PHIS 206 Lecture Notes - Lecture 21: Tubular Fluid, Clearance Rate, Reabsorption
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2 May 2018
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Lecture 21: Excretion & Balance
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• 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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