Blood Flow in the GI tract -
- aorta branches off mesenteric arteries that go into the intestins and spread along muscle bundles, into the intestinal viilli and into submucosal vessels under the epithelium to serve the secretory and absorptive functions of the gut
- during digestion, vasodilators are released, mostly peptide hormones like CCK, VIP, gastrin and secretin
- oxygen [ ] decreases in order to increase intestinal blood flow
Mitosis of differentiated intestinal cells accounts for much of the replacement, ~3 days -
- radiation causes a lot of harm to these cells b/c they are rapidly dividing
- countercurrent mechanism of the villis: arterial and venous flow, opposite directions and right next to each other allows for blood O2 diffusion ot of the arterioles into the adjacent venules w/o being carried in the blood to the tips of the villi
EX: during GI diseases, villi become damaged, leading to greatly diminished intestinal absorptive capacity
- also allows for rapid absorption of things going in and out of the liver
- stomach: storage, mixing and emptying, peristalic waves = “pyloric pump”
EX: gastrin increases activity of peristalic pump promotion of emptying of the stomach because it secretes highly acidic juice which stimulates motility
- note: pancreatic enzymes become active only when they come into contact w/ the acid in the chyme
- stomach “pseudo-sphincter: processes food, communicates w/ duodenum to control chyme transport
- when fat enters the duodenum, hormones (CCK) are released, and bind with receptors on the epithelial cells, CCK inhibits pyloric pump and increases pyloric sphincter strength contraction (fats are slower takes longer to digest)
- CCK then inhibits stomach motility that was initially caused by gastrin
- other inhibitors inc. secretin released from the duodenal mucosa in response to gastric acid that came from the stomach
- inhibitory hormones inhibit stomach emptying whn a lot of chyme (esp, acidic and fatty chyme) has entered the duodenum
EX: what do proton-pump inhibitors do to the stomach? – decrease acidity in the stomach leading to decreased functional ability of the duodenum
Mixing contraction in the SI -
- distension causes local peristalic contractions that are segmented in the SI
- promotes mixing w/ secretions in the SI
- even tho it is slow waves in the SM that cause segmentation contractions, these contractions are not as effective w/o other excitation, esp. from the myenteric nerve plexus
¬ caused by electrical slow waves – 12 contractions/min, 1cm
Propulsion -
- chyme is propelled thru the SI by peristaltic waves, slow from pylorus iliocecal valve
- peristaltic activity is increased by “gastroenteric reflex” that is initiated by stomach distension and conducted primary by the myenteric nerve plexus from the stomach to the SI
- but ALSO HORMONES – gastrin, CCK, insulin, serotonin enhance motility and are secreted at certain times
- but secretin and glucagon inhibit SI motility
¬ caused by peristaltic waves – 1cm/min, 0.5-2cm/sec
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