The excretory system removes cellular wastes and helps maintain the salt-water balance in an organism. In providing these functions, excretion contributes to the body's homeostasis, the maintenance of constancy of the internal environment. When cells break down proteins, they produce nitrogenous wastes, such as urea. The excretory system serves to remove these nitrogenous waste products, as well as excess salts and water, from the body. When cells break down carbohydrates during cellular respiration, they produce water and carbon dioxide as a waste product. The respiratory system gets rid of carbon dioxide every time we exhale. The digestive system removes feces, the solid undigested wastes of digestion, by a process called elimination or defecation.

The main excretory system in humans is the urinary system. The skin also acts as an organ of excretion by removing water and small amounts of urea and salts (as sweat). The urinary...