Why Are the Renal Artery and Vein Critical to the Process

Why are the renal artery and vein critical to the process of osmoregulation in mammals?

A) The kidneys require a constant and abnormally high oxygen supply to function.
B) The renal artery delivers blood with nitrogenous waste to the kidney, and the renal vein brings blood with less nitrogenous wastes away from the kidneys.
C) The kidneys require higher-than-normal levels of hormones.
D) The renal artery and vein are the main pathways regulating how much is produced by the kidneys.