The primary role of the kidneys is to maintain the homeostatic balance of bodily fluids by filtering and secreting metabolites (such as urea) and minerals from the blood and excreting them, along with water, as urine. The kidneys are also important regulators of blood pressure, glucose metabolism and erythropoiesis, since the kidneys are poised to sense plasma concentrations of ions such as sodium, potassium, hydrogen, oxygen, and compounds such as amino acids, creatinine, bicarbonate, and glucose.

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The kidneys are located in the posterior part of the abdominal cavity. There are two, one on each side of the spine; the right kidney sits just below the diaphragm and posterior to the liver, the left below the diaphragm and posterior to the spleen. Above each kidney is an adrenal gland. The asymmetry within the abdominal cavity caused by the liver results in the right kidney being slightly lower than the left one while the left kidney is located slightly more medial. The majority of the water re-absorption in the vertebrate kidney takes place in the loop of Henle. The kidneys are retroperitoneal and range from 9 to 13 cm in diameter; the left slightly larger than the right. The kidneys receive unfiltered blood directly from the heart through the abdominal aorta which then branches to the left and right renal arteries. Filtered blood then returns by the left and right renal veins to the inferior vena cava and then the heart. Renal blood flow accounts for 20-25% of the cardiac output.