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# Estimated renal plasma flow: PAH clearance

Because glomerular filtration rate accounts for only about 20% of blood flow through the kidneys, a substance that is completely eliminated from the plasma must be excreted in the renal tubules as well as by glomerular filtration.

In theory, if a substance is completely eliminated from the plasma, its clearance is equal to the renal plasma flow (RPF). In other words, the amount of that substance is transferred to the kidneys in the blood (RPF × Ps) will be equal to the amount of that substance excreted in the urine (Us x V).

Hence, the RPF can be calculated as follows:

RPF = (Us x V)/Ps = Cs

Because glomerular filtration rate accounts for only about 20% of blood flow through the kidneys, a substance that is completely eliminated from the plasma must be excreted in the renal tubules as well as by glomerular filtration. No substances are known to be completely eliminated from the body by the kidneys.

Figure. Calculates renal plasma flow through the clearance of para-amino hippuric acid (PAH). PAH is freely filtered through glomerular capillaries and excreted in the renal tubules. The amount of PAH in the plasma of the renal artery is equal to the amount of PAH in the urine. Therefore, plasma flows through the kidneys.

However, there is a substance, PAH, which is eliminated about 90% from plasma. Therefore, PAH clearance is approximately equal to RPF. To be more precise, we can determine the percentage of PAH remaining after it passes through the kidneys. The percentage of PAH that is eliminated from the bloodstream is called the excretion rate of PAH and is above 90 percent on average in the normal kidney. In the pathological kidney, this rate will decrease due to the failure of the renal tubule and its ability to excrete PAH into the lumen.

Calculate the RPF can be calculated as an example: Assuming the concentration of PAH in the serum is 0.01 mg/ml, the concentration in the urine is 5.85 mg/ml, the urine flow is 1 ml/min. The PAH clearance can be calculated by the PAH excretion flow (5.85 mg / ml x 1 ml / min) divided by the plasma concentration of PAH (0.01 mg / ml). Therefore, the PAH clearance will be 585 ml/min.

If the excretion rate of PAH was 90%, the actual renal blood flow would be calculated by dividing 585 ml/min by 0.9, the result would be 650 ml/min.

Thus, renal plasma flow (RFP) can be calculated using the formula:

Renal plasma flow = (PAH clearance) / (rate of PAH excretion)

The PAH excretion rate (EPAH) is calculated by the difference between the renal artery PAH concentration (PPAH) and the renal vein (VPAH), then divided by the renal artery PAH concentration:

EPAH = (PPAH - VPAH)/ PPAH

Renal blood flow can be calculated using RPF and haematocrit (the percentage of red blood cells in the blood):

If the haematocrit is 0.45 and the RPF is 650 ml/min, the renal blood flow will be calculated by 650 / (1-0.45) and by 1182 ml/min.

To calculate the filtration fraction, which is the percentage of plasma filtered through the glomeruli, we must first know the RPR (PAH clearance) and glomerular filtration rate (inulin clearance). If the RPF is 650 ml/min and the glomerular filtration rate is 125 ml/min, then the filtration fraction (FF) is calculated:

FF = GFR/RPF = 125/650 = 0.19