Ppah Cpah Apah Physiology

Clearance ratio

Cinulin

None

Effective renal plasma flow (ERPF)

PPAH

ml/min

Renal plasma flow (RPF)

RPF = CPAH = (UPAH x V/PPAH) EPAH (PPAH — VPAH V PPAH UPAH x VPAH PPAH — VPAH

ml/min

1 - Hematocrit

ml/min

Excretion rate

Excretion rate = Us x V

mg/min, mmol/min, or mEq/min

Reabsorption rate

Reabsorption rate = Filtered load - Excretion rate = (GFR x Ps) - (Us x V)

mg/min, mmol/min, or mEq/min

Secretion rate

Secretion rate = Excretion rate - Filtered load

mg/min, mmol/min, or mEq/min

S, a substance; U, urine concentration; V, urine flow rate; P, plasma concentration; PAH, para-aminohippuric acid; PPAH, renal arterial PAH concentration; Epah, PAH extraction ratio; VPAH, renal venous PAH concentration.

S, a substance; U, urine concentration; V, urine flow rate; P, plasma concentration; PAH, para-aminohippuric acid; PPAH, renal arterial PAH concentration; Epah, PAH extraction ratio; VPAH, renal venous PAH concentration.

clearance refers to the volume of plasma that would be necessary to supply the amount of substance excreted in the urine per unit time. Stated mathematically,

Cs x Ps = Us x V, where Cs is the clearance rate of a substance s, Ps is the plasma concentration of the substance, Us is the urine concentration of that substance, and V is the urine flow rate. Rearranging this equation, clearance can be expressed as

Thus, renal clearance of a substance is calculated from the urinary excretion rate (Us x V) of that substance divided by its plasma concentration.

5200. Inulin, which is not produced in the body, is found in the roots of certain plants and must be administered intravenously to a patient to measure GFR.

Figure 27-17 shows the renal handling of inulin. In this example, the plasma concentration is 1 mg/ml, urine concentration is 125 mg/ml, and urine flow rate is 1 ml/ min. Therefore, 125 mg/min of inulin passes into the urine. Then, inulin clearance is calculated as the urine excretion rate of inulin divided by the plasma concentration, which yields a value of 125 ml/min. Thus, 125 milliliters of plasma flowing through the kidneys must be filtered to deliver the inulin that appears in the urine.

Inulin is not the only substance that can be used for determining GFR. Other substances that have been used clinically to estimate GFR include radioactive iothalamate and creatinine.

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