Lesson # 16

1. Lesson # 16 The Digestive System 2 Chapter 24 Objectives: 1- To describe the macroscopic and microscopic anatomy of the stomach. 2- To describe the function of the stomach. 3- To discuss the significance of the low pH in the stomach. 4- To discuss the regulation of the gastric activity. 5- To describe the macroscopic and microscopic anatomy of the intestines. 6- To describe the macroscopic and microscopic anatomy of the pancreas.

2. Functions of Stomach • 1- Storage of ingested food. • 2- To produce the chyme by: • Mechanical breakdown of ingested food. • Disruption of chemical bonds in food material by acid and enzymes. • 3- Production of intrinsic factor, a glycoprotein required for absorption of vitamin B12 in small intestine.

3. Anatomy of the Stomach Fundus Cardia Serose Oblique layer Longitudinal layer Circular layer Mucosa Muscularis Externa: Pylorus: Lesser curvature Body Antrum Pyloric canal Pyloric sphincter It regulates the release of chyme to the duodenum. Greater curvature They let the gastric lumen expand. Duodenum Rugae

4. Simple columnar epithelium Mucosa Gastric pit Lamina propria Muscularis mucosae Submucosa Muscularis externa Oblique muscle Circular muscle Longitudinal muscle Serosa (visceral peritoneum) Histology of the Stomach It secrets alkaline protective mucus

5. Gastric pit Parietal cells Chief cells Gastric gland G cells Gastric Glands Mucous cells They produce alkaline protective mucus. They produce HCl and Intrinsic Factor. They produce Pepsinogen (In new born and infants produce rennin and gastric lipase). They are enteroendocrine cells that produce Gastrin, Somatostatin, Histamine and Serotonin.

7. Alkaline Tide: It is the increase in the blood pH produced by the influx of bicarbonate ions from the interstitial fluid to the blood stream, during the formation of HCl acid in the stomach. Functions of Hydrochloric Acid • 1- The acidity of gastric juice kills most of the micro-organisms ingested with food • 2- The acidity denatures proteins and inactivates most of the enzymes in food. • 3- The acidity helps break down plant cell walls in food and the connective tissue in meat.. • 4- An acidic environment is essential for the activation and function of pepsin, a protein-digesting enzyme secreted by chief cells.

8. The Production and Action of Pepsin Dietary proteins Parietal cells Pepsin (active enzyme) HCl Partially digested proteins Pepsinogen (zymogen) Zymogens: They are digestive enzymes secreted as inactive proteins. They are converted to active enzymes by removing some of their amino acids. HCl Pepsinogen: It is a zymogen secreted by the chief cells. Hydrochloric acid removes some of its amino acids and forms pepsin that digests proteins. Chief cells Pepsin digests dietary proteins into shorter peptide chains. Protein digestion is completed in the small intestine. Removed peptide

9. Regulation of Gastric Activity The nervous and endocrine systems gastric activity. Parasympathetic Division: Sympathetic Division: It increases secretions and motility in the digestive system. It decreases secretions and motility in the digestive system. The fibers of the parasympathetic division release acetylcholine (Ach). They are called cholinergic fibers. The fibers of the sympathetic division release epinephrine (adrenaline). They are called adrenergic fibers. CNS CNS _ + Parasympathetic division Sympathetic division Mucus Mucus Pepsinogen Pepsinogen ACh Adrenaline HCl HCl Gastrin Gastrin X Mucous cells Mucous cells X Chief cells Chief cells X Parietal cells Parietal cells X G cells G cells

10. Regulation of Gastric Activity The nervous and endocrine systems collaborate to increase gastric secretion and motility when food is eaten and to suppress them when the stomach empties. Gastric activity is divided into three phases: • Stimuli: Sight, smell, taste, or thought of food CNS • 1- The Cephalic Phase: The stomach is being controlled by brain. Vagus nerve • 2- The Gastric Phase: The stomach is controlling itself. Parasympathetic division 1 ACh • 3- The Intestinal Phase: The stomach is being controlled by the small intestine. + Mucous cells Mucus Chief cells Pepsinogen Parietal cells HCl G cells Gastrin Cephalic Phase • Vagus nerve stimulates gastric secretion even before food is swallowed. • The Cephalic Phase is directed by the CNS and prepares the stomach to receive food.

11. CEPHALIC PHASE Food Sight, smell, taste,or thoughts of food Central nervous system Vagus nerve (N X) Submucosal plexus Mucouscells Mucus Chiefcells Pepsinogen Parietalcells HCl Gastrin G cells KEY Stimulation

12. It begins with arrival of food in the stomach. • Stimuli Ingested food produces: • Distention of the stomach. Sensory fibers An increase in the pH of the gastric content. Hypothalamus Motor fibers Presence of undigestive materials in the stomach. Vagus nerve Responses: Parasympathetic division 1- Neural Response Gastrin Long reflex + + + 2- Hormonal Response pH ACh • Gastrin – a hormone produced by the enteroendocrine G cells of the pyloric antrum. Long reflex ACh 3- Local Response 2 • Stretching activates short reflex mediated through myenteric nerve plexus. Gastric Phase Short reflex Short reflex

13. GASTRIC PHASE Neural Response Stretchreceptors Submucosal andmyenteric plexuses Distension Elevated pH Chemoreceptors Mucouscells viabloodstream Mucus Chiefcells Pepsinogen Mixingwaves Parietalcells Gastrin HCl G cells Partly digested peptides

14. It begins when chyme first enters the duodenum. The function of the intestinal phase is controlling the rate of gastric emptying to ensure the secretory, digestive and absorptive functions of the small intestine. • Stimuli Distention of the duodenum by the chyme. Decrease in the pH of the duodenum by the chyme. Medulla oblongata Responses: Vagus nerve 1- Neural Response Stretch receptors and chemoreceptors in the duodenum trigger the Enterogastric Reflex. Sympathetic nerve • The medulla oblongata inhibits vagal nuclei, reducing vagal stimulation of the stomach. - 0 + • The medulla oblongata stimulates sympathetic neurons that send inhibitory signals to the stomach. Mucous cells Mucus Chief cells Pepsinogen The net result is that immediately after the chyme enters the duodenum, gastric contractions decrease, and further discharge of chyme is prevented, giving the duodenum time to neutralize and digest the acidic chyme. Parietal cells HCl X 3 Gastrin G cells Intestinal Phase X X X

15. 2- Hormonal Responses 1- The arrival of lipids and carbohydrates stimulates the duodenal enteroendocrine cells to release cholecystokinin (CCK) and Gastric Inhibitory Peptide (GIP), which inhibit gastric secretion. Medulla oblongata • As a result, a meal high in fats stays in the stomach longer to allow more time for lipids to be digested and absorbed in the small intestine Vagus nerve 2- A drop in pH below 4.5 stimulates the secretion of the hormone Secretin by the duodenal enteroendocrine cells: Sympathetic nerve CCK & GIP Secretin: It inhibits parietal and chief cells. - Mucous cells It stimulates the pancreas to produce bicarbonate to neutralize the acid in the chyme. Chief cells Mucus Parietal cells Pepsinogen G cells HCl • The pyloric sphincter contracts tightly to limit chyme entering duodenum in order to give duodenum time to work on chyme. The duodenum receives secretions from liver and pancreas 3 Intestinal Phase pH X Secretin Pancreas X Neutralizes the pH Bicarbonate Lipids & Carbohydrates

16. INTESTINAL PHASE Neural Responses Enterogastricreflex Myentericplexus Chiefcells via bloodstream Parietalcells Duodenalstretch andchemoreceptors Peristalsis Presence of lipids and carbohydrates CCK GIP KEY Inhibition Secretin Decreased pH

17. The Small Intestine It is the major digestive and absorptive organ of the body. It extends from the pyloric sphincter to the ileocecal valve (19.7 ft ) Duodenum (10 inches) It receives the pancreas and liver secretions that mix with the chyme. Jejunum (8.2 ft ) It is where most chemical digestion and absorption take place. Ileum (11.48 ft ) It is where digestion is completed. It ends at the ileocecal valve, which control the passage of undigestive materials to the large intestine. Cecum Ileocecal valve Appendix Functions: 1- It is where most chemical digestion takes place. 2- To absorb 99% of the digested nutrients.

18. Histology of the Small Intestine They increase the surface area for absorption. They increase the surface area for absorption. They slow movement to allow time for absorption. They make chyme spiral through the lumen to mix with intestinal juice . Plica circulares Villi Columnar epithelial cells with microvilli Villus Mucous cell Lacteal Intestinal crypt Capillary network Mucosa Highly folded lining where absorption occurs. Nerve Muscularis mucosae Submucosa Connective tissue containing blood vessels and nerves. Muscularis externa Circular and longitudinal muscle layers used for peristalsis Lymphatic vessel Venule Arteriole Serosa

19. Ileocecal valve The Large Intestine Transverse colon Supported by the transverse mesocolon. Haustra Ileum Cecum Taenia coli Ascending colon Functions: Descending colon 1- Reabsorb water. Supported by the sigmoid mesocolon. 2- Compact undigested food stuff into feces. Sigmoid colon Anus 3- Eliminate feces. Rectum Appendix 4- Absorb bacterial vitamins (vitamin K, biotin, and vitamin B5). 5- Store fecal matter until defecation.

20. The Pancreas • It is a spongy retroperitoneal gland posterior to the greater curvature of the stomach. • The head of the pancreas is encircled by the duodenum. • It is both an endocrine and exocrine gland. • The endocrine portion consists of the pancreatic islets that secrete insulin and glucagon. Pancreatic duct Lobules • The exocrine portion consists of the pancreatic acini, it is about 99% of pancreas and secretes 1200 to 1500 mL of pancreatic juice per day. Accessory pancreatic duct Body Tail Common bile duct Major duodenal papillae Minor duodenal papillae Head Duodenum

21. Endocrine pancreas Exocrine pancreas Islets of Langerhans Pancreatic acini • Acinar cells and epithelial cells of duct system secrete pancreatic juice. 1- Beta cells: Insulin 2- Alpha cells: Glucagon • They secret 1000 mL pancreatic juice per day, controlled by hormones from duodenum. 3- Delta cells: Somatostatin 4- F cells: Pancreatic polypeptide • The pancreatic juice contains the pancreatic enzymes: • Pancreatic alpha-amylase: It is a carbohydrase that breaks down starches. • It is similar to salivary amylase. • Pancreatic lipase: It breaks down complex lipids and releases products (e.g., fatty acids) that are easily absorbed. • Nucleases: They break down nucleic acids. • Proteolytic enzymes: They break certain proteins apart. • Proteases: They break large protein complexes. • Peptidases: Theybreak small peptides into amino acids. • They are the 70% of all pancreatic enzyme production, are secreted as inactive proenzymes and are activated after reaching small intestine.