Transport and Excretion and Water and Salt Balance

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Transport and Excretion and Water and Salt Balance

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University of Southern California
Bisc 307l - General Physiology
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BISC 307L 1st Edition Lecture 37 Current Lecture Transport and Excretion  Reabsorption of Na+, glucose HCO3- etc o o Already talked about Na+ going across the apical membrane (Na+/H+ anti- port main way) meaning that the protons must go into the lumen- where are they coming from?  Protons that are pumped into the lumen combined with bicarbonate ions (filtered) to make carbonic acid which dissociates to CO2 and H2O. Co2 permeable so easily permeates through the apical and basolateral membrane so there is always Co2 coming our. In the epithelial cell some CO2 enters where it combines with water which forms carbonic acid to forms protons and bicarbonate- it is these protons that provide the protons being pumped across the apical membrane To make this work must get rid of bicarbonate- leaves through basolateral membrane – there is a Na+/3bicarbonate symport which is driven by the bicarbonate gradient and pumps 3 bicarbonates and 1 Na+ ion out of cell.  Filtered bicarbonate not used up because it gets reabsorbed by the ECF  This is a mechanism for the reabsorption of Na+ and HCO3- because it is a 3 Bicarbonate Na+ symport it is electrogenic and makes the membrane more negative o Some of the H+ does not combine with bicarbonate so it is excreted in urine (way more protons are being secreted(excreted) than being reabsorbed)- our diets are acidic o The water ill follow by osmosis and things that are not transported will build up in concentration such as (Cl-) which creates a concentration gradient for Cl- to move across the epithelium- 2/3 paracellular route (between cells) and 1/3 goes through Cl- ion channels in both membranes o Urea is concentrated in tubule fluid and in the proximal tubule and distal tubule, urea isn’t permeable so as water is reabsorbed then the urea is left behind and the concentration goes up and in those places that do have urea transporters (collecting duct) it will passively be reabsorbed down its concentration gradient o Not much protein is taken up but once inside the cell they are degraded or put back across the basolateral side. This means there should be no protein in urine  Secretion of H+ ions o o H+ ions are constantly excreted by 3 mechanisms  1. Proton secretion in proximal tubule (previous slide)  sodium dependent  2. Distal tubule and collecting duct  2. Sodium independent  proton pump in apical membrane pumping H+ out by H2O dissociation in cell and the hydroxyl ions combine with CO2 to form HCO3 and is pumped out through a Cl-/HCO3 exchanger and Cl- doesn’t accumulate and is pumped right out  3. ATPase – H+/K+ ATPase  protons out and K+ in  Protons come from same place as above (dissociation of water) and also pumps K+ ions in which leave the cell through K+ channels in basolateral membrane  Mechanism for K+ reabsorption  “gets rid of acid by reabsorption of potassium” – acidosis ...

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