liveonearth (liveonearth) wrote,

Organ Systems II: Small & Large Intestine, Liver, Pancreas & Secretions

peristalsis and segmental contraction = not the same thing.

Peristalsis is the ordered and sequential contraction of alimentary canal smooth muscles via coordinated action potentials that causes food to be propelled along the alimentary canal. It works kind of like pulling on your socks, in that the canal (sock) bunches up over a section of foodstuff by contracting the longitudinal muscles, while the inner circular layer contracts behind the foodstuff to keep it from moving backward. When the muscles relax again the food has slid forward relative to the canal. The food moves slowest in the ileum, about 1 cm per minute. This is because the ileum is the last absorptive section of the small intestine--after this the waste drops into the cecum where the water is pulled out.

Hormones that increase peristalsis: gastrin, CCK, motilin, insulin, serotonin, prostaglandins

Segmental contraction is alternating and uncoordinated contractions of the inner circular muscle that cause the intestine to look like a string of sausages when in action. The contractions don't move forward to propel the food, to the contrary, they simply occur in alternating locations to mix the food and maximize absorbtion.

MMC = migrating motor complex = a sweeping peristaltic wave the goes through the digestive tract at about once every 90 minutes. This occurs primarily when you are not putting food in at the top, to keep the remainder of your last meal headed for the exit.

Gastroenteric reflex = distention of the stomach and duodenal activity provokes activity in the myenteric plexus that leads to muscle activity

Gastroileac reflex = the reflex that relaxes the ileocecal valve and releases food that has been in the ileum into the cecum--when the next meal hits your stomach

Function of the ileocecal valve = a contraction of the last severam centermeteres of the ileium that stops food. If the cecum is irritated or distended the tone of the valve increases; bother it and the food sticks around. It slows progress of chyme into the cecum to maximize absorption in the ileum. It also limits bacterial backflow from the large intestine into the small.


Haustra = the segmental mixing that occurs in the large intestine, only now the tube is so big it gets a new name. Occurs due to the contraction of longitudinal smooth muscle figers called tinia coli. It's slow, changing segments about every 30 seconds. This mixing allows the body to pull out the remainder of the water, and for the bacteria to get in there and digest what they need. Some of the bacteria provide nutrients to us, and they constitute the bulk of what we excrete.

Mass movements = my favorite thing and the main way that chyme is propelled through the large intestine. One to three times a day a 20cm section of the colon (often the transverse section) looses its haustra and contracts all at once, propelling a large segment of waste toward the exit. The contractions come in sets that last 10-30 minutes, forming every 2-3 minutes, and lasting about 30 seconds. Mass movements that are eliminated can clear the large intestine all the way up to the splenic flexure.

gastrocolic reflex = that urge to defecate that you get when you fill your stomach

orthocolic reflex = when you get up in the morning and stretch, it makes you take a dump (have a mass movement)

duodenal colic reflex = duodenal distention provokes a mass movement (that urge to defecate that you get AFTER a meal) (all autonomic reflexes)

innervation and control of defecation = simple. The innervation of the mechanics of defecation is parasympathetic, so it happens when you relax. This is why I sometimes have what I call the just got home colic reflex. There are two reflexes involved. The intrinsic reflex happens when the rectum is distended provoking peristalsis in the descending and sigmoid colon and then relaxation of the internal sphincter. The parasympathetic defecation reflex happens when pelvic parasympathetic afferent nerves intensify both actions of the intrinsic reflex. With these two reflexes you are ready to eliminate automatically, except for the last external sphincter, which you have intentional control of (pudendal nerve S2,3,4). If you hold it in using the external sphincter for too long the reflexes subside and you go around full of shit. Clenching it actually inhibits the rest of the reflexes.

Important digestive hormones:

gastrin = release is provoked by calcium, coffee, starch or protein in the stomach, also vagus nerve. Causes increase in stomach digestion including acid, motility, slowing of emptying and growth of mucosa.

cholecystokinin = CCK = Release from I-cells in the small intestine is provoked when the stomach contents hit the duodenum (esp acid). CCK kicks off and continues the intestinal phase of digestion, causing release of pancreatic ezymes from acinar cells, contraction of the gall bladder, increased intestinal motility and slowed gastric emptying.

secretin = also from the brush border of the small intestin, secretin is released by S cells when they sense acid and fat. Secretin causes the duct cells to release bicarbonate, buffering stomach acids. It also slows gastric emptying.

motilin = triggered from brush border by acid, fat and vagal stimulation, it causes the stomach and intestine to stir it up some more, ie, increases motility. It is the only one of this list that decreases stomach acid release, and it also provokes more motility in the form of MMC's, migrating motor complexes.

VIP = vasoactive intestinal peptide = not sure what stimulates it.....seems to just lay around in a hammock. Works via NO (nitric oxide) to dilate blood vessels in the GI tract, increasing absorption. VIP relaxes the smooth muscle and mucosa, allows the brush border to absorb sugar, increase pancreatic secretion to that end.

histamine = secreted from ECL cells provoked by gastrin, causes an increase in acid release. It "potentiates" gastrin and acetylcholine and is inhibited by H2 blockers. Histamine is also stored in mast cells and platelets, part of an allergic reaction. So when you take an antihistamine, does it affect your gastric acid levels????


proteases and precursors: how and where are they activated? -- The precursors come from external glands (pancreas) and enterocytes in the small intestinal brush border. The pancreas secretes inactive forms of trypsin, chymotrypsin and carboxypolypeptidase, and they are activated by trypsin and enterokinase. More specifically enterokinase activates trypsin which activates itself and all the rest. So they're activated in the duodenum, where they all get together, and digestion of proteins continues throughout the small intestine.

other pancreatic enzymes include amylase (carbs --> di & tri saccharides) and lipase, cholesterol esterase and phospholipase, which hydrolyze lipids. There is also a salivary version of amylase and lipase, but when the stomach acidifies everything these processes stop until the gastric contents are again neutralized enough that the pancreatic enzymes can work.

tight junctions = between the enterocytes of the brush border. These tight junctions are not as tight as once thought, and when a person has a lousy diet they can become quite loose --> leaky gut syndrome. Test for leaky gut by eating mannitol and lactulose together, then testing urine after 6 hours. Mannitol is absorbed transcellularly, through the cells, whereas lactulose is not absorbed, and only gets in via leaking through the tight junctions (ie paracellular). So if you have lactulose in your urine, your gut is leaky. But back to the general stuff, tight junctions are regulated by zonulin, not sure the mechanism or action. Leaks may cause food sensitivities, IBS, etc etc etc. Leaks are caused by: sugary diet, NSAIDS, etc etc etc. Paracellular absorbtion is probably used intentionally by the body for getting large amounts of water in and out, and also for macromolecules. Seems that there is lots more study to be done on this.

pacreatic acinar cells secrete all 3 kinds of enzymes when stimulated by CCK, vagus nerve

pancreatic duct cells secrete bicarbonate and water when stimulated by secretin

If a carb isn't digested sufficiently in the upper jejunum and enters the distal small instestine in hyperosmolar concentrations you get osmotic diarrhea, and an overgrowth of your commensal bacteria due to the food, which gives you bloating and nasty farts

in the cephalic phase of pancreatic digestion (the head phase) you see, smell, think about, taste food, chew or swallow, and this triggers the vagus nerve to stimulate pancreatic secretion.

gastric phase relatively unimportant

intestinal phase = most important for the pancreas. I cells in the duodenum sense fat and digested proteins, release CCK. S cells sense acid and fat, release secretin which stims bicarb and water from duct cells. Enzyme secretion adapts to diet; CCK up/down regulates protease & amylase secretions depending on proportion of proteins to carbs in diet.

lactose, amylopectin and sucrose digestion = lactose and sucrose have their own specific enzymes, lactase and sucrase, that digest them completely. Amylopectin and glycogen (starch) comprise 50% of the carbs that we digest and there are three enzymes that attack different parts of them. Some enzymes cut off branches of the starch, others chop up the long strands into shorter strands, another chops up the short strands into shorter strands.

glucose absorption at the enterocycte = transcellular with sodium, or paracellular with water.

brush border enzymes = chymotrypsin, elastase, carobxypeptidases A & B, trypsin, peptidases


liver anatomy

liver histology

flow of bile, blood and lypph in the lobule

significance of liver acinus

portal hypertension


functions of bile = 1) neutralize stomach acid 2) excrete iron (bilirubin) cholesterol, drugs, metals 3) digest and absorb fats

functions of liver = (the top few) 1) macrophages eat colonic bacteria 2) metabolize carbs to glycogen or glucose, lipids to lipoproteins, cholesterol & phospholipids, oxidize fatty acids, make lipids, metabolize proteins to albumin, hemoglobin, urea 3) store vitamins A, B12, D, K, folic and Fe2+, 4) detox drugs, hormones 5) secrete bile
Tags: digestion, disease, elimination, hormones, intestine, liver, vitamins

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