Inulin Clearance Equals the Glomerular Filtration Rate

An important measurement in the evaluation of kidney function is the glomerular filtration rate (GFR), the rate at fo[o[o|°l°[o|°I°l° ° i " * ----

The principle behind the measurement of glomerular filtration rate (GFR). PIN =

plasma [inulin], UIN = urine [inulin], V = urine flow rate, CIN = inulin clearance.

The principle behind the measurement of glomerular filtration rate (GFR). PIN =

plasma [inulin], UIN = urine [inulin], V = urine flow rate, CIN = inulin clearance.

which plasma is filtered by the kidney glomeruli. If we had a substance that was cleared from the plasma only by glomerular filtration, it could be used to measure GFR.

The ideal substance to measure GFR is inulin, a fructose polymer with a molecular weight of about 5,000. Inulin is suitable for measuring GFR for the following reasons: It is freely filterable by the glomeruli. It is not reabsorbed or secreted by the kidney tubules. It is not synthesized, destroyed, or stored in the kidneys. It is nontoxic.

Its concentration in plasma and urine can be determined by simple analysis.

The principle behind the use of inulin is illustrated in Figure 23.6. The amount of inulin (IN) filtered per unit time, the filtered load, is equal to the product of the plasma [inulin] (Pin) X GFR. The rate of inulin excretion is equal to UIN X V. Since inulin is not reabsorbed, secreted, synthesized, destroyed, or stored by the kidney tubules, the filtered inulin load equals the rate of inulin excretion. The equation can be rearranged by dividing by the plasma [inulin]. The expression UINV/PIN is defined as the inulin clearance. Therefore, inulin clearance equals GFR.

Normal values for inulin clearance or GFR (corrected to a body surface area of 1.73 m2) are 110 ± 15 (SD) mL/min for young adult women and 125 ± 15 mL/min for young adult men. In newborns, even when corrected for body surface area, GFR is low, about 20 mL/min per 1.73 m2 body surface area. Adult values (when corrected for body surface area) are attained by the end of the first year of life. After the age of 45 to 50 years, GFR declines, and is typically reduced by 30 to 40% by age 80.

If GFR is 125 mL plasma/min, then the volume of plasma filtered in a day is 180 L (125 mL/min X 1,440 min/day). Plasma volume in a 70-kg young adult man is only about 3 L, so the kidneys filter the plasma some 60 times in a day. The glomerular filtrate contains essential constituents (salts, water, metabolites), most of which are reabsorbed by the kidney tubules.

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