The conversion of foodstuff to energy and various body processes produces excess compounds that cannot be used by the body. They must be removed or else the body will become toxic. The process of excretion involves finding and removing waste materials produced by the body.

The main organs in the human body responsible for excretion are the lungs, kidneys and the skin. The lungs dissipate waste gases that are carried to them in blood via veins. The gases diffuse across the membrane of the many alveoli in the lung and are subsequently passed out of the body during exhalation. The skin is used to remove dead cells, by sloughing off, and sweat, by evaporation. Liquid waste is removed from the body through the kidneys.

The pair of kidneys are small, about 3.9 in. (10 cm) long, and are shaped like beans. Blood is routed through the...