# Is the glomerular filtration rate, per kidney or for both?

We say in a healthy human being the average glomerular filtration rate is about 120 mL/min. Is that for both kidneys together (60+60), or just one kidney?

Pardon me if this makes me sound like an idiot, because I have a feeling I'm thinking about this wrong.

The definition of GFR given in Wikipedia -

Glomerular filtration rate (GFR) is the volume of fluid filtered from the renal (kidney) glomerular capillaries into the Bowman's capsule per unit time.

Since it is difficult to measure the volume entering into each Bowman's capsule, as a proxy, we measure the clearance of a solute that we know isn't going to be absorbed or secreted. For that particular solute,

GFR = (Urine concentration x Urine flow)/ Plasma concentration

The GFR you obtain here is the sum total of volume of all the fluid filtered from all the renal (kidney) glomerular capillaries into all of the Bowman's capsule per unit time.

Obviously, a single Bowman's capsule can't receive 120ml every minute, it is microscopic!!

So yes, the GFR or eGFR we use in clinical practice is the sum total of the function of both the kidneys. In fact, before transplantation, although the left kidney is preferred from the donor (since it is easier for surgery having a longer renal vein), it is more ethical to estimate individual GFR for each kidney and preserving the better functioning kidney. And yes, the total GFR drops after donating a kidney, but the body has sufficient reserve to manage.

If you still have a tough time figuring that out, think of them as many separate taps opened to fill a bucket. One tap for each glomerulus. The net rate of flow is the sum of flow rate in all the taps. The kidney has million such taps! And the taps are being clogged one at a time in chronic kidney disease - decreasing the GFR. And in a transplant, half the taps are suddenly shut down in the donor! But since this is a healthy kidney, the donor adapts and slowly reaches ~70% of original GFR.

• Exactly what I was looking for. Thank you, that was very succinctly put! May 20 '18 at 4:50

It's a rate, not an absolute amount, so you wouldn't divide it up like that. It's like if a car is traveling 60 mph, each tire is going 60 mph, not 15.

You'd divide it up to describe the absolute amount, so if the kidneys filter 120 mL, then you could say each kidney filtered 60 mL (although even then each kidney isn't going to function identically).

• Thank you, I understand how it's a rate and used as an indicator of the function of the excretory system. My point is, if you removed one kidney, gradually the remaining one may grow to work more. But theoretically, the day that you remove the first kidney, the GFR is halved? May 19 '18 at 6:24