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The validity of the above GFR formula depends on:

1)A substance being freely filterable
2)Not reabsorbed
3)Not secreted
4)Not synthesized by the tubules
5)Not broken down by the tubules

Now creatinine:
1) Since its secreted by the tubules it overestimates true GFR.
2)But its a close approximation
3)Its indigenous , formed from muscle creatine

1)At low pasma concentrations it escapes filtration and is secreted
2)PAH clearance actually measure the ERPF which is about 90% of the
true total plasma fow
3)ERBF =ERPF/ 1- hematocrit
4)When PAH exceeds its Tm it is not completey removed from the plasma and its clearance as a measure of ERPF is invaid.

Can anyone name a substance that is both secreted and reabsorbed??


It is UREA.
a)secreted in loop of henle
b)reabsorbed in collecting tubule??
It maintain hyperosmolarity of interstitium in medulla for conc: urine formation.


PCT-2/3 of Na, K, Cl, and h20 are reabsorbed and maintain the same osmolarity and conc. as plasma

Descending limb- is permeable to H2O, causing the highest osmolarity at the tip of the Loop of Henle(1200mOsm/)(1/3 of original vol) Fluid at the end of the collecting dut can equal this only with ADH.

Ascending limb is called diluting segment and is impermeable to H2O.Here
tubular fluid is diluted leaving the Loop of henle as hypotonic fluid.Nacl is pumped from the tubule into the interstitium in the Loop of henle. Flow thru the loop is slow. Anything that increases flow thru the Loop or vas recta will decrease the ability of the sytem to maintain high medullary osmolarity and reduce the conentrating abiityof the kidney to form concentrated urine. For example if the proximal tubule fails to reabsorb 2/3s of fluid and electolytes the excess load willl overwhelm the Loop decrease the interstitial osmolarity and reduce maximum urine osmolarity.

Collecting duct- Without ADH it is completely impermeable to H2O. And the dilute fluid leaving the Loop of Henle becomes dilute urine. ADH increases the permeability of the colecting duct to H2O and allows pasive reabsorbtion of H2O and some urea.

1)An exampleof passive transport
2)Freely filtered at the glomerulus
3) Its concentration =s peritubular capillary in the very first portion of the tubule.
4)In the descending limb water reabsorbtion occurs, increasing the conc. of any tubular solute.
5)So urea conc. increases to a greater level then peritubular plasma
6)*Urea is able to passivey diffuse down this conc. gradient from tubular lumen to peritubular fluid completely dependent on the reaborbtion of water, which estblishes this concentration gradient. In man reabsorbtion of urea is about 50%.
7)*Urea is known more for being passively reabsorbed in the medullary collecting duct. Looks like it helps maintain a critical diffusion gradient.

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