Human Physiology, 14th edition (2016)

602 Chapter 17

Clearance of Urea

Urea may be used as an example of how the clearance calcula-

tions can reveal the way the kidneys handle a molecule. Urea

is a waste product of amino acid metabolism that is released

by the liver into the blood and filtered into the glomerular

capsules. Using the formula for renal clearance previously

described and these sample values, the urea clearance can be

obtained:

V 5 2 ml/min

U 5 7.5 mg/ml of urea

P 5 0.2 mg/ml of urea

Urea clearance 5

(2 ml/min)(7.5 mg/ml)

___________________

0.2 mg/ml

5 75 ml/min

If a substance is not reabsorbed, all of the filtered amount
will be cleared. If this substance is, in addition, secreted by active
transport into the renal tubules from the peritubular blood, an
additional amount of plasma can be cleared of that substance.
Therefore, the renal plasma clearance of a substance that is fil-
tered and secreted is greater than the GFR ( table 17.5 ). In order
to compare the renal “handling” of various substances in terms
of their reabsorption or secretion, the renal plasma clearance is
calculated using the same formula used for determining the GFR:

Renal plasma clearance 5

V 3 U

P
where

V 5  urine volume per minute

U 5  concentration of substance in urine

P 5  concentration of substance in plasma

Figure 17.23 The renal
clearance of PAH. Some of the
para-aminohippuric acid (PAH) in
glomerular blood ( a ) is filtered into
the glomerular (Bowman’s) capsules
( b ). The PAH present in the unfiltered
blood is secreted from the peritubular
capillaries into the nephron ( c ) , so that
all of the blood leaving the kidneys is
free of PAH ( d ). The clearance of PAH
therefore equals the total renal blood
flow.

To peritubular

capillaries

PAH

Filtration

Renal artery

containing PAH

Renal vein

almost no PAH

Ureter

urine containing almost

all PAH that was in

renal artery

To renal vein

Filtration plus

Peritubular capillariessecretion

(a) (b)

(c) (d)

Table 17.5 | Renal “Handling” of Different Plasma Molecules

If Substance Is: Example Concentration in Renal Vein Renal Clearance Rate

Not filtered Proteins Same as in renal artery Zero

Filtered, not reabsorbed or secreted Inulin Less than in renal artery Equal to GFR (115–125 ml/min)

Filtered, partially reabsorbed Urea Less than in renal artery Less than GFR

Filtered, completely reabsorbed Glucose Same as in renal artery Zero

Filtered and secreted PAH Less than in renal artery; approaches zero Greater than GFR; up to total plasma

flow rate (~ 625 ml/min)

Filtered, reabsorbed, and secreted K^1 Variable Variable