Renal Clearance of Inulin Measurement of GFR

If a substance is neither reabsorbed nor secreted by the tubules, the amount excreted in the urine per minute will be equal to the amount that is filtered out of the glomeruli per minute. There does not seem to be a single substance produced by the body, however, that is not reabsorbed or secreted to some degree. Plants such as artichokes, dahlias, onions, and garlic, fortunately, do produce such a compound. This compound, a polymer of the monosaccharide fructose, is inulin. Once injected into the blood, inulin is filtered by the glomeruli, and the amount of in-ulin excreted per minute is exactly equal to the amount that was filtered per minute (fig. 17.22).

If the concentration of inulin in urine is measured and the rate of urine formation is determined, the rate of inulin excretion can easily be calculated:

where

Quantity excreted per minute =

V

x U

(mg/min) |

f ml N

\ fmg'

V min,

I V ml.

rate of urine formation inulin concentration in urine

The rate at which a substance is filtered by the glomeruli (in milligrams per minute) can be calculated by multiplying the milliliters of plasma filtered per minute (the glomerular

To peritubular capillaries

Renal vein inulin concentration lower than in renal artery

Ureter urine containing all inulin that was filtered

To peritubular capillaries

Renal vein inulin concentration lower than in renal artery

Ureter urine containing all inulin that was filtered

■ Figure 17.22 The renal clearance of inulin. (a) Inulin is present in the blood entering the glomeruli, and (b) some of this blood, together with its dissolved inulin, is filtered. All of this filtered inulin enters the urine, whereas most of the filtered water is returned to the vascular system (is reabsorbed). (c) The blood leaving the kidneys in the renal vein, therefore, contains less inulin than the blood that entered the kidneys in the renal artery. Since inulin is filtered but neither reabsorbed nor secreted, the inulin clearance rate equals the glomerular filtration rate (GFR).

filtration rate, or GFR) by the concentration of that substance in the plasma, as shown in this equation:

Quantity filtered per minute =

GFR x

P

(mg/min)

f—1 (

mg 1

V min J '

P = inulin concentration in plasma

Since inulin is neither reabsorbed nor secreted, the amount filtered equals the amount excreted:

GFR x P = V x U (amount filtered) (amount excreted)

If the preceding equation is now solved for the glomerular filtration rate,

Suppose, for example, that inulin is infused into a vein and its concentrations in the urine and plasma are found to be 30 mg per ml and 0.5 mg per ml, respectively. If the rate of urine formation is 2 ml per minute, the GFR can be calculated as:

This equation states that 120 ml of plasma must have been filtered each minute in order to excrete the measured amount of inulin that appeared in the urine. The glomerular filtration rate is thus 120 ml per minute in this example.

Measurements of the plasma concentration of creatinine are often used clinically as an index of kidney ^ function. Creatinine, produced as a waste product of muscle creatine, is secreted to a slight degree by the renal tubules so that its excretion rate is a little above that of in-ulin. Since it is released into the blood at a constant rate, and since its excretion is closely matched to the GFR, an abnormal decrease in GFR causes the plasma creatinine concentration to rise. Thus, a simple measurement of blood creatinine concentration can indicate whether the GFR is normal and provide information about the health of the kidneys.

Clinical Investigation Clues

Remember that Emily had mild oliguria, edema, and an elevated plasma creatinine concentration. What does an elevated plasma creatinine concentration suggest? How might this be related to Emily's oliguria and edema?

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