The renal system is also known as the urinary system. Located in the retroperitoneal upper area of the abdominal cavity, the renal system is a collection of the organs and the structures that function in the removal of waste material from the body. In humans the renal system consists of the pair of kidneys, ureters, bladder, and the urethra.

The conversion of incoming nutrients to energy for the body's various processes generates waste. Much of this waste is soluble, that is, it can dissolve in water (and so in blood, which is largely comprised of water). This waste must be removed from the water and expelled from the body.

Waste removal takes place in the kidneys. These are the principle filtration units of the renal system. There are normally two kidneys in the human body. They are located on each side of the abdomen in the...