Final answer:
ERPF is measured with the substance para-aminohippuric acid (PAH), which enables the calculation of the rate of plasma flow through the kidneys based on its clearance and the hematocrit value.
Step-by-step explanation:
The effective renal plasma flow (ERPF) is measured using a substance called para-aminohippuric acid (PAH). PAH filters freely through the glomeruli, and any PAH that does not get filtered is typically secreted into the proximal convoluted tubules. Essentially all of the PAH that passes through the kidneys is excreted in the urine, which makes it an excellent marker for calculating the rate of plasma flow through the kidneys. After obtaining the rate of PAH clearance, by knowing the hematocrit, healthcare professionals can then calculate the rate of total blood flow through the kidneys.