The kidneys are paired organs of the urinary system that have many homeostatic functions, including regulation of electrolytes, acid-base balance, and blood pressure; excretion of wastes such as urea and ammonium; reabsorption of glucose and amino acids; and production of hormones including vitamin D and erythropoietin.

Located behind the abdominal cavity in the retroperitoneum, the kidneys receive blood from the paired renal arteries, and drain into the paired renal veins. Each kidney excretes its waste product—urine—into a ureter, itself a paired structure that empties into the urinary bladder.

The medical field that studies the kidneys and diseases of the kidney is called nephrology. The prefix nephro- meaning kidney is from the Ancient Greek word nephros (νεφρός); the adjective renal meaning related to the kidney is from Latin rēnēs, meaning kidneys.

Location

In humans, the kidneys are located behind the abdominal cavity, in a space called the retroperitoneum. There are two, one on each side of the spine; they are approximately at the vertebral level T12 to L3. 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 (also called the suprarenal gland). The asymmetry within the abdominal cavity caused by the liver typically results in the right kidney being slightly lower than the left, and left kidney being located slightly more medial than the right.Fact: date=April 2009 The upper (cranial) parts of the kidneys are partially protected by the eleventh and twelfth ribs, and each whole kidney and adrenal gland are surrounded by two layers of fat (the perirenal and pararenal fat) and the renal fascia. Each adult kidney weighs between 125 and 170 g in males and between 115 and 155 g in females. The left kidney is typically slightly larger than the right.Fact: date=April 2009

Blood supply

The kidneys receive blood from the renal arteries, left and right, which branch directly from the abdominal aorta. Despite their relatively small size, the kidneys receive approximately 20% of the cardiac output.

Each renal artery branches into segmental arteries, dividing further into interlobar arteries which penetrate the renal capsule and extend through the renal columns between the renal pyramids. The interlobar arteries then supply blood to the arcuate arteries that run through the boundary of the cortex and the medulla. Each arcuate artery supplies several interlobular arteries that feed into the afferent arterioles that supply the glomeruli.

After filtration occurs the blood moves through a small network of venules that converge into interlobar veins. As with the arteriole distribution the veins follow the same pattern, the interlobar provide blood to the arcuate veins then back to the interlobar veins which come to form the renal vein exiting the kidney for transfusion for blood.