this way PAH, at low doses, is almost completely cleared from the blood during a single pass through the kidney. (Accordingly, the plasma concentration of PAH in renal venous blood is approximately zero.) Setting Pv to zero in the equation for RPF yields which is the equation for renal clearance. For PAH, this is commonly represented as Since the venous plasma concentration of PAH is not exactly zero (in fact, it is usually 10% of the PAH arterial plasma concentration), eRPF usually underestimates RPF by approximately 10%. This margin of error is generally acceptable considering the ease with which PAH infusion allows eRPF to be measured. Finally, renal blood flow (RBF) can be calculated from a patient's renal plasma flow (RPF) and hematocrit (Hct) using the following equation: . Autoregulation and kidney failure If the kidney is methodologically perfused at moderate pressures (90–220 mm Hg performed on an experimental animal; in this case, a dog), then, there is a proportionate increase of: -Renal Vascular Resistance Along with the increase in pressure. At low perfusion pressures, Angiotensin II may act by constricting the efferent arterioles, thus mainlining the GFR and playing a role in autoregulation of renal blood flow. People with poor blood flow to the kidneys caused by medications that inhibit angiotensin-converting enzyme may face kidney failure. References Bibliography External links Renal Clearance Techniques Renal physiology