Question

Para aminohippuric acid (PAH) clearance test is employed to measure:

• A. Renal blood flow
• B. Liver blood flow
• C. Cerebral blood flow
• D. Venous blood flow

Answer 1

The Correct Option is A

The Para-Aminohippuric Acid (PAH) clearance test is primarily used to measure renal blood flow. Let's break down the reasoning behind this:

1. Understanding PAH clearance: PAH is a compound that, when introduced into the bloodstream, is almost entirely excreted by the kidneys in the urine. This occurs because PAH is both filtered by the glomerulus and secreted by the renal tubules.
2. Renal Handling of PAH: Since PAH is effectively removed from the plasma by the kidneys, the rate at which it appears in the urine can provide a measure of renal plasma flow (the volume of plasma delivered to the kidneys per unit time).
3. Relation to Renal Blood Flow: Renal blood flow can be calculated by knowing the renal plasma flow and the hematocrit (the volume fraction of red blood cells in blood). The formula used is: \text{Renal Blood Flow} = \frac{\text{Renal Plasma Flow}}{1-\text{Hematocrit}} This formula assumes that only the plasma portion of the blood is cleared of PAH, as red blood cells do not participate in its elimination.
4. Ruling Out Other Options:
   • Liver blood flow: PAH clearance is not related to liver function, as PAH is not specifically processed by the liver.
   • Cerebral blood flow: PAH does not provide information about blood flow in the brain since it is not specifically cleared by cerebral tissues.
   • Venous blood flow: The test specifically measures how well blood is delivered to and processed by the kidneys, not overall venous circulation.

Therefore, the correct answer is Renal blood flow.