PHIS 206 Lecture Notes - Lecture 21: Tubular Fluid, Clearance Rate, Reabsorption

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Lecture 21: Excretion & Balance
All Together
Filtration is simple and passive, driven by the pressures of the capillary.
Reabsorption, for the most part, is an active process (proteins recognize components of
filtrate and transport them to the blood - carrier-mediated transport)
Secretion is an active process (carrier-mediated transport from blood into tubular fluid)
Excretion
Following filtration, reabsorption, and secretion, the tubular fluid (urine) achieves its
final form. This urine is excreted from the kidney and body. Don’t confuse secretion with
excretion.
An excretion rate speaks to amount of substance x that leaves the kidney with respect to
time.
Excretion rate of x = (Urine Flow Rate) x (Urine Concentration of x)
Know formula
Excretion Rate
So, if nearly everything is filtered (with exceptions), it truly is the amount (the rates) of
reabsorption and secretion that controls the excretion of a substance.
For example, if something is nearly completely reabsorbed, there will be none of the
substance remaining in the urine. Excretion rate would be zero.
But, if something is filtered freely, and then heavily secreted as well, it will have a very
high excretion rate.
Secretion is getting rid of a substance
This Suggests Math…
Physiologists have been able to figure out how the kidney handles a substance by
measuring the filtration of the substance and the excretion of the same substance...
If the excretion of a substance is less than the filtration of the same substance → the
substance must have been reabsorbed***
And, if the excretion of a substance is greater than the filtration of the same substance →
the substance must have been secreted***
But Isn’t This About the Blood?
The blood was the original focus - the kidney conditions the blood. So
measurements/descriptions can focus on the substance (excretion rate), or we can focus
on the blood flowing through the kidney instead.
This creates the odd concept of clearance.
The clearance of a substance is the amount of blood that the kidney clears of the
substance with respect to time.
Only place the kidney can find a place to get rid of is the blood
Clearance rate speaks to the blood, secretion rate speaks to the substance
If excretion is high, than clearance is high
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Document Summary

Excretion: following filtration, reabsorption, and secretion, the tubular fluid (urine) achieves its final form. This urine is excreted from the kidney and body. Don"t confuse secretion with excretion: an excretion rate speaks to amount of substance x that leaves the kidney with respect to time, excretion rate of x = (urine flow rate) x (urine concentration of x, know formula. Excretion rate would be zero: but, if something is filtered freely, and then heavily secreted as well, it will have a very high excretion rate, secretion is getting rid of a substance. This suggests math : physiologists have been able to figure out how the kidney handles a substance by measuring the filtration of the substance and the excretion of the same substance But isn"t this about the blood: the blood was the original focus - the kidney conditions the blood. If excretion is high, than clearance is high.

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