Excretion is the physiological process that eliminates metabolic waste substances from the body. In the vertebrate organisms, it is primarily carried out by the lungs, that excrete the carbon dioxide, skin that eliminates the excess water and salt in the form of sweat and the kidneys that eliminate nitrogenous wastes.

The urethra can be characterised as a short, tube-like muscular structure that extends from the lower end of the urinary bladder to the outside of an individual's body. The base of the urethra is surrounded by circular muscles called sphincter muscles. These muscles contract tightly to close the opening of the urethra. When these muscles relax, the urethra opens and the urinary bladder then contracts to force the urine out. This process is called urination.