Cant understand: Urine Flow = Free Water Clearance + Osmolar Clearance

[pizza100]

Hi everyone. I don't understand the equation above. Hope someone can shed some light on it for me.

Here is what I understand so far:
Osmolar Clearance is the theoretical volume of plasma per unit time that the kidney has to filter in order to extract all the solutes out of that volume of plasma. In Osmolar Clearance, the solutes are excreted, but the fluids are returned to the body. Similarly, Free Water Clearance is the theoretical volume of plasma that the kidney has to filter in a certain amount of time in order to extract all the free water from that volume of plasma. In Free Water Clearance, the free water gets excreted while the rest of the plasma gets returned to the body.

Here is what I don't understand:
Since Free Water Clearance and the Osmolar Clearance are rate of fluids that eventually get returned to the body, why are their sum equal to the flow of Urine? If Free Water Clearance and Osmolar Clearance are rate of fluids that get excreted, then I can understand why their sum can be equal to the flow of Urine. However, I don't think Free Water Clearance and Osmolar Clearance are rate of fluids that get excreted. It seems like I am comparing two sides of the equation that has nothing to do with each other - like comparing apples to oranges.


Thanks in advance for any help.

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[tallman24]

Clearance means an amount clearing the body. Pretty sure your second assumption is correct. Then again i haven't looked renal in a while.

 

[CrimsonKing]

The free water clearance is how much water you are filtering (ie removing) from the body for some given time and the osmolar clearance is the total osmolarity that is being cleared per unit time, which will bring an equal amount of water with it via passive secretion elsewhere in the nephron.

So, the total volume of your urine is the total amount of water freely filtered PLUS the amount of water secreted.

 

[pizza100]

osmolar clearance is the total osmolarity that is being cleared per unit time.

Thanks for answering my question. I wish the concept of clearance were that simple. Osmolar Clearance or Water Clearance almost sounds like the amount of solute or water respectively that is being excreted per unit time (by the way you define it). I don't think that is right.

I've inserted a picture to illustrate what I mean. The Green line represents the volume of plasma per unit time that gets filtered into the Glomerular space and eventually gets reabsorbed back into the plasma. If that volume of plasma (green line) flowing at that rate drops off all its solutes to be excreted (blue dots), then that volume of plasma that gets filtered at that rate is called the Osmolar Clearance. Osmolar Clearance is not the amount of solute that gets excreted to urine, and most of the Osmolar Clearance will be reabsorbed back into the plasma. The blue dots are the amount of solute that gets excreted to urine.

So, please allow me to state more clearly my confusions with that equation in a different way:

Here is what I understand so far:
The term "Clearance" refers to the volume of plasma that the kidneys have to clean in order to remove all of a certain material per unit time from that volume of plasma. So, Osmolar Clearance is the volume of plasma that the kidneys have to clean in order to remove all of the solutes (X) from that volume of plasma per unit time. Similarly, Free Water Clearance is the volume of plasma per unit time that the kidney has to clean in order to remove all of the free water (W) from that volume of plasma. I call the amount of solutes per unit time that are removed from plasma to be X and the amount of water that are removed from the plasma per unit time to be W.

Here is my confusion:
I don't understand why Urine Flow = Free Water Clearance + Osmolar Clearance. To me, the following equation makes more sense: X + W = Urine Flow.
X and W are products of the cleaning process per unit time that actually gets excreted to become urine. Free Water Clearance and Osmolar Clearance are the volumes of plasma per unit time that have been cleaned of water and solutes respectively. Free Water Clearance and Osmolar Clearance are eventually returned to plasma. Free Water Clearance and Osmolar Clearance are not substances that get excreted, so how can their sum be Urine Flow? That's like saying: Phil + Tim = furniture. The more accurate picture is Phil makes chairs and Tim makes tables, and chairs + tables = furniture. The sum of Phil and Tim are not furniture. Rather, it's the sum of the products that Phil and Tim make that equal furniture.

Please help.

 

[calvnandhobbs68]

I think you're forgetting that osmolar clearance already takes into account the reabsorption you're concerned about. Remember the formula for osmolar clearance is:

Cosm= (Uosm * V)/Posm

Where Uosm is the urine osmolarity, V is urine flow rate and Posm is plasma osmolarity. You've already measured Uosm directly (you can't do the calculation without it). So you already know what the final urine osmolarity is when calculating the osmolar clearance.