Kidneys are dark reddish-brown coloured, bean-shaped organs of the excretory system, located in the back of the abdomen. Kidneys filter liquid waste and work as a disposal system of the body. Each kidney is connected with a renal artery and a renal vein. Nephrons are the basic functional units of a kidney. A kidney houses a large number of nephrons (coiled tubes) in it. Kidneys also help in the regulation of water balance, concentration of salt in blood and acid-base balance, along with the formation of red blood cells.