BIO 313 Details of Digestive System

Functions of Large Intestine

Absorb some of remaining H2O and salt. Vitamin K and some B vitamins produced by bacteria in colon. Store and concentrate remaining residues until elimination. Capacity to absorb water is limited.

Role of HCl

Activates pepsinogen. Provides optimal pH for pepsin activity. Kills ingested bacteria.

Role of Pepsingogen

After conversion to pepsin, begins protein digestion

How Intestinal Phase Inhibits Gastric Secretion

As chyme leaves the stomach, a major stimulus for gastric secretion-protein-is removed. When pH of chyme drops below 2, release of gastrin is inhibited thus less stimulation of gastric secretions. The "enterogastric reflex" is initiated by the presence of chyme in the duodenum. Presence of chyme in duodenum stimulates release of intestinal hormones (CCK, secretin, VIP) that inhibit gastric secretion.

Emulsion of Fat

Bile salts increase the surface area of fat droplets for pancreatic lipase.

Pancreatic Amylase

Breaks down polysaccharides into disaccharides and monosaccharides

Micelles

Carry the water insoluble products of fat digestion (and lipid soluble vitamins) to their site of absorption. Composed of bile salts, cholesterol, fatty acids, monoglycerides and fat soluable vitamins.

Control of Gastric Secretions

Cephalic Phase: stimulates gastric secretion Gastric Phase: stimulates gastric secretion Intestinal Phase: inhibits gastric secretion

Surface Area of Small Intestine

Circular folds increase membrane surface area. Villi on circular folds further increase surface area. Lumenal Membrane has microvilli called brush border.

Function of Bile

Composed of H2O, HCO3, cholesterol, bilirubin, & bile salts. Bile salts aid in the digestion of fat by emulsifying fats. Bile salts aid in the absorption of the digested fats -> fatty acids by participating in the formation of micelles. Bile salt consists of lipid soluble portion that dissolves in fat droplet.

What Increases Strength of Mixing in the Stomach

Distension of stomach wall by food activates short and long reflexes to stimulate motility. Gastrin is released which stimulates motility.

Pacreatic Exocrine Secretions

Duct cells secrete aqueaous NaHCO3 solution. Acinar Cells secrete digestive enzymes.

Pancreatic Juice Secretion

During cephalic and gastric phases, stimulation ny vagal nerve fibers causes release of pancreatic juices. Acidic chyme entering the duodenum causes enterendocrine cells of duodenal wall to release secretin, whereas fatty, protein rich chyme induces release of CCK. The hormones enter the blood stream and reach pancreas, inducing release of pancreatic juices.

Gastric Secretion

Gastric juice is produced by gastric glands in mucosa of stomach. Gastrin is released by G cells into the capillaries which comes back to the stomach and stimulates more secretion.

Pancreatic Lipase

Hydrolyzes triglycerides (fat) into monoglycerides and free fatty acids

Liver & Gall Bladder in Digestion

Liver function most important to digestion is the production and secretion of bile. The components of bile are digestive enzymes. Between meals bile diverted to and stored in the gall bladder.

Function of the Liver

Metabolism of CHO, fat, protein, glycogenesis, lipogenesis, formation of urea. Cholesterol metabolism. Storage for Vit A, B, D, and Fe. Transfroms & excretes many hormones, drugs, toxins. Synthesis of most plasma proteins. Clears bilirubin from blood.

Motility of Small Intestine

Mix chyme with secretions. Bring chyme into contact with absorptive surfaces. Propel chyme toward colon (rate of propulsion slow to allow time for digestion & absorption)

Gastric Ulcer Formation

Occurs when barrier is broken.

Digestion & Absorption of Fats

Pancreatic lipase breaks down ingested triglycerides into fatty acids and monoglycerides. Fatty acids and monoglycerides are not water soluble thus would float to surface and be difficult to absorb. They interact with bile salts, cholesterol, and fat soluble vitamins to form "micelles".

Pancreatic Proteolytic Enzymes

Proteases; break down protein and polypeptides into amino acids and smaller peptide chains. Don't want to turn on protein digesting enzyme until in the small intestine otherwise the enzyme would breakdown the pancreas itself.

Gastric Mucosal Barrier

Protects against damage from HCl and pepsin. A mucus coating rich in HCO3-.

Alkaline Solution

Rich in NaHCO3 which neutralizes acidic chyme. Necessary to (1) prevent damage by acid to intestinal mucosa and (2) pancreatic digestive enzymes work best in neutral or alkaline environment

G Cells

Secrete Gastrin (a hormone)

Parietal Cells

Secrete HCl and intrinsic factor

Cheif Cells

Secrete Pepsinogen (inactive protein)

Mucous Cells

Secrete mucus and HCO3- (coat the inside of the stomach)

Chemical Digestion in Stomach

Some protein digestion by pepsin in the antrum where mixing is the greatest. In fundus and body where little mixing occurs, CHO digestion continues under influence of salivary amylase. Some drugs and alcohol is absorbed in the stomach.

Major Functions of Stomach

Store ingested food. Secrete HCl and pepsinogen (inactive protein). Mixing and mechanical breakdown of ingested food to produce chyme. Secretes intrinsic factor. Limited absorption.

Physiology of Small Intestine

Very acidic; CHO and protein paritally digested; NO fat digestion yet; essentially no absorption. When chyme enters duodenum, it is immediately mixed with secretions from: pancreas, liver (by way of gall bladder) , duodenal glands.

Control of Bile Secretion

(1) Acidic, fatty chyme entering duodenum causes release of CCK and secretin from duodenal wall enteroendocrine cells. (2) CCK and secretin enter the bloodstream (3) Bile salts and secretin transported via bloodstream stimulate liver to produce bile more rapidly. (4) Vagal stimulation causes weak contraction of gall bladder (5) CCK causes gall bladder to contract and Sphincter of Oddi to relax, bile enters duodenum