A single substance may sometimes be an excretion product and at other times, an indispensible metabolite.
Water is a by-product of metabolism and must often be excreted in large amounts to avoid a serious condition of edema.
On the other hand in some animals the metabolic water is the only available source of water, therefore, in them it must be rigorously conserved.
The carbon dioxide is a metabolic by-product of cellular respiration but it is also an important component in the synthetic and regulatory machinery of animals and plants.
The same is true about urea, a prominent constituent of the urine in many animals, sometimes discharges useful physiological functions.
If the blood urea in man rises above about 0-05% (normal values 0 01 to 0 03) a pathological condition of uremia develops, but the elasmobranch fishes actively retain urea for the purpose of osmotic regulation and have normal blood urea values of 2 0 to 2 5% (Smith 1953).
The above facts show that no concise definition of excretion can be based solely on the chemical nature of the material removed and it is better to define excretion in very general way as the process of separation and elimination of water soluble waste products of cellular metabolism.
In other words excretion can be defined as a process by which the byproducts of cellular metabolism are so treated that they take no further part in the metabolism.
These by-products or waste products are removed from the body in aqueous solutions, therefore, water constitutes the bulk of the excreta by weight.
The excretory processes play a most important role in maintaining the relative constancy of the body’s internal environment without which life is impossible.
If the excretory processes fail to eliminate these metabolic wastes from the body, the same may be accumulated in’ the body.
This accumulation disturbs certain delicate acid-base balances in the body and also upsets the osmotic-relationships between blood and lymph and the tissues.
This may lead to even the death of the individual after a short period of their accumulation.
Disorders of the excretory processes kill higher animals much faster than do food deficiencies.
In single-celled animals like protozoans and animals like sponges and jelly fishes excretion occurs directly, i.e., their metabolic wastes are discharged directly into the surrounding medium through their body surface.
To some extent, excretion occurs in a more indirect way by the secretion of waste products into a vacuole which is later extruded from the cell.
In higher animals these products are not discharged directly into surrounding medium because every cell of the body is not in a direct contact with the surrounding environment, therefore, in them each cell discharges its metabolic wastes into the tissue fluid, which in turn reaches the blood stream.
The blood transports these metabolic wastes in the excretory organs which eliminate them outside the body.