杨宇 Department of physiology Shenyang Medical College

Section 6 Use of Clearance Methods to Quantify Kidney Function

• Definition of clearance: The volume of plasma from

which a substance is completely removed by the kidney in a given amount of time (usually a minute).

杨宇 Department of physiology Shenyang Medical College

Clearance is a useful concept for assessing elements of renal functionsuch as:1. Glomerular filtration rate (GFR)2. Renal plasma flow (RPF)3. Filtration fraction (FF)

杨宇 Department of physiology Shenyang Medical College

Mathematically: • Cs × Ps = Us × V • Where: Cs is the clearance rate of a substance S. Ps is the plasma concentration of the substance S. Us is the urine concentration of substance S. V is the urine flow rate.

杨宇 Department of physiology Shenyang Medical College

Rearranging this equation, clearance can be expressed as

Us × V Cs = Ps

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

杨宇 Department of physiology Shenyang Medical College

Inulin Clearance Can Be Used To Estimate GFR

Inulin, a polysaccharide molecule (polymer of fructose) with a molecular weight of about 5200 D.

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.

Inulin, neither reabsorbed, secreted, nor metabolized by the cells of the nephron.

杨宇 Department of physiology Shenyang Medical College

杨宇 Department of physiology Shenyang Medical College

PAH (para-amino hippurate ) Clearance Can Be Used To Estimate Renal Plasma Flow

The PAH that filters freely through the glomerular membranes. However, unlike inulin, any PAH remaining in the peritubular capillary plasma after filtration is secreted into the proximal convoluted tubules.

Therefore, essentially all PAH passing through the kidneys appears in the urine.

杨宇 Department of physiology Shenyang Medical College

The amount of the substance delivered to the kidneys in the blood (renal plasma flow × Ps) would be equal to the amount excreted

in the urine (Us × V). As Us × V RPF = = Cs

Ps

PAH Clearance Can Be Used To Estimate Renal Plasma Flow

杨宇 Department of physiology Shenyang Medical College

杨宇 Department of physiology Shenyang Medical College

PAH is about 90 per cent clearance from the plasma

• Therefore, the clearance of PAH can be used as an approximation of renal plasma flow.

• If the extraction ratio for PAH is 90 per cent, the actual renal plasma flow can be calculated by diving 585ml/min by 0.9, yielding a value of 650ml/min.

• Thus, total renal plasma flow = clearance of PAH / extraction ratio of PAH.

杨宇 Department of physiology Shenyang Medical College

杨宇 Department of physiology Shenyang Medical College

Filtration fraction (FF)

GFR CIN 125FF = = = ERPF CPAH 650

 FF = 0 .20