The meaning of excretion is most easily understood in the context of vertebrate physiology. The animal swallows food (ingestion). In the stomach and intestine some of the food is broken down into soluble products (digestion) that are absorbed into the body (assimilation). In the body these soluble products undergo further chemical change (metabolism); some are used by the body for growth, but most provide energy for the various activities of the body.
Excretion is 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.
The excretory system is a passive biological system that removes excess, unnecessary materials from the body fluids of an organism, so as to help maintain internal chemical homeostasis and prevent damage to the body. The dual function of excretory systems is the elimination of the waste products of metabolism and to drain the body of used up and broken down components in a liquid and gaseous state. In humans and other amniotes (mammals, birds and reptiles) most of these substances leave the body as urine and to some degree exhalation, mammals also expel them through sweating.
Only the organs specifically used for the excretion are considered a part of the excretory system. In the narrow sense, the term refer to the urinary system. However, as excretion involves several functions that are only superficially related, it is not usually used in more formal classifications of anatomy or function.
As most healthy functioning organs produce metabolic and other wastes, the entire organism depends on the function of the system. Breaking down of one of more of the systems is a serious health condition, for example renal failure.