The kidney is one of the major homeostatic devices of the body. The organ maintains water balance and expels metabolic wastes in vertebrates and some invertebrates. In primal and embryonic organisms, kidneys have sets of specialized tubules that empty into collecting ducts that pass urine into a primitive bladder.

In humans, the kidneys are two, bean-shaped organs that are located in the back part of the abdomen, on either side of the spine. Due to the presence of many blood vessels, the kidneys appear dark reddish brown. In an average adult, each kidney is 5 to 6 inches long and weighs approximately 5 ounces. The left kidney is frequently located slightly higher in the body than the right.

The functional unit of the human kidney is the nephron. There are about one million nephrons present in the cortex of each kidney and together they are responsible for the purification and filtration of the blood. As blood flows through the kidneys, the nephrons determine the amount of water, salts, glucose, urea and other minerals it contains. When excess substances are present, the surfeit quickly passes out in the urine. However, the kidneys accelerate their reclamation of these same substances when they are present in the blood in amounts that are inadequate.