin • Stimulates secretion by parietal and chief cells • Stimulates contractions of gastric wall – D cells release somatostatin • Inhibits release of gastrin

. Chemical digestion in stomach – Some digestion of carbohydrates (by salivary amylase) and lipids (by lingual lipase) – As stomach contents become more fluid, • pH approaches 2.0 • Preliminary digestion of proteins by pepsin increases – Nutrients are not absorbed in stomach

. Regulation of gastric activity – Production of acid and enzymes by gastric mucosa can be controlled by • CNS • Short reflexes of ENS • Hormones of digestive tract – Three overlapping phases of gastric control • Cephalic phase • Gastric phase • Intestinal phase

Food

Sight, smell, taste, or thoughts of food

Central nervous system

Vagus nerve (X)

Submucosal plexus

Mucous Mucus cells Chief Pepsinogen cells Parietal HCl cells

Gastrin KEY Neural G cells stimulation Secretion 109 Figure 24–15 The Regulation of Gastric Activity (Part 2 of 4). 2 GASTRIC PHASE

Neural Response

Submucosal and Distension Stretch myenteric plexuses receptors Elevated pH Chemoreceptors carried by Mucous Mucus bloodstream cells Chief Pepsinogen cells Mixing Parietal waves Gastrin HCI cells

G cells Partly KEY digested Neural peptides stimulation Hormonal stimulation

110 Figure 24–15 The Regulation of Gastric Activity (Part 3 of 4). 3 INTESTINAL PHASE

Neural Responses

Enterogastric Myenteric reflex plexus

carried by bloodstream Chief cells Parietal Duodenal cells stretch and Peristalsis chemoreceptors

CCK Presence of lipids and KEY carbohydrates GIP Neural inhibition Secretin Decreased pH Hormonal inhibition 111 24-5 Accessory Digestive Organs

. Pancreas – Lies posterior to stomach – Extends from duodenum toward spleen – Retroperitoneal • Bound to posterior wall of abdominal cavity – Wrapped in thin, connective tissue capsule

. Gross anatomy of pancreas – Head • Broad; in loop formed by duodenum – Body • Slender; extends toward spleen – Tail • Short and rounded – Pancreatic duct • Delivers secretions of pancreas to duodenum

. Histology of pancreas – Lobules separated by connective tissue partitions – Ducts in lobules branch repeatedly and end in blind pockets (pancreatic acini) • Acini are lined with simple cuboidal epithelium – Pancreatic islets • Endocrine tissues of pancreas • Scattered among pancreatic acini • Account for about 1 percent of pancreatic cells

Accessory Bile Pancreatic Lobules pancreatic duct duct Tail of duct pancreas

Body of pancreas

Head of pancreas

Duodenal Duodenum papilla

a The gross anatomy of the pancreas. The head of the pancreas is tucked into a C-shaped curve of the duodenum that begins at the pylorus of the stomach. 115 Figure 24–16b Anatomy of the Pancreas.

Pancreatic duct

Connective tissue septum

Exocrine cells in pancreatic acini

Endocrine cells in pancreatic islet

b Diagram of the cellular organization of the pancreas.

116 Figure 24–16c Anatomy of the Pancreas.

Duct

Pancreatic islet (endocrine)

Pancreatic acini (exocrine)

Pancreas LM × 75

c Light micrograph of the cellular organization of the pancreas.

117 24-5 Accessory Digestive Organs

. Endocrine cells of pancreatic islets – Secrete insulin and glucagon into bloodstream . Exocrine cells – Acinar cells and epithelial cells of duct system – Secrete alkaline pancreatic juice into small intestine • About 1000 mL per day • Contains digestive enzymes, water, and ions – Controlled by hormones from duodenum

. Pancreatic enzymes – Pancreatic alpha-amylase – Pancreatic lipase – Nucleases – Proteolytic enzymes

. Pancreatic alpha-amylase – A carbohydrase – Breaks down certain starches – Almost identical to salivary amylase . Pancreatic lipase – Breaks down certain complex lipids – Releases products (e.g., fatty acids) that are easily absorbed

. Nucleases – Break down RNA or DNA . Proteolytic enzymes – Break apart proteins • Proteases break apart large protein complexes • Peptidases break small peptide chains into individual amino acids – 70 percent of all pancreatic enzyme production – Secreted as inactive proenzymes • Activated after reaching small intestine

. Proenzymes secreted by pancreas – Trypsinogen • Converted to active trypsin in duodenum – Chymotrypsinogen • Converted to active chymotrypsin by trypsin – Procarboxypeptidase • Converted to active carboxypeptidase by trypsin – Proelastase • Converted to active elastase by trypsin

. Liver – Largest visceral organ (1.5 kg) – Lies in right hypochondriac and epigastric regions – May extend into left hypochondriac and umbilical regions – Performs essential metabolic and synthetic functions

. Gross anatomy of liver – Wrapped in tough, fibrous capsule – Covered with visceral peritoneum – Divided into • Left and right lobes • Caudate lobe • Quadrate lobe – Falciform ligament • Marks division between left and right lobes • Thickening in posterior margin is round ligament

. Gross anatomy of liver – Blood vessels converge at porta hepatis – Nearly one-third of blood supply is arterial blood from hepatic artery proper • Remaining two-thirds is venous blood from hepatic portal vein – Hepatocytes • Liver cells • Adjust circulating levels of nutrients through selective absorption and secretion

Liver

Sternum Falciform ligament Left lobe Porta hepatis of liver

Right lobe of liver Stomach Lesser Caudate lobe omentum of liver Inferior vena cava Aorta Pleural cavity Spleen Cut edge of diaphragm Peritoneal cavity

a A transverse section through the superior abdomen (diagrammatic view)

126 Figure 24–17b Gross Anatomy of the Liver.

Coronary ligament

Right lobe Left lobe

Falciform Round ligament ligament Gallbladder b The anterior surface of the liver

127 Figure 24–17c Gross Anatomy of the Liver.

Left hepatic vein Inferior vena cava Coronary ligament

Lobes of Liver

Porta Hepatis Left lobe Bile duct Caudate lobe Hepatic portal vein Right lobe Hepatic artery proper Quadrate lobe

Gallbladder c The posterior surface of the liver

128 24-5 Accessory Digestive Organs

. Histology of liver – Each lobe is divided by connective tissue • Into approximately 100,000 lobules – Lobules are basic functional units of liver • Roughly 1 mm in diameter • Hepatocytes form a series of irregular plates arranged like wheel spokes • Sinusoids between plates empty into central vein • Many stellate macrophages (Kupffer cells) in lining of sinusoids

1 mm

Interlobular Interlobular Interlobular Bile septum bile duct vein Portal triad ductules

a A diagram of liver structure, showing relationships among lobules

130 Figure 24–18b Histology of the Liver.

Central Hepatocytes vein

Sinusoids Stellate macrophages

Bile canaliculi

Portal Triad Interlobular bile duct Interlobular vein Interlobular artery

b A single liver lobule and its cellular components

131 Figure 24–18c Histology of the Liver. Portal Triad Interlobular Interlobular vein Interlobular bile duct (containing blood) artery

Hepatocytes

Sinusoids

Portal area LM × 320

c A micrograph showing the vessels and ducts within a portal triad 132 24-5 Accessory Digestive Organs

. Hepatic portal system – Liver lobules are hexagonal in cross section • Each of six corners has a portal triad containing • Interlobular vein • Interlobular artery • Interlobular bile duct

. Bile duct system – Liver secretes bile • Into a network of narrow channels (bile canaliculi) between adjacent liver cells – Right and left hepatic ducts • Collect bile from all bile ducts of liver lobes • Unite to form common hepatic duct – From common hepatic duct, bile enters either • Bile duct, which empties into duodenal ampulla • Cystic duct, which leads to gallbladder

. Bile duct – Formed by union of • Cystic duct • Common hepatic duct – Passes within lesser omentum toward stomach – Penetrates wall of duodenum – Meets pancreatic duct at duodenal ampulla

135 © 2018 Pearson Education, Inc. Figure 24–19a Anatomy and Physiology of the Gallbladder and Bile Ducts. Round ligament Left hepatic duct Right hepatic duct Left hepatic artery Cystic duct Common hepatic Gallbladder duct Fundus Cut edge of lesser Body omentum Bile duct Neck Hepatic portal vein

Common hepatic artery Liver

Duodenum Right gastric artery

Stomach

Pancreas

a A view of the inferior surface of the liver, showing the position of the gallbladder and ducts that transport bile from the liver to the gallbladder and duodenum. A portion of the lesser omentum has been cut away. 136 Figure 24–19b Anatomy and Physiology of the Gallbladder and Bile Ducts.

Pancreatic Bile duct duct

Hepatopancreatic sphincter Duodenal ampulla Duodenal papilla Pancreas Intestinal lumen

b A sectional view through a portion of the duodenal wall, showing the duodenal ampulla and related structures.

137 Figure 24–19c Anatomy and Physiology of the Gallbladder and Bile Ducts.

Left hepatic duct Right hepatic duct Common hepatic duct Gallbladder Neck

Body Fundus

Duodenum Bile duct

c A radiograph (cholangiogram, anterior-posterior view) of the hepatic ducts, gallbladder, and bile duct.

138 Figure 24–19d Anatomy and Physiology of the Gallbladder and Bile Ducts.

1 The liver secretes bile continuously— 2 about 1liter Bile becomes more per day. concentrated the longer it remains in the gallbladder. Liver

Duodenum 3 CCK The release of CCK by the 4 duodenum triggers dilation In the lumen of the of the hepatopancreatic digestive tract, bile Lipid sphincter and contraction salts break the lipid droplet of the gallbladder. This ejects droplets apart by bile into the duodenum emulsification. through the duodenal ampulla.

d Physiology of the gallbladder. 24-5 Accessory Digestive Organs

. Physiology of liver – Liver has over 200 functions in three categories • Metabolic regulation • Hematological regulation • Bile production

. All blood leaving absorptive surfaces of digestive tract enters hepatic portal system – Flows into liver . Liver cells extract nutrients or toxins from blood – Before blood reaches systemic circulation through hepatic veins . Liver removes and stores excess nutrients – Corrects nutrient deficiencies by mobilizing stored reserves or performing synthetic activities

. Regulatory activities of liver affect – Carbohydrate metabolism – Lipid metabolism – Amino acid metabolism – Waste removal – Vitamin storage – Mineral storage – Drug inactivation

. Liver receives about 25 percent of cardiac output – Largest blood reservoir in body . Hematological regulation by liver involves – Phagocytosis and antigen presentation – Synthesis of plasma proteins – Removal of circulating hormones – Removal of antibodies – Removal or storage of toxins

. Production and functions of bile – Bile salts in bile break lipid droplets apart (emulsification) in duodenum • Creates tiny emulsion droplets coated with bile salts • Increases surface area exposed to enzymes – Necessary because mechanical digestion in stomach creates large droplets of lipids • Pancreatic lipase can interact only at surface – Enterohepatic circulation • Cycling of bile salts between liver and small intestine

. Gallbladder – Hollow, pear-shaped muscular sac – Stores and concentrates bile prior to secretion into small intestine – Located in fossa in posterior surface of liver’s right lobe

. Regions of gallbladder – Fundus – Body – Neck . Cystic duct – Extends from gallbladder – Unites with common hepatic duct to form bile duct – Bile duct joins pancreatic duct before emptying into duodenal ampulla • Opens into duodenum at duodenal papilla

. Gallbladder releases bile into duodenum – Only when stimulated by cholecystokinin (CCK) . Without CCK – Hepatopancreatic sphincter encircling lumen of bile duct remains closed – Bile exiting liver in common hepatic duct enters cystic duct and is stored in gallbladder . When chyme enters duodenum, CCK is released – Hepatopancreatic sphincter relaxes – Gallbladder contracts

. Physiology of gallbladder – Full gallbladder contains 40–70 mL bile – Bile composition gradually changes in gallbladder • Water is absorbed • Bile salts and other components become increasingly concentrated

. Small intestine – Long, muscular tube where • Chemical digestion is completed • 90 percent of nutrient absorption occurs – Consists of three segments • Duodenum • Jejunum • Ileum

. Duodenum – Segment of small intestine closest to stomach – 25 cm long – “Mixing bowl” that receives chyme from stomach and digestive secretions from pancreas and liver . Jejunum – Middle segment of small intestine – 2.5 meters long – Site of most chemical digestion and nutrient absorption

. Ileum – Final segment of small intestine – 3.5 meters long – Ends at ileocecal valve • Sphincter that controls flow of material from ileum into cecum of large intestine

Segments of the Small Intestine

Duodenum

Jejunum

Ileum

Large intestine

Rectum

a The positions of the duodenum, jejunum, and ileum in the abdominopelvic cavity 152 152 Figure 24–24b Anatomy of the Large Intestine.

Ileocecal valve

Cecum (cut open)

Appendix

b The cecum and appendix

153 24-6 The Small Intestine

. Histology of small intestine – Circular folds • Transverse folds in intestinal lining • Permanent features that do not disappear when small intestine fills – Intestinal villi • Fingerlike projections in mucosa of small intestine • Covered by simple columnar epithelium • Carpeted with microvilli that form brush border

Circular folds

Gross anatomy of the jejunum

b A representative view of the jejunum

155 24-6 The Small Intestine

. Histology of small intestine – Lacteal • Lymphatic vessel in each villus • Transports chylomicrons that are too large to enter blood capillaries – Intestinal glands (intestinal crypts) • Extend deep into lamina propria • Stem cells near base produce new epithelial cells • Paneth cells at base function in innate immunity • Contain enteroendocrine cells

a Segment of the small intestine

157 Figure 24–21b Histology of the Intestinal Wall.

Villi Intestinal Lymphoid gland nodule

Lamina propria

Layers of the Lacteal Small Intestine

Mucosa

Muscularis Lymphatic mucosae vessel Submucosal plexus Submucosa Circular layer of smooth muscle Muscular Myenteric layer plexus

Serosa Longitudinal layer of smooth muscle

Submucosal artery and vein b The organization of the intestinal wall 158 Figure 24–21c Histology of the Intestinal Wall.

Epithelial cell

Stem cell

Paneth cell

c Cellular components of an intestinal gland

159 Figure 24–21d Histology of the Intestinal Wall.

Columnar epithelial cell Goblet cell Lacteal

Nerve

Capillary network Lamina propria

Lymphatic vessel Smooth muscle cell Arteriole Venule

d Internal structures in a single villus, showing the capillary and lymphatic supplies 160 Figure 24–21e Histology of the Intestinal Wall.

Capillaries

Goblet cells

Lacteal

Brush border

Tip of villus LM × 250

e A villus in sectional view

161 24-6 The Small Intestine

. Duodenal submucosal glands – Produce copious quantities of mucus • When chyme arrives from stomach – Mucus protects epithelium from acidity of chyme • Contains bicarbonate ions that raise pH

. Physiology of small intestine – 1.8 liters of intestinal juice enters intestinal lumen each day – Intestinal juice • Moistens chyme • Assists in buffering acids • Keeps digestive enzymes and products of digestion in solution

. Mucosa of small intestine produces few enzymes involved in chemical digestion – Brush border enzymes • Integral membrane proteins on intestinal microvilli • Break down materials in contact with brush border – Enteropeptidase • A brush border enzyme • Activates pancreatic trypsinogen

. Intestinal motility – After chyme arrives in duodenum • Weak peristaltic contractions move it slowly toward jejunum – Contractions are myenteric reflexes • Not under CNS control • Parasympathetic stimulation accelerates local peristalsis and segmentation

. Reflexes of small intestine – Gastroenteric reflex • Stimulates motility and secretion along entire small intestine – Gastroileal reflex • Triggers opening of ileocecal valve • Allows materials to pass from small intestine into large intestine – Opposite effect from that of enterogastric reflex • Which causes constriction of pyloric sphincter

Central Gastric Reflexes

The gastroenteric reflex stimulates motility and secretion along the entire small intestine.

The gastroileal (gas-tro-IL-e-al) reflex triggers the opening of the ileocecal valve, allowing materials to pass from the small intestine The ileocecal valve controls into the large intestine. the passage of materials into the large intestine. 167 24-6 The Small Intestine

. Neural and hormonal mechanisms – Coordinate activities of digestive glands – Centered on duodenum • Where acids are neutralized and enzymes are added

. Neural mechanisms involving CNS – Prepare digestive tract for activity • Through parasympathetic innervation – Inhibit gastrointestinal activity • Through sympathetic innervation – Coordinate movement of materials along digestive tract • Through reflexes – Motor neuron synapses in digestive tract release neurotransmitters

. Intestinal hormones – Intestinal tract secretes peptide hormones with multiple effects • In several regions of digestive tract • In accessory glandular organs

. Major hormones of duodenum – Gastrin – Secretin – Gastric inhibitory peptide (GIP) – Cholecystokinin (CCK) – Vasoactive intestinal peptide (VIP) – Enterocrinin

. Gastrin – Secreted by G cells in duodenum • When exposed to incompletely digested proteins – Promotes increased stomach motility – Stimulates production of acids and enzymes . Secretin – Released when chyme arrives in duodenum – Increases secretion of buffers by pancreas and bile by liver – Reduces gastric motility and secretory rates

. Gastric inhibitory peptide (GIP) – Secreted when fats and carbohydrates enter small intestine . Cholecystokinin (CCK) – Secreted when chyme arrives in duodenum – Accelerates pancreatic production and secretion of digestive enzymes – Relaxes hepatopancreatic sphincter and contracts gallbladder • Ejecting bile and pancreatic juice into duodenum

. Vasoactive intestinal peptide (VIP) – Stimulates secretion of intestinal glands – Dilates regional capillaries – Inhibits acid production in stomach . Enterocrinin – Released when chyme enters duodenum – Stimulates alkaline mucus production by submucosal glands

Hormone Action

Food in stomach

Acid production by KEY parietal cells Stimulates Inhibits Gastrin Stimulation of gastric motility; mixing waves increase in intensity

Release of insulin GIP from pancreas Pancreas

Release of pancreatic enzymes and buffers Chyme in duodenum Secretin and CCK Bile secretion and ejection of bile from gallbladder

Dilation of intestinal VIP capillaries

facilitates facilitates facilitates

NUTRIENT Material Nutrient absorption UTILIZATION arrives in BY ALL TISSUES jejunum

175 24-6 The Small Intestine

. Absorption in small intestine – Movements of mucosa increase absorptive effectiveness • Stir and mix intestinal contents • Quickly eliminate local differences in nutrient concentration

. Large intestine (large bowel) – Horseshoe shaped – Extends from end of ileum to anus – Lies inferior to stomach and liver – Frames the small intestine – About 1.5 meters long and 7.5 cm wide

. Parts of large intestine – Cecum • Pouchlike first portion – Colon • Largest portion – Rectum • The last 15 cm and end of digestive tract

. Cecum – Expanded pouch – Receives and stores materials arriving from ileum – Begins compaction . Appendix (vermiform appendix) – Slender, hollow structure about 9 cm long – Attached to posteromedial surface of cecum – Dominated by lymphoid nodules – Meso-appendix (small mesentery) connects appendix to ileum and cecum

. Colon – Larger diameter and thinner wall than small intestine – Haustra • Pouches in wall of colon • Permit expansion and elongation

. Teniae coli – Three longitudinal bands of smooth muscle – Run along outer surfaces of colon, deep to serosa – Similar to outer layer of muscular layer – Muscle tone in teniae coli creates haustra . Omental appendices – Numerous teardrop-shaped sacs of fat in serosa of colon

. Four regions of colon – Ascending colon – Transverse colon – Descending colon – Sigmoid colon

. Ascending colon – Begins at superior border of cecum – Ascends along right lateral and posterior wall of peritoneal cavity • To inferior surface of liver – Bends sharply to the left at right colic flexure (hepatic flexure)

. Transverse colon – Crosses abdomen from right to left – Turns at left colic flexure (splenic flexure) – Supported by transverse mesocolon – Separated from anterior abdominal wall by greater omentum . Descending colon – Proceeds inferiorly along left side to iliac fossa (inner surface of left ilium) – Retroperitoneal, firmly attached to abdominal wall

. Sigmoid colon – S-shaped segment, about 15 cm long – Lies posterior to urinary bladder – Suspended from sigmoid mesocolon – Empties into rectum

. Large intestine – Receives blood from branches of superior mesenteric and inferior mesenteric arteries – Venous blood is collected by superior mesenteric and inferior mesenteric veins

186 © 2018 Pearson Education, Inc. Figure 24–24a Anatomy of the Large Intestine. Aorta Splenic vein Hepatic portal vein Superior mesenteric artery Right colic Superior mesenteric vein Inferior mesenteric vein (hepatic) flexure Inferior vena cava Left colic (splenic) flexure Greater omentum (cut) Transverse colon Descending colon

Middle colic Left colic vein artery and vein Inferior Right colic mesenteric artery and vein artery Ascending colon Left colic artery Haustra Omental appendices Intestinal arteries and veins

Ileocecal valve Ileum Rectal artery Cecum Sigmoid arteries and veins Appendix Teniae coli

Sigmoid colon a The gross anatomy and regions of the large intestine

Rectum

. Rectum – Forms last 15 cm of digestive tract – Expandable organ for temporary storage of feces . Anal canal – Last portion of rectum – Contains small longitudinal folds (anal columns) . Anus – Exit of anal canal – Keratinized epidermis like skin

. Internal anal sphincter – Circular muscle layer – Smooth muscle cells – Not under voluntary control . External anal sphincter – Encircles distal portion of anal canal – Skeletal muscle fibers – Under voluntary control

Rectum

Anal canal Anal columns Internal anal sphincter External anal sphincter Anus

c The rectum and anus

190 24-7 The Large Intestine

. Histology of large intestine – Lacks villi – Abundance of goblet cells – Distinctive intestinal glands • Deeper than glands of small intestine • Dominated by goblet cells – Mucus provides lubrication for fecal material – Large lymphoid nodules scattered throughout lamina propria and submucosa

Teniae coli Omental appendices

Haustrum

Simple Aggregated lymphoid nodule columnar epithelium Layers of the Large Intestine Goblet Mucosa cells Intestinal gland Muscularis Muscularis mucosae mucosae

Submucosa Submucosa Muscular layer Circular layer Longitudinal layer (teniae coli) Serosa a Diagrammatic view of the colon wall 192 Figure 24–25b Histology of the Colon.

Simple columnar epithelium

Goblet cells Intestinal gland Muscularis mucosae

Submucosa

The colon LM × 110

b Colon histology showing detail of mucosal layer 193 24-7 The Large Intestine

. Functions of large intestine – Absorption or reabsorption of • Water • Nutrients (less than 10 percent) • Bile salts • Organic wastes • Vitamins and toxins produced by bacteria – Compaction of intestinal contents into feces – Storage of fecal material prior to defecation

. Microbiome – Microbes (bacteria, fungi, and viruses) that live in and on human body • Including those that inhabit large intestine . Vitamins – Organic molecules – Important as cofactors or coenzymes in metabolism – Normal bacteria in colon make three vitamins that supplement diet

. Vitamins produced by bacteria in colon – Vitamin K (fat soluble) • Required by liver for synthesizing four clotting factors, including prothrombin – Biotin (water soluble) • Important in glucose metabolism

– Vitamin B5 (pantothenic acid; water soluble) • Required in manufacture of steroid hormones and some neurotransmitters

. Organic wastes – Bacteria convert bilirubin to urobilinogens and stercobilinogens • Some urobilinogens are absorbed into bloodstream and excreted in urine • Urobilinogens and stercobilinogens remaining in colon are converted to urobilins and stercobilins – By exposure to oxygen

. Organic wastes – Bacteria break down peptides in feces and generate • Ammonia, as soluble ammonium ions • Indole and skatole (nitrogen-containing compounds responsible for odor of feces) • Hydrogen sulfide gas that produces “rotten egg” odor – Bacteria feed on indigestible carbohydrates • Produce flatus (intestinal gas) in large intestine

. Motility of large intestine – Gastroileal and gastroenteric reflexes • Move materials into cecum while you eat – Movement from cecum to transverse colon is very slow, allowing hours for water absorption – Peristaltic waves move material along length of colon – Segmentation movements (haustral churning) mix contents of adjacent haustra

. Motility of large intestine – Movement from transverse colon through rest of large intestine • Results from powerful peristaltic contractions (mass movements) – Stimulus is distension of stomach and duodenum • Relayed over intestinal nerve plexuses – Distension of rectal wall initiates defecation reflex • Involves two positive feedback loops • Both triggered by stretch receptors in rectum

. Positive feedback loops – Intrinsic myenteric defecation reflex • Short reflex • Triggers peristaltic contractions in sigmoid colon and rectum – Parasympathetic defecation reflex • Long reflex • Coordinated by sacral parasympathetic neurons • Stimulates mass movements

. Elimination of feces – Requires relaxation of internal and external anal sphincters • Reflexes open internal sphincter – Somatic nervous system must be activated to consciously open external sphincter • Pudendal nerves carry somatic motor commands

Spinal cord

3 The long reflex is a spinal reflex coordinated by the sacral parasympathetic system. This reflex stimulates mass movements that push feces toward the rectum from the descending colon and sigmoid colon. It 1 also further relaxes the Feces move into internal anal sphincter. rectum causing distension, stimulating stretch receptors 2 The first loop is a short reflex that triggers a series of peristaltic contractions in the rectum that move feces toward the anus. This reflex is mediated by the myenteric plexus in the sigmid colon and rectum. The internal anal sphincter relaxes. DISTENSION OF RECTUM

Stretch receptors

Internal anal sphincter

External anal sphincter 203 24-8 Chemical Digestion

. Nutrients – A balanced diet contains • Carbohydrates • Lipids • Proteins • Vitamins • Minerals • Water

. Processing and absorption of nutrients – Digestive system breaks down physical structure of food • Then disassembles component molecules – Molecules released into bloodstream are absorbed by cells and either • Broken down to provide energy for ATP synthesis • Used to synthesize carbohydrates, proteins, and lipids

. Digestive enzymes – Break molecular bonds in large organic molecules • Carbohydrates, proteins, lipids, and nucleic acids • In a process called hydrolysis – Divided into classes by their specific substrates • Carbohydrases break bonds between simple sugars • Proteases break bonds between amino acids • Lipases separate fatty acids from glycerides

. Digestive enzymes – Secreted by • Salivary glands • Tongue • Stomach • Pancreas – Brush border enzymes • Break down digestive fragments further

. Carbohydrate digestion and absorption – Complex carbohydrates are digested in two steps 1. Salivary amylase and pancreatic alpha-amylase • Carbohydrases from salivary glands and pancreas 2. Brush border enzymes of intestinal microvilli • Maltase splits bonds between maltose • Sucrase breaks apart sucrose • Lactase hydrolyzes lactose – Insufficient lactase leads to lactose intolerance

. Absorption of monosaccharides – Via facilitated diffusion and cotransport, which differ in • Number of transported substances • Concentration gradients • ATP requirement

REGION and Hormonal Controls CARBOHYDRATES

Salivary ORAL CAVITY amylase

ESOPHAGUS

STOMACH Stimulus: Anticipation or arrival of food Hormone: Gastrin Source: G cells of stomach Disaccharides Trisaccharides Proenzyme released: Pepsinogen by chief cells, activated to pepsin by HCl

SMALL INTESTINE Stimulus: Arrival of chyme in duodenum Pancreatic Hormone: CCK alpha-amylase Proenzymes released: Chymotrypsinogen, procarboxypeptidase, proelastase, trypsinogen. Enteropeptidase Disaccharides Trisaccharides activates trypsin, which activates other enzymes. Enzymes released: Pancreatic amylase, pancreatic lipase, nuclease, enteropeptidase. Lactase Maltase, sucrase INTESTINAL Brush border MUCOSA

FACILITATED DIFFUSION AND COTRANSPORT

Monosaccharides Cell body

FACILITATED DIFFUSION

ROUTE TO BLOODSTREAM Capillary Carbohydrates and amino acids are absorbed and transported by intestinal capillaries. Lipids form Monosaccharides chylomicrons that diffuse into lacteals and are delivered to the left subclavian vein by the thoracic duct. 210 24-8 Chemical Digestion

. Lipid digestion and absorption – Involves lingual lipase and pancreatic lipase – Pancreatic lipase breaks triglycerides into monoglycerides and fatty acids • Interact with bile salts to form micelles (lipid–bile salt complexes) – New triglycerides are assembled by intestinal cells • Join with steroids, phospholipids, vitamins, and proteins to form chylomicrons • Most chylomicrons diffuse into intestinal lacteals

REGION and Hormonal Controls LIPIDS

Lingual ORAL CAVITY lipase

ESOPHAGUS

STOMACH Stimulus: Anticipation or arrival of food Hormone: Gastrin Source: G cells of stomach Proenzyme released: Pepsinogen by chief cells, activated to pepsin by HCl

SMALL INTESTINE Bile salts Stimulus: Arrival of chyme and in duodenum pancreatic Hormone: CCK lipase Proenzymes released: Chymotrypsinogen, procar- Monoglycerides, boxypeptidase, proelastase, fatty acids in trypsinogen. Enteropeptidase micelles activates trypsin, which activates other enzymes. Enzymes released: Pancreatic amylase, pancreatic lipase, nuclease, enteropeptidase. DIFFUSION INTESTINAL Brush border MUCOSA

Monoglycerides, fatty acids

Triglycerides Cell body

Chylomicrons

EXOCYTOSIS

ROUTE TO BLOODSTREAM Lacteal Carbohydrates and amino acids are absorbed and transported by intestinal capillaries. Lipids form Chylomicrons chylomicrons that diffuse into lacteals and are delivered to the left subclavian vein by the thoracic duct. 212 24-8 Chemical Digestion

. Protein digestion and absorption – Complex and time consuming – Involves mechanical digestion, hydrochloric acid, and proteases – Dipeptidases in epithelial surfaces of small intestine • Break short peptide chains into individual amino acids

REGION and Hormonal Controls PROTEINS

ORAL CAVITY

ESOPHAGUS

STOMACH Pepsin Stimulus: Anticipation or arrival of food Hormone: Gastrin Source: G cells of stomach Polypeptides Proenzyme released: Pepsinogen by chief cells, activated to pepsin by HCl

SMALL INTESTINE Trypsin Stimulus: Arrival of chyme Chymotrypsin in duodenum Elastase Hormone: CCK Carboxypeptidase Proenzymes released: Chymotrypsinogen, procarboxypeptidase, proelastase, Short peptides, trypsinogen. Enteropeptidase amino acids activates trypsin, which activates other enzymes. Enzymes released: Pancreatic amylase, pancreatic lipase, nuclease, enteropeptidase. Dipeptidases INTESTINAL Brush border MUCOSA

FACILITATED DIFFUSION AND COTRANSPORT

Amino acids Cell body

FACILITATED DIFFUSION AND COTRANSPORT

ROUTE TO BLOODSTREAM Capillary Carbohydrates and amino acids are absorbed and transported by intestinal capillaries. Lipids form Amino acids chylomicrons that diffuse into lacteals and are delivered to the left subclavian vein by the thoracic duct. 214 24-8 Chemical Digestion

. Nucleic acid digestion and absorption – Nucleic acids are broken down into nucleotides – Brush border enzymes digest nucleotides into sugars, phosphates, and nitrogenous bases • Absorbed by active transport . Water absorption – Cells cannot actively absorb or secrete water – Movement of water across digestive tract involves passive water flow down osmotic gradients

Digestive Dietary Input Secretions

Food and drink Saliva 2200 mL 1500 mL

Gastric secretions 1500 mL

5200 mL

Liver (bile) 1000 mL Pancreas (pancreatic juice) 1000 mL Water Reabsorption Intestinal 9200 mL secretions Small intestine 2000 mL reabsorbs 8000 mL

1200 mL

Colonic mucous Colon reabsorbs secretions 1250 mL 1400 200 mL mL

150 mL lost in feces 216 24-8 Chemical Digestion

. Ion absorption – Osmosis does not distinguish among solutes • Determined by total concentration of solutes – To maintain homeostasis • Concentrations of specific ions must be regulated – Rate of sodium ion uptake • Generally proportional to concentration of Na+ in intestinal contents • Increased by aldosterone

. Ion absorption – Calcium ion absorption • Involves active transport at epithelial surface • Rate is increased by calcitriol – Potassium ions • Increase in concentration as other solutes move out of lumen • Diffuse into epithelial cells along concentration gradient

. Ion absorption – Absorption of magnesium, iron, and other cations • Involves specific carrier proteins • Cell must use ATP to transport ions to interstitial fluid – Anions (chloride, iodide, bicarbonate, and nitrate) • Absorbed by diffusion or carrier-mediated transport – Phosphate and sulfate ions • Enter epithelial cells only by active transport

. Vitamins – Organic compounds required in very small quantities – Water-soluble vitamins • Include B vitamins and vitamin C – Fat-soluble vitamins • Vitamins A, D, E, and K

. Age-related changes – Division rate of epithelial stem cells declines – Smooth muscle tone decreases – Effects of cumulative damage become apparent – Cancer rates increase – Dehydration is common among the elderly – Changes in other systems have direct or indirect effects on the digestive system

. Digestive system – Extensive anatomical and physiological connections to several other systems • Nervous • Cardiovascular • Endocrine • Lymphatic – Digestive tract is an endocrine organ • Produces a variety of hormones – Produces neurotransmitters

• The Integumentary System provides • The Nervous System regulates movement

vitamin D3 needed for the absorption of and secretion with the ANS and ENS; calcium and phosphorus reflexes coordinate passage of materials along tract; control over skeletal muscles • The digestive system provides lipids for regulates ingestion and defecation; storage by adipocytes in hypodermis hypothalamic centers control hunger, satiation, and feeding

Skeletal System • The digestive system provides compounds essential for neurotransmitter synthesis • The Skeletal System (axial division and pelvic girdle) supports and protects Endocrine System parts of digestive tract; teeth are used in mechanical processing of food • The Endocrine System produces epinephrine and norepinephrine that • The digestive system absorbs calcium stimulate constriction of sphincters and and phosphate ions for use in bone depress digestive activity; hormones matrix; provides lipids for storage in coordinate activity along digestive tract yellow marrow • The digestive system provides nutrients Cardiovascular System and substrates to endocrine cells; endocrine cells of pancreas secrete • The Cardiovascular System insulin and glucagon; liver produces distributes hormones of the digestive angiotensinogen tract; carries nutrients, water, and Lymphatic System ions from sites of absorption; delivers nutrients and toxins to liver • The Lymphatic System defends against • The digestive system absorbs fluid infection and toxins absorbed from the to maintain normal blood volume; digestive tract; lymphatic vessels carry absorbs vitamin K; liver excretes absorbed lipids to the general circulation heme (as bilirubin), synthe- sizes blood clotting proteins • The digestive system secretes acids and enzymes that provide innate (nonspecific) immunity against Respiratory System pathogens

• The Respiratory System can assist in defecation by producing increased Digestive System thoracic and abdominal pressure through contraction of respiratory The digestive system provides organic muscles substrates, vitamins, minerals, electrolytes (inorganic ions), and water required by all • The digestive system produces pressure cells. It: with digestive organs against the • ingests solid and liquid materials into the diaphragm that can assist in exhalation oral cavity of the digestive tract and limit inhalation • mechanically digests solid materials to ease their movement and propels them Muscular System down the digestive tract • chemically digests food into smaller • The Muscular System protects and molecules in the digestive tract supports digestive organs in • secretes water, acids, enzymes, and abdominal cavity; controls entrances buffers into the digestive tract and exits of digestive tract • absorbs small organic molecules (organic substrates), ions, vitamins, and water • The digestive system regulates blood across the digestive epithelium glucose and fatty acid levels and • eliminates wastes and indigestible metabolizes lactate from active materials from the body by defecation muscles with the liver 223