Role of Other Organs in Excretion

Although the kidneys carry out the great bulk of the body's excretory work, several other organs also contribute to removing waste from the body, each specialising in a different class of by-product, and together they meaningfully reduce the load the kidneys would otherwise have to handle alone.

The skin contributes to excretion chiefly through the activity of its sweat glands, which are distributed over most of the body surface. Sweat is a watery secretion containing dissolved sodium chloride and other salts, small amounts of urea, and lactic acid (a by-product of muscular activity), so that sweating removes a modest but genuine quantity of nitrogenous and other waste from the body even as its primary physiological role is thermoregulation, cooling the body through the evaporation of water from the skin surface. The sebaceous glands of the skin also secrete sebum, which carries away some fatty waste products along with its main role of lubricating and waterproofing the skin and hair.

The lungs contribute to excretion on a very large scale, though the substances they remove are gaseous rather than dissolved in a liquid. With every exhalation, the lungs expel carbon dioxide, the chief waste product of cellular (aerobic) respiration throughout the body, produced in enormous quantity continuously by essentially every living cell; a considerable amount of water vapour is also lost with each breath as the air passing over the moist respiratory surfaces picks up water and carries it away. Because carbon dioxide is generated so continuously and in such large amounts, the lungs in fact remove a far greater mass of waste from the body over the course of a day than either the skin or, in absolute terms, even the kidneys handle as solid or dissolved waste, though the kidneys remain indispensable for the specific task of clearing nitrogenous waste and regulating the fluid and ionic composition of the blood. …