[memory]

i am confused... yet again.
about the clearance of PAH:
in a certain qn of kaplan(textbk) as the concentration of PAH in the blood rises, it's excretion increases. then shouldn't the clearance curve for PAH, with rising plasma levels, begin ascending? in the graph, the clearance curve descends, tending to approach the GFR.
IS THIS STATEMENT TRUE? "with rising plasma PAH levels, the venous levels of PAH begin to increase as the carriers for the secretion of PAH become saturated."
i just can't fix these facts together.
could the underlying answer be that a constant fraction of PAH is secreted...
in which case, the excretion of PAH increases with the increasing blood levels of PAH.
BUT THEN HOW DOES THE CLEARANCE CURVE DESCEND?
or does it descend because of greater number of carriers becoming saturated?
and have they forgotten to show that the curve begins ascending again?
as you can see, i've worked myself into a corner. please explain this. it's driving me up the kitchen wall...

 

[khanar]

Clearance = volume of plasma from which a substance is removed per minute.

At low plasma concentration of PAH, all of PAH is excreted (20% filtered and 80% secreted and 0% reabsorbed) if all PAH is excreted this means that what ever plasma coming to tubules is being completely cleared of PAH therefore clearance is maximum and represents the renal plasma flow and no PAH is seen in renal venous blood as you have secreted it all.

As the concentration of PAH increases in the plasma the transport carriers begin to saturate and so not all PAH can be secreted therefore it's less excreted and therefore less volume of plasma is being cleared of PAH per minute and therefore less clearance therefore your curve must descend and therefore PAH starts to appear in renal venous blood.

Hope that you stop hitting the kitchen wall after this

 

[memory]

Clearance = volume of plasma from which a substance is removed per minute.

At low plasma concentration of PAH, all of PAH is excreted (20% filtered and 80% secreted and 0% reabsorbed) if all PAH is excreted this means that what ever plasma coming to tubules is being completely cleared of PAH therefore clearance is maximum and represents the renal plasma flow and no PAH is seen in renal venous blood as you have secreted it all.

As the concentration of PAH increases in the plasma the transport carriers begin to saturate and so not all PAH can be secreted therefore it's less excreted and therefore less volume of plasma is being cleared of PAH per minute and therefore less clearance therefore your curve must descend and therefore PAH starts to appear in renal venous blood.

Hope that you stop hitting the kitchen wall after this

thank you.