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GI Physiology

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9000 mL
Amount of fluid that is ingested and secreted in the gut
100 mL
Amount of fluid that is excreted from the gut
7000 mL
Amount of fluid absorbed in the Small Intestine
1900 mL
Amount of fluid absorbed in the Colon
Baroreflex
How does hypovolemia stimulate absorption
Norepinephrine and Angiotensin II
Two stimulators of the sodium pump in gut epithelial cells used to conserve salt and water
Jejunum
This area absorbs most of the products of digestion
Duodenum
This area has the greatest absorptive capacity
Active Na Transport plays this part in the gut/kidney
What fuels fluid absorption in the gut and kidney
1. Symport (Glucose-SGLT1), Antiport (H+) 2. Na pump at basolateral membrane 3. Negative filtration pressure for absorption from interstitial spaces into the blood are three requirements for this.
Requirements for sodium absorption.
Which molecules follow Na Paracellularly
How do water, k, & cl get absorbed
HCO3 absorption is driven by this
H secretion drives this process.
Diarrhea
During inflammation, leaky capillaries decrease the oncotic pressure gradient between capillaries and interstitium, slowing fluid absorption and contributing to this.
HC03 in the ileum
capacity for symport of sodium and organic solutes and paracellular permeabilty is less than in the jejunum but NaCl is absorbed and net secretion of this molecule occurs.
K and HCO3 in the colon. Aldosterone
These two ions are secreted in the colon. This opens sodium potassium channels and stipulates Na/K ATPase synthesis (to increase k secretion and sodium absorption).

Paracellular permeability to ions and H20 in the colon are less than the small intestine and the lumen is negative compared to the interstitium
Secreting (crypt) to Absorbing (villus)
As enterocytes in the small intestine mature, they migrate towards the tip of the villi and eventually slough off at the tip. There cell type changes as the mature in this manner (snot nose kid --> medical student)
Immature cells of intestinal crypts
In this area, a resting cell has a [cl] above equilibrium which allows for active transport across this type of cell into the lumen via the NaK2Cl and CFTR pumps to make the lumen negative. This negative lumen causes paracellular diffusion of sodium and h20. Mediated by an increase in cAMP intracellularly.
HCO3 and Jejunum
This area is the only area of the GI tract that absorbs HC03.
HC03 and Duodenum
HC03 neutralizes stomach acid in this area of the GI tract
Bacterial metabolism of protein and carbs
Why is there a high level of short chain fatty acids in the stool (acetate, propionate, and butyrate)
Secretory diarrhea
This type of diarrhea is caused by stimulation of Cl secretion by crypt cells and can be caused by E.coli, cholera, or inflammatory diseases
Osmotic Diarrhea
This type of diarrhea is caused by the presence of non-absorbable solutes in the lumen as seen in lactose intolerance, some laxatives and sugar substitutes
Inflammatory process leading to intestinal salt and water secretion and diarrhea
Mast cells, macrophages, eosinophils, and neutrophils
1. Activate secretomotor neurons to increase secretion of chloride (sodium and water follows) and activate motor and vasodilatory neurons to increase motility and blood flow
2. Increase vessel permeability, diminishing gradients and decrease capillary oncotic pressure leading to a decrease in sodium, chloride, and water absorption
Bradykinin, PGE2, and oxidants
These directly stimulate net epithelial secretion and inhibit absorption in an acute setting
PGI2, serotonin, oxidants
These activate enteric secretomotor neurons acutely
IL-1, histamine, and oxidants
These activate arachidonic acid metabolism acutely
Endotoxin and Inflammatory bowel disease
In a chronic situation, these decrease sodium pump activity, decrease sodium absorption, while increasing potassium secretion and mucosal permeability
gamma interferon and insulin like growth factor 1
In a chronic situation, these increase mucosal permeability and increase the proportion of secretory crypt cells (immature enterocytes).
Slow waves
intrinsic rhythmic spontaneous depolarizations of smooth muscle membrane potential occuring all over the GI tract
Increases force of contraction of GI smooth muscle
Increasing the amplitude of a slow wave does this to a contraction
Increases the length of contraction of GI smooth muscle
Increasing duration of plateau phase
Resting and threshold potentials of GI smooth muscle
-60- -80 mV and -55- -45 mV
interstitial cells of Cajal
This is where pacemaker potential originates. They occur between slow waves.
Stomach- 3/min
Duodenum 12/min
ileum 7/min
colon 5/min
Frequency of slow waves determines the frequency of segmentation and peristalsis. Where are slow waves most frequent and least frequent.
1. Cranial Division of parasympathetic pathway
2. Sacral division of parasympathetic pathway
Divisions of parasympathetic nervous system for gut

1. Esophagus&Stomach, Liver&Gall bladder, Pancreas, Small intestine&proximal colon

2. Rectosigmoidal colon
Celiac and superior mesenteric sympathetic ganglia
Innervates sympathetically the stomach, liver, pancreas, small intestine and proximal colon
Inferior mesenteric sympathetic ganglion
Innervates distal colon and rectosigmoid colon