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Lecture 17

BIOLOGY 288 Lecture 17: Digestive System Feedback Loops

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Diane Kelly

Next case study: osmoregulation • Stomach o Connects to esophagus at the top o Lower esophageal sphincter ▪ Opens to let food into the stomach & prevents reflux o Body (Outer wall) ▪ Secretes mucus, pepsinogen, and HCl ▪ Lots of folds ▪ Mucous cells – secretes mucus that coats stomach and bicarbonate that acts as buffer • Stomach wall doesn’t get digested because its cells are covered by neutralizing mucus & through negative feedback loops that limit the amount of acid secreted ▪ Parietal cells – secretes acid and intrinsic factor ▪ Chief cells – secretes pepsinogen – precursor to pepsin – endopeptidase (breaks bonds in the middle of protein aa chains) • Makes inactive version of pepsin to avoid autodigestion • Activated by HCl – broken down to Pepsin – a piece is cut off • Pepsin can break down proteins into smaller peptides o Only active in a small range of pH o Fundus o Antrum ▪ Mixing and grinding, secretes mucus, pepsinogen, and gastrin o Pyloric sphincter ▪ Controls rate of emptying o Connects to duodenum at the bottom • Negative feedback loop to limit acid production: Parietal cells and chief cells o Protein enters stomach ▪ Sensors in stomach wall detect protein ▪ Stomach distension (fills/stretches) o Stomach wall secretes gastrin (hormone) o Gastrin stimulates parietal cells to release HCl o Gastrin stimulates chief cells to release pepsinogen ▪ (carbonic anhydrase makes H CO2fr3m CO and 2 O) 2 o Secretion of HCl has a negative feedback effect on the secretion of gastrin ▪ As levels of HCl increase, secretoin of gastrin decrease o Higher levels of HCl stimulations somatostatin secretion ▪ Inhibits acid secretion from parietal cells o Both are negative feedback loops • The Small Intestine o Pepsin is not active in the pH of the small intestine o Duodenum, Jejunum, Ileum empties into cecum (large intestine) o Duodenum Next case study: osmoregulation ▪ Secretory ▪ Digestion ▪ Absorptive surface o Jejunem ▪ Mostly absorptive (at the surface) ▪ Intracellular digestion o Ileum ▪ Mostly absorptive (at the surface) ▪ Intracellular digestion o Surface Area matters ▪ Inner layer (mucosa) is convoluted – has a lot of folds • Each circular fold has many villi o Each villus is a fold of its own ▪ Has capillary and lymph vessel • Lymph vessel (lacteal) takes removes plasma fluid from villus and returns it to the circulatory system – decreases pressure – how fats enter circulatory system – don’t enter capillaries ▪ The outer surface of each cell is also folded • Micro-villi • Magnifies overall absorptive surface area o Secretin is released by cells in the wall of the duodenum when food enters ▪ Ac
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