Kidney

Organs · Latin: Ren

Open in 3D

The kidneys are two bean-shaped organs, each about 11 cm long, on the back wall of the abdomen behind the peritoneum, level with the twelfth thoracic to third lumbar vertebrae. The right one sits a little lower because the liver is above it. Together they receive about a fifth of all the blood the heart pumps. Each holds about a million nephrons, tiny filters that strain the blood and then take back the water, salt and sugar the body still needs, leaving urine. The kidneys also keep the blood's salt, water and acid in balance, help set blood pressure (through the hormone renin), and make erythropoietin, which tells the bone marrow to make red blood cells, and the active form of vitamin D. The surface shown here is the fibrous capsule, the thin tough coat around the kidney.