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.

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.

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

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

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

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. 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.

Partially digested proteins The Production and Action of Pepsin 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.

Regulation of Gastric Activity Parasympathetic division 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. Mucous cells Chief cells Parietal cells G cells Mucus Pepsinogen HCl Gastrin CNS CNS Parasympathetic division Sympathetic division _ + ACh Adrenaline X Mucous cells Mucus Chief cells Pepsinogen Parietal cells HCl Gastrin G cells

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 + ACh 3- The Intestinal Phase: The stomach is being controlled by the small intestine. Mucous cells Mucus Chief cells Pepsinogen Parietal cells HCl 1 Cephalic Phase Gastrin G cells 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.

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

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

GASTRIC PHASE Neural Response Stretch receptors Submucosal and myenteric plexuses Distension Elevated pH Chemoreceptors via bloodstream Mucous cells Mucus Chief cells Pepsinogen Mixing waves Parietal cells Gastrin HCl G cells Partly digested peptides 13

- X Intestinal Phase Stimuli Responses: 3 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: 1- Neural Response Vagus nerve 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. + The medulla oblongata stimulates sympathetic neurons that send inhibitory signals to the stomach. X 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 Gastrin G cells

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

INTESTINAL PHASE Neural Responses KEY Enterogastric reflex Myenteric plexus via bloodstream Chief cells Parietal cells Duodenal stretch and chemoreceptors Peristalsis CCK Presence of lipids and carbohydrates GIP KEY Secretin Inhibition Decreased pH 16

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. Ileocecal valve Cecum Appendix Functions: 1- It is where most chemical digestion takes place. 2- To absorb 99% of the digested nutrients. 17

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

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

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. 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 Pancreatic duct Lobules Common bile duct Body Tail Minor duodenal papillae Major duodenal papillae Head Duodenum

Endocrine pancreas Exocrine pancreas Islets of Langerhans Pancreatic acini Acinar cells and epithelial cells of duct system secrete pancreatic juice. 1- Beta cells: 2- Alpha cells: 3- Delta cells: 4- F cells: Insulin Glucagon Somatostatin Pancreatic polypeptide They secret 1000 mL pancreatic juice per day, controlled by hormones from duodenum. 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: They break 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.