Lesson # 16The 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.

Functions of Stomach 1- Storage of ingested food. 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 B 12 in small intestine. 2- To produce the chyme by:

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

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

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

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. Functions of Hydrochloric Acid 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.

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

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

Vagus nerve 1 Cephalic Phase Parasympathetic division CNS 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: 1- The Cephalic Phase: The stomach is being controlled by brain. 2- The Gastric Phase: The stomach is controlling itself. 3- The Intestinal Phase: The stomach is being controlled by the small intestine.+ Stimuli: Vagus nerve stimulates gastric secretion even before food is swallowed. Sight, smell, taste, or thought of food The Cephalic Phase is directed by the CNS and prepares the stomach to receive food. Mucous cells Chief cells Parietal cells G cells MucusPepsinogenHClGastrin ACh

CEPHALIC PHASE Central nervous system Food Vagus nerve (N X) Sight, smell, taste, or thoughts of food Gastrin Mucous cells Chief cells Parietal cells G cells Mucus Pepsinogen HCl Stimulation KEY Submucosal plexus

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

GASTRIC PHASE Submucosal and myenteric plexuses Stretch receptors Chemoreceptors Gastrin Mucous cells Chief cells Parietal cells G cells Mucus Pepsinogen HCl Distension Elevated pH Mixing waves Partly digested peptides via bloodstream Neural Response

3 Intestinal Phase It begins when chyme first enters the duodenum. Vagus nerve 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.+ Stretch receptors and chemoreceptors in the duodenum trigger the Enterogastric Reflex. Sympathetic nerve 0 The medulla oblongata inhibits vagal nuclei, reducing vagal stimulation of the stomach. The medulla oblongata stimulates sympathetic neurons that send inhibitory signals to the stomach. - Medulla oblongata Stimuli Distention of the duodenum by the chyme. Decrease in the pH of the duodenum by the chyme. Responses: 1- Neural Response Mucous cells Chief cells Parietal cells G cells MucusPepsinogenHClGastrin X X X X 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.

Vagus nerve Sympathetic nerve 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. - 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 Medulla oblongata 2- Hormonal Responses 3 Intestinal Phase Lipids & Carbohydrates pH 2- A drop in pH below 4.5 stimulates the secretion of the hormone Secretin by the duodenal enteroendocrine cells: Mucous cells Chief cells Parietal cells G cells MucusPepsinogenHCl X X CCK & GIP Secretin Secretin: It inhibits parietal and chief cells. It stimulates the pancreas to produce bicarbonate to neutralize the acid in the chyme. Pancreas Bicarbonate 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 Neutralizes the pH

INTESTINAL PHASE Inhibition KEY Decreased pH Presence of lipids and carbohydrates Peristalsis Secretin GIP CCK Duodenal stretch and chemoreceptors via bloodstream Enterogastric reflex Myenteric plexus Chief cells Parietal cells Neural Responses

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 ) Functions: 1- It is where most chemical digestion takes place. 2- To absorb 99% of the digested nutrients. It receives the pancreas and liver secretions that mix with the chyme. It is where most chemical digestion and absorption take place. It is where digestion is completed. It ends at the ileocecal valve, which control the passage of undigestive materials to the large intestine. Jejunum (8.2 ft ) Duodenum (10 inches) Ileum (11.48 ft ) Ileocec al valve Appendix Cecum

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

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. Accessory pancreatic duct Head Body Tail It is both an endocrine and exocrine gland. The endocrine portion consists of the pancreatic islets that secrete insulin and glucagon. 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. Pancreatic duct Major duodenal papillae Minor duodenal papillae Common bile duct Duodenum Lobules