Kidney, cross section
The kidneys are a pair of bean-shaped, fist-sized organs found in the lower back. They are protected by layers of fat and muscle and partially sheltered by the ribs. Kidneys have a hollow middle called the renal sinus. The renal sinus is filled with ducts, or calyces, which drain the urine out of the kidneys, and with the renal artery and vein and their branches, which bring blood to and from the kidney.