Functions of the GI Tract. Chapter 18. Functions of the GI Tract (continued)

Transcription

1 Functions of the GI Tract Chapter 18 The Digestive System Motility: Movement of of food through the GI tract. Ingestion: Taking food into the mouth. Mastication: Chewing the food and mixing it with saliva. Deglutition: Swallowing the food. Rhythmic wave-like contractions that move food through GI tract. Functions of the GI Tract (continued) Secretion: Includes both exocrine and endocrine secretions. Exocrine: HCl, H 2 0, HC0 3-, bile, lipase, pepsin, amylase, trypsin, elastase, and histamine are secreted into the lumen of the GI tract. Endocrine: and small intestine secrete hormones to help regulate the GI system. Gastrin, secretin, CCK, GIP, GLP-1, guanylin, VIP, and somatostatin. Functions of the GI Tract (continued) Digestion: Breakdown of food particles into subunits (chemical structure change). Absorption: Process of the passage of digestion (chemical subunits) into the blood or lymph. Storage and elimination: Temporary storage and elimination of indigestible food. Digestive System (GI) GI tract divided into: Alimentary canal. Accessory digestive organs. GI tract is 30 long and extends from mouth to anus. Layers of GI Tract Composed of 4 tunics: Mucosa. Muscularis. Submucosa. Serosa. 1

2 From Mouth to Mastication (chewing): Mixes food with saliva which contains salivary amylase. Enzyme that can catalyze the partial digestion of starch. Deglutition (swallowing): Begins as a voluntary activity. Involves 3 phases: Oral phase is voluntary. Pharyngeal and esophageal phases are involuntary. Cannot be stopped. Larynx is raised. Epiglottis covers the entrance to respiratory tract. Esophagus Produced by a series of localized reflexes in response to distention of wall by bolus. Wave-like muscular contractions: Circular smooth muscle contract behind, relaxes in front of the bolus. Followed by longitudinal contraction (shortening) of smooth muscle. Rate of 2-4 cm/sec. Insert 18.4a Most distensible part of GI tract. Functions of the stomach: Stores food. Initiates digestion of proteins. Kills bacteria. Moves food (chyme) into intestine. (continued) Contractions of the stomach churn chyme. Mix chyme with gastric secretions. Push food into intestine. Insert fig Gastric Glands Secrete gastric juice: Goblet cells: mucus. Parietal cells: HCl and intrinsic factor (B12). Chief cells: pepsinogen. G cells: gastrin. Several others Digestion and Absorption in the Proteins partially digested by pepsin. Carbohydrate digestion by salivary amylase is soon inactivated by acidity. Alcohol and aspirin are the only commonly ingested substances absorbed. 2

3 Protective Mechanisms of Parietal and chief cells impermeable to HCl. Alkaline mucus contains HC0 3-. Tight junctions between adjacent epithelial cells. Rapid rate of cell division (entire epithelium replaced in 3 days). Small Intestine Each villus is a fold in the mucosa. Covered with columnar epithelial cells interspersed with goblet cells. Lamina propria contain lymphocytes, capillaries, and central lacteal. Insert fig Absorption in Small Intestine Intestinal Contractions and Motility Majority of absorption (90%) takes place here Duodenum and jejunum: Carbohydrates, amino acids, lipids, iron, and Ca 2+. Ileum: Bile salts, vitamin B 12, electrolytes, and H major types of contractions occur in the small intestine: Slow movement. Pressure at the pyloric end of small intestine is greater than at the distal end. Segmentation: Major contractile activity of the small intestine. Contraction of circular smooth muscle. Mix chyme. Insert fig Large Intestine Outer surface bulges outward to form haustra. Little absorptive function. Absorbs H 2 0, electrolytes, several vitamin B complexes, vitamin K, and folic acid. Intestinal microbiota (bacteria) produce significant amounts of folic acid and vitamin K. Bacteria ferment indigestible molecules to produce short-chain fatty acids. Does not contain villi. Defecation Waste material passes to the rectum. Occurs when rectal pressure rises and external anal sphincter relaxes. Defecation reflex: Longitudinal rectal muscles contract to increase rectal pressure. Relaxation of internal anal sphincter. Excretion is aided by contractions of abdominal and pelvic skeletal muscles. Push feces from the rectum. 3

4 Major Categories of Liver Function Accessory Organs of Digestion Bile Production and Secretion Gallbladder The liver produces and secretes ml of bile/day. Bile pigment (bilirubin) is produced in spleen, bone marrow, and liver. Derivative of the heme groups (without iron) from hemoglobin. Converted by bacteria in intestine to urobilinogen. Urobilogen is absorbed by intestine and enters the hepatic vein. Recycled, or filtered by kidneys and excreted in urine. Sac-like organ attached to the liver. Stores and concentrates bile. Contraction of the gallbladder ejects bile into the common bile duct into duodenum. Pancreas Pancreatic Juice Exocrine: Acini: Secrete pancreatic juice. Endocrine: Islets of Langerhans: Secrete insulin and glucagon. Insert fig Contains H 2 0, HC0 3- and digestive enzymes. 4

5 Regulation of Gastric Function Gastric motility and secretion are automatic. Extrinsic control of gastric function is divided into 3 phases: Cephalic phase. Gastric phase. Intestinal phase. Cephalic Phase Stimulated by sight, smell, and taste of food. Activation of vagus: Stimulates chief cells to secrete pepsinogen. Directly stimulates G cells to secrete gastrin. Directly stimulates ECL cells to secrete histamine. Indirectly stimulates parietal cells to secrete HCl. Continues into the 1 st 30 min. of a meal. Gastric Phase Arrival of food in stomach stimulates the gastric phase. Gastric secretion stimulated by: Distension. Chemical nature of chyme (amino acids and short polypeptides). Stimulates G cells to secrete gastrin. Stimulates chief cells to secrete pepsinogen. Stimulates ECL cells to secrete histamine. Histamine stimulates secretin of HCl. Intestinal Phase Inhibits gastric activity when chyme enters the small intestine. Arrival of chyme increases osmolality and distension. Activates sensory neurons of vagus and produces an inhibitory neural reflex: Inhibits gastric motility and secretion. In the presence of fat, enterogasterone inhibits gastric motility and secretion. Hormone secretion: Inhibit gastric activity: Somatostatin, CCK, and GLP-1. Intestinal Reflexes Gastroileal reflex: Increased gastric activity causes increased motility of ileum and movement of chyme through ileocecal sphincter. Ileogastric reflex: Distension of ileum, decreases gastric motility. Intestino-intestinal reflex: Overdistension in 1 segment, causes relaxation throughout the rest of intestine. Secretion of Pancreatic Juice Secretion of pancreatic juice and bile is stimulated by: Secretin: Occurs in response to duodenal ph < 4.5. Stimulates production of HC0 3- by pancreas. Stimulates the liver to secrete HC0 3- into the bile. CCK: Occurs in response to fat and protein content of chyme in duodenum. Stimulates the production of pancreatic enzymes. Enhances secretin. Stimulates contraction of the sphincter of Oddi. 5