Clearance of Urea

Urea may be used as an example of how the clearance calculations can reveal the way the kidneys handle a molecule. Urea is a waste product of amino acid metabolism that is secreted 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 = 2 ml/min U = 7.5 mg/ml of urea P = 0.2 mg/ml of urea

Urea clearance =-2-= 75 ml/min

The clearance of urea in this example (75 ml/min) is less than the clearance of inulin (120 ml/min). Thus, even though 120 ml of plasma filtrate entered the nephrons per minute, only the amount of urea contained in 75 ml of filtrate is excreted. The kidneys therefore must reabsorb some of the urea that is filtered. Despite the fact that it is a waste product, a significant portion of the filtered urea (ranging from 40% to 60%) is always reabsorbed. This is a passive transport process that occurs because of the presence in the tubules of carriers for the facilitative diffusion of urea.

Table 17.4

Effects of Filtration, Reabsorption, and Secretion on Renal Plasma Clearance

Term

Definition

Effect on Renal Clearance

Filtration

A substance enters the glomerular ultrafiltrate.

Some or all of a filtered substance may enter the urine and be "cleared" from

the blood.

Reabsorption

A substance is transported from the filtrate, through

Reabsorption decreases the rate at which a substance is cleared; clearance

tubular cells, and into the blood.

rate is less than the glomerular filtration rate (GFR).

Secretion

A substance is transported from peritubular blood,

When a substance is secreted by the nephrons, its renal plasma clearance is

through tubular cells, and into the filtrate.

greater than the GFR.

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+

Variable

Variable

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Responses

  • severo loggia
    How does kidney handle urea?
    8 years ago
  • yolanda
    What is urea clearance?
    11 months ago

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