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Physio: GI - Absorption and Secretion (Ions, Water, Vitamins)


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What is the average daily volume of ingested or secreted fluids, and from what sources?
2-diet, 1-saliva, 2-gastric juice, 3-pancreatic juice and bile, 1-small intestine
How much fluid is absorbed versus excreted on average daily?
small intestine r/a's ~7L, colon r/a's ~2L

100-200mL e/c in feces
What is there net absorption of in the jejunum?
NaHCO3 (also monosaccharides, AAs)
Transport in jejunal epithelial cells parallels that of which part of the kidney?
early proximal tubule
What are the 4 main ways Na is reabsorbed in the small and large intestines?
1. nutrient-stimulated: Na-monosacc and Na-AA cotransport (w/facilitated diffusion into blood)
2. high pH stimulated: apical Na-H exchange (duod and jej)
3. interdigestive: parallel Na-H and HCO3-Cl exchangers (ileum and prox colon)
4. diffusion via apical ENaC channels (distal colon)
What is there net absorption of in the ileum?
NaCl (and sugar and AAs)
Transport in epithelial cells of the colon parallels that of which part of the kidney?
principal cells of the late distal tubule and collecting ducts
What effect does aldosterone have on Na transport in the colon?
induces synthesis of ENaC channels, to increase Na r/a and K s/c (like in principal cells of LDT and CDs)
What are the 3 main ways Cl is reabsorbed in the small and large intestines?
1. voltage-dependent: passive r/a (follows Na), cellular or paracellular
2. electroneutral: Cl-HCO3 exchanger in ileum and colon
3. interdigestive: parallel Na-H, HCO3-Cl channels in ileum and prox colon
What is the mechanism of K reabsorption and/or secretion in the small intestine?
K r/a passively via solvent drag (no s/c)
What is the mechanism of K reabsorption and/or secretion in the large intestine?
1. passive paracellular (net) s/c of K
2. active K s/c via Na-K-2Cl pump (stimulated by aldosterone)
Does diarrhea cause hypo- or hyperkalemia?
hypokalemia; flow rate-dependent secretion of K (like in distal tubules), causes excessive K s/c, e/c
What is the mechanism of Cl secretion in the intestinal crypts?
in response to secretagogues (i.e. ACh, VIP); NaK2Cl pump, and diffusion through apical Cl channels (CFTR)
What substances are secreted by the crypts, in addition to Cl?
Na and water passively follow Cl s/c
What type of diarrhea results from cholera toxin?
secretory diarrhea (excessive secretion overwhelms absorptive capacities of intestine/colon)
What is the mechanism by which cholera toxin causes diarrhea?
cholera toxin binds to apical receptors of crypt cells, activates AC, cAMP, permanently opening apical Cl channels; Na and water follow
What are the fat soluble vitamins?
A, D, E, K
How are fat soluble vitamins absorbed?
incorporated into micelles, diffuse across apical membrane, incorporated into chylomicrons and extruded into lymph
What are the water soluble vitamins?
B1, B2, B6, B12, C, biotin, folic acid, nicotinic acid, pantothenic acid
How are most water soluble vitamins absorbed? What is the exception?
via Na-dependent cotransport mechanisms; except B12
What is the mechanism of B12 absorption?
B12 is transferred to intrinsic factor in duodenum (resistant to degradation by panc proteases), binds to receptor in ileum and is absorbed
What is a critical consequence of a gastectomy? How is this remedied?
loss of gastric parietal cells = no intrinsic factor = pernicious anemia; give B12 injections (to bypass need for GI r/a)
What does Ca depend on for intestinal absorption?
active vitamin D (1,25-dihydroxycholecalciferol) (activated in kidney)
How does 1,25-dihydroxycholecalciferol promote Ca r/a?
it induces synthesis of vitamin D-dependent Ca-binding protein (calbindin D-28K) in intestinal epithelial cells
What happens in vitamin D deficiency or chronic renal failure?
inadequate Ca absorption = osteomalacia (adults) or rickets (kids)
In what forms is iron absorbed across the apical membrane of intestinal epithelial cells?
as free iron or heme iron (heme iron is then converted to free iron in cell)
How is iron transported across the basolat membrane of intestinal epithelial cells?
free iron binds to apoferritin and is transported across memb
What is the form in which free iron circulates in the blood, and where does it go to?
bound to transferrin, goes to storage sites in liver, then to bone marrow for synthesis of hemoglobin

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