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Chapter 23: The Digestive System

Part 1: Overview of the Digestive System 1. Describe the function of the digestive system, and differentiate between organs of the alimentary canal and accessory digestive organs.

Digestive Processes 2. List and define the major processes occurring during digestive system activity.

Basic Functional Concepts 3. Describe stimuli and controls of digestive activity.

Digestive System Organs: Relationships 4. Describe the location and function of the peritoneum. 5. Define retroperitoneal and name the retroperitoneal organs of the digestive system. 6. Define splanchnic circulation. 7. Indicate the importance of the hepatic portal system. 8. Describe the tissue composition and general function of each of the four layers of the alimentary canal.

Part 2: Functional Anatomy of the Digestive System

The Mouth and Associated Organs 9. Describe the gross and microscopic anatomy and the basic functions of the mouth, pharynx, and esophagus. 10. Describe the composition and functions of saliva, and explain how salivation is regulated. 11. Explain the dental formula and differentiate clearly between deciduous and permanent teeth.

Digestive Processes: Mouth to Esophagus 12. Describe the mechanisms of chewing and swallowing.

The Stomach 13. Identify structural modifications of the wall of the stomach that enhance the digestive process. 14. Name the cell types responsible for secreting the various components of gastric juice and indicate the importance of each component in stomach activity. 15. Describe stomach structure and indicate changes in the basic alimentary canal structure that aid its digestive function. 16. Explain how gastric secretion and stomach motility are regulated. 17. Define and account for the alkaline tide.

The Small Intestine and Associated Structures 18. Identify and describe structural modifications of the wall of the small intestine that enhance the digestive process. 19. Differentiate between the roles of the various cell types of the intestinal mucosa. 20. Describe the function of intestinal hormones and paracrines. 21. Describe the histologic anatomy of the liver. 22. State the role of bile in digestion. 23. Describe the role of the gallbladder. 24. State the role of pancreatic juice in digestion. 25. Describe how bile and pancreatic juice secretion into the small intestine are regulated.

The Large Intestine 26. List the major functions of the large intestine. 27. Describe the regulation of defecation.

Part 3: Physiology of Digestion and Absorption 28. List the enzymes involved in digestion; name the foodstuffs on which they act. 29. List the end products of protein, fat, carbohydrate, and nucleic acid digestion. 30. Describe the process by which breakdown products of foodstuffs are absorbed in the small intestine.

Developmental Aspects of the Digestive System 31. Describe embryonic development of the digestive system. 32. Describe abnormalities of the gastrointestinal tract at different stages of life.

Suggested Lecture Outline

Part 1: Overview of the Digestive System (pp. 850–855; Figs. 23.1–23.6) A. Digestive system organs fall into two main groups: the alimentary canal and the accessory organs (pp. 850– 851; Fig. 23.1). 1. The alimentary canal, or gastrointestinal (GI) tract, is the continuous muscular digestive tube that winds through the body digesting (dissolving) and absorbing foodstuffs; its organs include: the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. 2. Accessory digestive organs aid digestion physically and produce secretions that break down foodstuffs in the GI tract; the organs involved are the teeth, tongue, gallbladder, salivary glands, liver, and pancreas. I.Digestive Processes (pp. 851–852; Figs. 23.2–23.3) A. Ingestion is the simple act of putting food into the mouth (p. 851; Fig. 23.2). B. Propulsion moves food through the alimentary canal and includes both swallowing and peristalsis involuntary alternating waves of contractions causing squeezing of contents thru GI (p. 851; Figs. 23.2–23.3). C. Mechanical digestion is the physical process of preparing the food for digestion and involves chewing, mixing, churning, and segmentation rhythmic local constrictions of SI → mixes food with digestive juices (pp. 851–852; Fig. 23.2). Increases surface area of food. D Digestion is a series of catabolic steps in which complex food molecules are broken down to their chemical building blocks (monomers) by enzymes (p. 852; Fig. 23.2). E. Absorption is the passage of digested end products from the lumen of the GI tract through the mucosal cells into the blood or lymph via active or passive transport (p. 852; Fig. 23.2). (includes vitamins, minerals, & water) F. Defecation eliminates indigestible substances from the body via the anus as feces (p. 852; Fig. 23.2). II. Basic Functional Concepts (p. 852; Fig. 23.4) A. The digestive system creates an optimal internal environment for its functioning in the lumen of the GI tract, an area that is technically outside of the body (p. 852; Fig. 23.4). 1. Digestive activities within the GI tract are triggered by mechanical and chemical stimuli. Stretch is the most important; others include osmolality, pH, presents of substance → leads to reflexes that (1) activate or inhibit gland secretions into lumen or hormones into blood (2) stimulate smooth muscle to mix & move 2. Controls of the digestive activity are both extrinsic and intrinsic “in-house” (nervous and hormonal). enteric “gut” plexus = short reflexes; long reflexes involve CNS & extrinsic autonomic nerves (ex. sight, smell, taste, thought of food) III. Digestive System Organs: Relationships (pp. 852–855; Figs. 23.5–23.6) A. Relationship of the Digestive Organs to the Peritoneum (pp. 852–854; Fig. 23.5) 1. The visceral peritoneum covers the external surfaces of most of the digestive organs, and the parietal peritoneum lines the body wall of the abdominopelvic cavity. Slippery serous membranes 2. The peritoneal cavity is located between the visceral and parietal peritoneums and is filled with serous fluid. 3. Mesentery is a double layer of peritoneum that extends to the digestive organs from the body wall. It (1) allows blood vessels, lymphatics, and nerves to reach the digestive organs; (2) holds the organs in place; and (3) stores fat. 4. Retroperitoneal organs are found posterior to the mesentery, lying against the dorsal abdominal wall. Includes: pancreas, duodenum & parts of LI HI: peritonitis B. The splanchnic circulation serves the digestive system and includes those arteries that branch off the abdominal aorta to serve the digestive organs and the hepatic portal circulation (p. 854). Celiac trunk serves spleen, liver & stomach; small & large intestine thru mesenteric arteries; hepatic portal → collects nutrient -rich venous blood from digestive organs to liver. C. Histology of the Alimentary Canal (pp. 854–855; Fig. 23.6) 1. Mucosa (aka. mucous membrane) is the innermost, moist, epithelial membrane that lines the entire digestive tract. It (1) secretes mucus, digestive enzymes, and hormones; (2) absorbs digestive end products into the blood; and (3) protects against infectious disease.  Epithelium - simple columnar with goblet cells; SI & stomach also have enzyme & hormonal cells; esophagus is stratified squamous + goblet cells  Lamina propria - LCT that nourish epithelium & absorb nutrients. MALT - defend against pathogens (large collections in tonsils & appendix.  Muscularis mucousa - smooth muscle tone creates folds that increase surface area. 2. Submucosa is a moderately dense connective tissue layer containing blood and lymphatic vessels, lymphoid follicles, and nerve fibers. abundant elastic fibers enables organs to return to their original shape 3. Muscularis externa typically consists of smooth muscle and is responsible for peristalsis and segmentation. inner circular layer & outer longitudinal layer; sphincters (valves) prevent backflow. 4. Serosa, the protective outer layer of the intraperitoneal organs, is the visceral peritoneum. (mesothelium) **esophagus - adventitia instead; retroperitoneal organs (serous anteriorly & adventitia posteriorly) D. The alimentary canal has its own nerve supply made up of enteric neurons that communicate widely with each other to regulate digestive activity (p. 855). Sympathetic & parasympathetic control of 2 unmyelinated intrinsic plexuses:  Submucosal nerve plexus - occupies submucosa  Myenteric nerve plexus - lies between circular & longitudinal muscle layers of the muscularis

Part 2: Functional Anatomy of the Digestive System (pp. 856–892; Figs. 23.7–23.31; Tables 23.1–23.3) IV. The Mouth and Associated Organs (pp. 856–861; Figs. 23.7–23.11) A. The mouth is a stratified squamous epithelial mucosa-lined cavity with boundaries of the lips, cheeks, palate, and tongue (pp. 856–861; Figs. 23.7–23.11). 1. The lips (labia) and cheeks have a core of skeletal muscle covered externally by skin that helps to keep food between the teeth when we chew. a. The fleshy lips are formed by the orbicularis oris muscle; the cheeks are formed largely by the buccinators. b. The oral vestibule (porch) is the space between the lips and cheeks and gums and teeth. c. The area within the teeth and gums is the oral cavity proper. 2. The palate forms the roof of the mouth and has two parts: a. The anterior hard palate, is formed by the palatine process of the maxilla and the palatine bones, forms a rigid surface that the tongue manipulates food against. b. The posterior soft palate is formed by skeletal muscle (uvula) that rises to close off the nasopharynx during swallowing. 3. The tongue is made of interlacing bundles of skeletal muscle and is used to reposition food when chewing, mix food with saliva (forms bolus) , initiate swallowing, and help form consonants for speech. a. A fold of mucosa, the lingual frenulum, anchors the tongue to the floor of the mouth. HI: ankyloglossia - “tied-tongue” b. Several types of papillae are located on the superior surface of the tongue: filiform papillae that are used to provide friction against foods, fungiform papillae that contain taste buds, and vallate and foliate papillae that house taste buds and secrete lingual lipase. 4. Major and minor salivary glands produce saliva, which (1) cleanses the mouth, (2) dissolves food chemicals for taste, (3) moistens food, (4) contains antibodies and defensins to protect against microorganisms, and is a (5) source of chemicals that begin the breakdown of starches. Saliva contents: amylase, lingual lipase, mucin, lysozymes, IgA, & defensins. 1. Parotid gland (serous) HI: Mumps (myxovirus); submandibular (serous), buccal (equal serous & mucous), sublingual (mucus). 2. Salivation Control: parasympathetic ANS; salivary nuclei (pons & medulla) → CN VII & IX HI: halitosis 5. The teeth tear and grind food, breaking it into smaller pieces. a. The primary dentition, also called deciduous, or baby teeth consists of 20 teeth that are lost to make way for the permanent dentition. b. The permanent dentition consists of 32 teeth, including the wisdom teeth, or third molars. c. Teeth are classified by their shapes and functions: incisors are used for cutting, canines tear or pierce, and premolars & molars are used for grinding. HI: tooth embedded in jawbone = impacted d. Each tooth has two regions: the enamel covered crown extending above the gingiva, or gum, and the root, which is embedded in the jawbone. e. Cement, a calcified connective tissue, anchors the root to the periodontal ligaments, which hold the tooth in the bony socket of the jaw. f. Dentin, a bonelike material, underlies the enamel and surrounds the pulp cavity, containing blood and nerve supply. HI: dental caries, gingivitis, peridontal disease (peridontitis) V. The Pharynx (pp. 861–862) A. The pharynx (oropharynx and laryngopharynx) provides a common passageway for food, fluids, and air (pp. 861–862). Stratified squamous, mucus glands; skeletal muscle - propels food to esophagus VI. The Esophagus (pp. 862–863; Fig. 23.12) A. The esophagus provides a passageway for food and fluids from the laryngopharynx to the stomach where it joins at the cardial orifice (pp. 862–863; Fig. 23.12). Pierces diaphragm thru esophageal hiatus; joins stomach @ cardial orifice which has sphincter. Diaphragm keeps sphincter closed when no food is passing 1. 4 layers - Mucosa: NK stratified squamous plus mucus for protection. Changes to simple columnar @ stomach; Submucosa: ACT + esophageal glands; Muscularis: circular & longitudinal layers, superior ⅓ is skeletal; lower ⅓ is smooth & middle ⅓ is mixed; Adventitia: (no serosa) fibrous CT blends with surrounding tissues. 2. HI: Heartburn, GERD, hiatal hernia VII. Digestive Processes: Mouth to Esophagus (pp. 863–864; Fig. 23.13) A. Mastication, or chewing, begins the mechanical breakdown of food and mixes the food with saliva (p. 863). B. Deglutition, or swallowing, is a complicated process that involves two major phases (p. 863; Fig. 23.13). 1. The buccal phase is voluntary and occurs in the mouth where the bolus is forced into the oropharynx. 2. The pharyngeal-esophageal phase is involuntary and occurs when food is squeezed through the pharynx and into the esophagus. Controlled by swallowing centers of medulla & lower pons via CNX 3. Preventative measures taken to ensure food goes to esophagus: soft palate (uvula) blocks nasopharynx, epiglottis blocks trachea, & upper espohageal sphincter prevents regurgitation. VIII. The Stomach (pp. 864–874; Figs. 23.14–23.20; Tables 23.1–23.2) A. The stomach is a temporary storage tank where the chemical breakdown of proteins is initiated and food is converted to chyme (pp. 864–865; Fig. 23.14). 1. The adult stomach varies from 15–25 cm long; its diameter and volume vary depending on the amount of food it contains. Rugae - large longitudinal folds of mucosa & submucosa a. The major regions of the stomach include the cardial part (cardia), fundus (dome-shaped), body (mid- portion), & pyloric part (continuous with duodenum) has sphincter which controls stomach emptying. b. The convex lateral surface of the stomach is its greater curvature (fatty apron with large amounts of lymph nodes for protection), and its convex medial surface is its lesser curvature. c. Extending from the curvatures are the lesser omentum and the greater omentum, which help to tie the stomach to other digestive organs and the body wall. d. ANS Control: Sympathetic fibers via thoracic splanchnic thru celiac plexus; parasympathetic fibers supplied by CN X. Blood via ciliac trunk; blood returned via hepatic portal system. B. Microscopic Anatomy (pp. 866–869; Figs. 23.15–23.16; Table 23.1) 1. The surface epithelium of the stomach mucosa is a simple columnar epithelium composed of goblet cells (gastric pits + gastric glands), which produce a protective two-layer coat of alkaline mucus. Muscularis externa has additional “oblique” muscle layer to assist in food breakdown. 2. The gastric glands of the stomach produce gastric juice, which may be composed of a combination of mucus (mucous neck cells) , hydrochloric acid (parietal cells) , intrinsic factor (parietal cells) , pepsinogen (chief cells) , and a variety of hormones (enteroendocrine cells - histamine, serotonin, somatostatin & gastrin ). Gastrin is essential for regulating stomach secretions & motility. Refer to p. 868: know CCK, GIP, Gastrin 3. HI: peptic (gastric) ulcers - H. pylori C. Digestive Processes in the Stomach (pp. 869–874; Figs. 23.17–23.20; Table 23.2) involved in all digestion system functions except ingestion & defecation. Protein digestion begins in stomach: HCl denatures proteins → pepsin (renin) in infants; alcohol & aspirin pass easily thru stomach; intrinsic factor is essential for vitamin B12 absorption needed for erythropoiesis (pernicious anemia if deficient) . 1. Gastric secretion is controlled by both neural and hormonal mechanisms and acts in three distinct phases:  Cephalic (Reflex) phase: before food consumption; triggered by aroma, taste, sight &/or thought of food o Sensory input → hypothalamus→ vagal nuclei (medulla) → CNX (parasympathetic motor response)  Gastric phase: food reaches stomach; most important stimuli are distention, peptides & low acidity (high pH). o Distension → stretch receptors → myenteric (short) & vagovagal (long) reflexes = Ach release → stimulates gastric output. o Peptides, caffeine or rising pH → activate G cells (antrum) → increase HCl release (parietal cells); low pH (below 2) inhibits Gastrin release; proteins cause pH to increase (acts as a buffer) = Gastrin to increase HCl = lower pH → inhibits gastrin-secreting cells (negative feedback) o Stimulation of HCl release: (1) ACh (2) Gastrin (3) Histamine o Formation of HCl: https://highered.mheducation.com/sites/9834092339/student_view0/chapter48/hydrochloric_ acid_production____of_the_stomach.html . CO2 (blood) diffuses into parietal cell + H2O + carbonic anhydrase → H2CO3 → HCO3- + H+ . HCO3- / Cl- exchange (bicarbonate ion diffuses into blood while Cl- enters the parietal cell) . H+ are actively transported into the ducts of gastric glands while Cl- diffuses with H+ . H+/ K+ exchange (H+ into duct while K+ into parietal cell) . H+ + Cl- → HCl o Alkaline Tide: HCO3- / Cl- antiporter Cl- moves into lumen as bicarbonate ion  Intestinal phase: 2 components: o Stimulatory & Inhibitory Action: partially digested food (duodenum) contains H+, fats, proteins, etc. → enteric gastrin release → enterogastric reflex (inhibitory effect); short lived process and is used to protect SU from excessive acidity & to decrease volume of chyme. Dumping syndrome - unpleasant feeling due to reduced stomach volume. . Inhibits vagal nucli (medulla) . Inhibits local reflex . activates sympathetic fibers → pyloric sphincter tightening . Release of enterogastones (histamine, secretin, CCK & VIP = gastric secretion inhibition 2. The reflex-mediated relaxation of the stomach muscle (Mediated by Vagus nerve acting on serotonin & NO via swallowing centers of brain stem.) and the plasticity of the visceral smooth muscle allow the stomach to accommodate food and maintain internal pressure. 3. The enteric pacemaker cells establish the stomach’s basic electrical rhythm of peristaltic waves. 4. The rate at which the stomach empties is determined by both the contents of the stomach and the processing that is occurring in the small intestine. IX. The Small Intestine and Associated Structures (pp. 874–887; Figs. 23.21–23.28; Tables 23.2–23.3) A. The small intestine is the site of the completion of digestion and absorption of nutrients (pp. 875–878; Figs. 23.21–23.23). 1. It extends from the pyloric sphincter to the ileocecal valve where it joins the large intestine. It has three subdivisions: the duodenum, the jejunum, and the ileum. 2. It is highly adapted for absorption with three microscopic modifications: circular folds, villi, and microvilli. 3. The intestinal crypts, secrete intestinal juice that serves as a carrier fluid for absorbing nutrients from chyme. B. The liver and gallbladder are accessory organs associated with the small intestine (pp. 878–882; Figs. 23.24– 23.25). 1. The liver is the largest gland in the body and has four lobes. 2. The liver is composed of liver lobules, which are made of plates of liver cells (hepatocytes). 3. The digestive function of the liver is to produce bile, which is a fat emulsifier. 4. Bile is a yellow-green, alkaline solution containing bile salts, bile pigments (primarily bilirubin), cholesterol, neutral fats, phospholipids, and a variety of electrolytes. 5. The gallbladder stores and concentrates bile that is not needed immediately for digestion. 6. Bile does not usually enter the small intestine until the gallbladder contracts when stimulated by cholecystokinin. C. The pancreas is an accessory gland that is retroperitoneal (pp. 882–884; Figs. 23.26–23.28). 1. Pancreatic juice consists mainly of water and contains enzymes that break down all categories of foodstuffs and electrolytes. 2. Secretion of pancreatic juice is regulated by local hormones and the parasympathetic nervous system. D. Digestive Processes in the Small Intestine (pp. 884–887; Tables 23.2–23.3) 1. Food takes 3–6 hours to complete its digestive path through the small intestine, the site of virtually all nutrient absorption. 2. Most substances required for digestion within the small intestine are imported from the pancreas and the liver. 3. Optimal digestive activity in the small intestine depends on a slow, measured delivery of chyme from the stomach. 4. Segmentation is the most common motion of the small intestine. X. The Large Intestine (pp. 887–892; Figs. 23.29–23.31; Table 23.2) A. The large intestine absorbs water from indigestible food residues and eliminates the latter as feces (pp. 887– 890; Figs. 23.29–23.30). 1. The large intestine exhibits three unique features: teniae coli, haustra, and epiploic appendages, and has the following subdivisions: cecum, appendix, colon, rectum, and anal canal. 2. The mucosa of the large intestine is thick and has crypts with a large number of mucus-producing goblet cells. 3. Bacteria entering the colon via the small intestine and anus colonize the colon and ferment some of the indigestible carbohydrates. B. Digestive Processes in the Large Intestine (pp. 890–892; Fig. 23.31) 1. The movements seen in the large intestine include haustral contractions and mass movements. 2. Feces forced into the rectum by mass movements stretch the rectal wall and initiate the defecation reflex.

Part 3: Physiology of Digestion and Absorption (pp. 892–901; Figs. 23.32–23.36) XI. Digestion (pp. 892–895; Figs. 23.32–23.34) A. Digestion is a catabolic process in which large food molecules are broken down to chemical building blocks (monomers), which are small enough to be absorbed by the GI tract lining (pp. 892–895; Figs. 23.32–23.24). 1. Digestion is accomplished by enzymes, secreted by intrinsic and accessory glands of the alimentary canal, used in hydrolysis reactions. 2. Carbohydrates a. Monosaccharides are simple sugars that are absorbed immediately (glucose, galactose, and fructose). b. Disaccharides are composed of two monosaccharides bonded together (maltose, lactose, and sucrose). c. The digestible polysaccharide found in the diet is starch; other polysaccharides, such as cellulose, are not able to be broken down by humans. d. Digestion of carbohydrates begins in the mouth, where salivary amylase breaks large polysaccharides into smaller fragments. 3. Proteins digested into amino acids in the GI tract include not only dietary proteins but also enzyme proteins secreted into the GI tract lumen. a. Pepsin, secreted by the chief cells, begins the digestion of proteins in the stomach. b. Rennin is produced in infants and breaks down milk proteins. c. Pancreatic enzymes, such as trypsin and chymotrypsin, further break down proteins in the small intestine. d. The brush border enzymes carboxypeptidase, aminopeptidase, and dipeptidase work on freeing single amino acids in the small intestine. 4. The small intestine is the primary site for lipid digestion. a. Lipases are secreted by the pancreas and are the enzymes that digest fats after they have been pretreated with bile. 5. Nucleic acids (both DNA and RNA) are hydrolyzed to their nucleotide monomers by pancreatic nucleases present in pancreatic juice. XII. Absorption (pp. 895–898; Fig. 23.35) A. Absorption occurs along the entire length of the small intestine, and most of it is completed before the chyme reaches the ileum (pp. 895–898; Fig. 23.35). 1. Absorption of Specific Nutrients a. Glucose and galactose are transported into the epithelial cells by common protein carriers and are then moved by facilitated diffusion into the capillary blood. b. Several types of carriers transport the different amino acids before entering the capillary blood by diffusion. c. Monoglycerides and free fatty acids of lipid digestion become associated with bile salts and lecithin to form micelles, which are necessary for lipid absorption. d. Pentose sugars, nitrogenous bases, and phosphate ions are transported actively across the epithelium by special transport carriers in the villus epithelium. e. The small intestine absorbs dietary vitamins, while the large intestine absorbs vitamins B and K. f. Electrolytes are actively absorbed along the entire length of the small intestine, except for calcium and iron, which are absorbed in the duodenum. g. Water is the most abundant substance in chyme and 95% of it is absorbed in the small intestine by osmosis. 2. Malabsorption of nutrients can result from anything that interferes with the delivery of bile or pancreatic juices, as well as factors that damage the intestinal mucosa. XIII. Developmental Aspects of the Digestive System (pp. 898–901; Fig. 23.36) A. Embryonic Development (pp. 898–899; Fig. 23.36) 1. The epithelial lining of the developing alimentary canal forms from the endoderm with the rest of the wall arising from the mesoderm. 2. The anteriormost endoderm touches the depressed area of the surface ectoderm where the membranes fuse to form the oral membrane and ultimately the mouth. 3. The end of the hindgut fuses with an ectodermal depression, called the proctodeum, to form the cloacal membrane and ultimately the anus. 4. By week 5, the alimentary canal is a continuous tube stretching from the mouth to the anus. B. Aging (pp. 899, 901) 1. GI tract motility declines, digestive juice production decreases, absorption is less efficient, and peristalsis slows, resulting in less frequent bowel movements and, often, constipation. 2. Diverticulosis, fecal incontinence, and cancer of the GI tract are fairly common problems in the elderly.