Excretion

excretion, the process by which animals rid themselves of waste products and of the nitrogenous by-products of metabolism. Through excretion organisms control osmotic pressure—the balance between inorganic ions and water—and maintain acid-base balance. The process thus promotes homeostasis, the constancy of the organism’s internal environment.

Every organism, from the smallest protist to the largest mammal, must rid itself of the potentially harmful by-products of its own vital activities. This process in living things is called elimination, which may be considered to encompass all of the various mechanisms and processes by which life forms dispose of or throw off waste products, toxic substances, and dead portions of the organism. The nature of the process and of the specialized structures developed for waste disposal vary greatly with the size and complexity of the organism.

Four terms are commonly associated with waste-disposal processes and are often used interchangeably, though not always correctly: excretion, secretion, egestion, and elimination.

Excretion is a general term referring to the separation and throwing off of waste materials or toxic substances from the cells and tissues of a plant or animal.

The separation, elaboration, and elimination of certain products arising from cellular functions in multicellular organisms is called secretion. Though these substances may be a waste product of the cell producing them, they are frequently useful to other cells of the organism. Examples of secretions are the digestive enzymes produced by intestinal and pancreatic tissue cells of vertebrate animals, the hormones synthesized by specialized glandular cells of plants and animals, and sweat secreted by glandular cells in the skins of some mammals. Secretion implies that the chemical compounds being secreted were synthesized by specialized cells and that they are of functional value to the organism. The disposal of common waste products should not, therefore, be considered to be of a secretory nature.

Egestion is the act of excreting unusable or undigested material from a cell, as in the case of single-celled organisms, or from the digestive tract of multicellular animals.

As defined above, elimination broadly defines the mechanisms of waste disposal by living systems at all levels of complexity. The term may be used interchangeably with excretion.