Excretion is the Process of eliminating waste products of metabolism and other non-useful materials. It is an essential process in all forms of life.
In single-celled organisms, waste products are discharged directly through the surface of the cell. Multicellular organisms utilize more complex excretory methods. Higher plants eliminate gases through the stomata, or pores, on the surface of leaves. Animals have special excretory organs. In humans the main organs of excretion are the kidneys and accessory urinary organs, through which urine is eliminated, and the large intestines, from which solid wastes are expelled. The skin and lungs also have excretory functions: the skin eliminates water and salts in sweat, and the lungs expel water vapor and carbon dioxide.

Etymology

Countercurrent exchange
Eructation
Flatulence
Homeostasis
Osmoregulation
Respiration (physiology)