Chapter 6 The Digestive System The Excretory System.pdf

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University of Florida
Biology - Biological Sciences
BSC 3096

Chapter 6 The Digestive System; The Excretory System Thursday, July 11, 2013 7:14 PM 1. 2. The Mouth and Esophagus a. Digestion begins in the mouth with α-amylase contained in the saliva b. α-amylase is capable of breaking down the complex carbs starch and glycogen polymersinto smaller glucose chains c. Chewing food increases the surface area of food, which enables more enzymes to act on the food at any one time d. Chewed food forms a clump in the mouth called a bolus e. The bolus is pushed into the esophagus by swallowing, where it is then moveddown the esophagus by peristaltic action f. NO DIGESTION OCCURS IN THE ESOPHAGUS 3. The Stomach a. The bolus moves into the stomachthrough the lower esophageal sphincter (aka cardiac sphincter) b. c. The stomach is a flexible pouch that mixes and stores food, reducing it to a semifluid mass called chyme i. Also protects the body by destroying bacteria d. The stomach beings protein digestion by secreting the enzyme pepsin e. NO ABSORBITON OCCURS IN THE STOMACH (except for lipid soluble stuff like alcohol and aspirin) f. The stomach contains exocrine glands called gastric glands/pits i. g. There are four types of cells deep down in the gastric glands i. Mucous cells 1) Secrete mucus that lines the stomachwall and the necks of the exocrine glands 2) The mucus lubricates the stomach wall so food can slide along its surface without causing damage and protects the epithelium lining from the low pH ii. Chief (peptic) cells 1) Secrete pepsinogen, the zymogen precursor to pepsin (degrades proteins) iii. Parietal (oxyntic) cells 1) Secrete HCl which diffuses to the lumen 2) Also secrete intrinsic factor, which binds to B and helps its absorption in the 12 intestine iv. G cells 1) Secrete gastrin into the interstitium 2) Gastrin is a peptide hormonethat is absorbed into the blood via the endocrine system and stimulates parietal cells to secrete HCl h. The hormone acetylcholineincreases the secretionsof all cell types i. Histamine increases HCL secretion (along with Ach and Gastrin) 4. The Small Intestine 4. The Small Intestine a. About 90% of digestion and absorption occurs in the small intestine b. Divided into three parts, from smallest to largest i. Duodenum, jejunum, and ileum c. Most digestion occurs in the duodenum and most absorption occurs in the jejunum and ileum d. The wall of the small intestine is similar to the wall of the stomach except the small intestine wall has finger like projectionscalled villi on its outermostlayer i. Within each villus are a capillary networkand a lymph vessel, called a lacteal ii. e. Nutrients absorbed through the wall of the small intestine pass into the capillary network and the lacteal f. On the tip of each villi are much smaller projections called microvilli i. Increase the surface area of the intestinal wall ii. Appear as a "fuzzy cover" under a microscopecalled a brush border g. The brush border contains membranebound digestive enzymes h. Goblet cells (black thing in picture), found on the villi, secrete mucus to lubricate the intestine and help protect the brush border from mechanical and chemical damage 5. The pancreas a. The chyme is squeezed out of the stomach through the pyloric sphincter into the small intestine b. The fluid inside the first compartmenthas a pH of 6 due to bicarbonate acid secreted from the pancreas c. Acts as an endocrine gland producing hormoneslike insulin and glucagon d. Acts as an exocrine grand secreting enzymes through the pancreatic duct i. Trypsin, chymotrypsin,pancreatic amylase,lipase ,ribonuclease and deoxyribonuclease e. All enzymes are released as zymogens f. Trypsin is activated by the brush border i. Once activated, it actives the other enzymes g. Trypsin and chymotrypsin'sjob is to degrade proteins into small polypeptides i. The polypeptides are further cleaved by carboxypolypeptidase h. Most proteins reach the brush border as small polypeptides i. Once there, they are reduced to AA and absorbed into the enterocytes j. Since intestinal fluid is an aqueous solution, fat tends to clump together, reducing its surface area i. This problem is solved with the addition of bile ii. Bile emulsifies the fat, breaking it up into small particleswithout changing it ii. Bile emulsifies the fat, breaking it up into small particleswithout changing it chemically k. Bile is produced in the liver and stored in the gall bladder i. When needed, the gall bladder releases bile through the cystic duct, which empties into the commonbile duct shared with the liver ii. The commonbile duct empties in the pancreatic duct before connecting to the duodenum iii. l. Once used, most of the bile is reabsorbed by the small intestine and transported back to the liver m. Chyme is movedthrough the intestines by peristalsis n. Segmentationmixes the chyme with the digestive juices 6. The large intestine a. Consists of the ascending colon, the transversecolon, descending colon, and sigmoid colon b. The major function of the large intestine is water absorption and electrolyteabsorption i. If this fails, diarrhea results 7. Gastrointestinalhormones help regulate the process of digestion by not allowing a bunch of food to pass through the stomach a. Instead, the stomach releases a little bit at a time so it all get digested 8. The Liver a. b. Positionedto receive blood from the capillary beds of the intestines, stomach, spleen and pancreas via the hepatic portal vein i. The blood is then worked upon by the liver c. A second blood supply, used to oxygen
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