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RS 12
Glucose Reabsorption, Reabsorption of Water, HCO3– Reabsorption,…
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Reabsorption of Water
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Obligatory reabsorption.
- About 85% of the filtered water is always reabsorbed, irrespective of the
body water balance.
- This reabsorption occurs by osmosis in response to a transtubular osmotic
gradient and is called obligatory (must occur) reabsorption.
- About 67% of obligatory reabsorption occurs in the proximal tubules
and
- About 15–18% of obligatory reabsorption occurs in the descending thin
segment of loop of Henle.
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HCO3– Reabsorption
It is important to note reabsorption of HCO3 – alone cannot maintain normal acid base balance, therefore, kidney is capable of generating new HCO3 – as well.
HCO3– reabsorption occurs throughout the nephron,
except in the descending limb of loop of Henle.
- i. Approx. 90% of the filtered HCO3–
is reabsorbed into the PCT by
secondary active transport (antiport) via the Na+- H+ exchanger,
this represents more than 4000 mEq/day.
- ii. The secreted H+ reacts with filtered HCO3– to form carbonic acid(H2CO3).
The presence of carbonic anhydrase (CA) on the microvilli of the luminal border of the PCT catalyzes the rapid dehydration of
H2CO3 to form CO2 and water.
- iii. Both CO2 and H2O diffuse back into the PCT cells which together with CO2 derived from cell metabolism is rehydrated by intracellular ‘CA’ into H2CO3 ; which dissociates to form H+ and HCO3-. This HCO3 is termed as newly synthesized HCO3- by renal tubular cells.
a. This newly synthesized HCO3- is reabsorbed across the basolateral membrane by a 'secondary active transport system'
into the peritubular blood along with equimolar amounts of Na+ and
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- Reabsorption from the DCT and CT
- i. The remaining 10-15% of the filtered HCO3 -is reabsorbed by DCT and CT via a mechanism that involves the exchange of Na+ for K+ or H+ (as in PCT).
- ii. Unlike the PCT, the DCT and CT have H+- K+ -ATPase at the luminal membrane and the lack of ‘CA’ from that site.
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