Excretory system aids in elimination of harmful waste products formed in the body. The main excretory organs are lungs, kidneys, skin and large intestine. Lungs throw out carbon dioxide and water vapur. Large intestine excretes waste matter of digestion as faeces and kidneys excrete unite. Skin excretes sweat.

Kidneys take away the nitrogenous end product of the metabolism, chiefly urea. The blood entering kidneys brings urea, uric acid, large quantities of water, sugar and various salts which, by filtration, are passed out with urine. Kidneys also absorb certain useful constituents such as glucose, water and salts. Skin protects the body, regulates the temperature of the body, excretes waste matter, gives the sense of touch and is a store­house of reserve food.