Final answer:
The contrast material used to assess blood flow to the kidneys is called para-aminohippuric acid (PAH). PAH is freely filtered through the glomerular membranes and secreted into the proximal convoluted tubules.
Step-by-step explanation:
A contrast material used to assess blood flow to the kidneys is called para-aminohippuric acid (PAH). PAH is a substance that filters freely through the glomerular membranes and is secreted into the proximal convoluted tubules. By measuring the rate of PAH clearance, the rate of plasma flow through the kidneys can be calculated.