The object of excretion is to remove the unwanted, harmful metabolic waste products from the body in order to maintain the equilibrium of the body.
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If these products are allowed to accumulate instead of their removal from the body, the equilibrium of the body will be disturbed by their mass-action effect, therefore, their removal is very necessary and they must be removed from the body by any son of method.
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In the animal bodies, the following structures are concerned in the excretion of harmful metabolic waste products:
(i) The lungs and gills are the paths for excretion of gaseous wastes,
(ii) The kidneys or some homologous nephritic organs are the paths for excretion of urea, uric acid, ammonia and other nitrogenous compounds which are usually removed in the form of urine,
(iii) The skill also serves as an excretory organs in certain animals and is a path for excretion of some gaseous wastes and inorganic ions,
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(iv) The liver also effects some excretion, chiefly of cholesterol and bile-pigments,
(v) The intestinal epithelium of certain animals also functions as an excretory organ because it excretes certain inorganic constituents which are present in excess in the body,
(vi) The salivary, mammary and tear glands upto some extent also serve as vehicles of excretion of traces of wastes and may excrete a significant amount of foreign substances.
No doubt all the mechanisms are of equal importance because all operate, of course, so as to tend to preserve the steady state of the blood and the body as a whole but the kidneys deserve a special discussion as they can eliminate any excess of non-volatile substances that tend to make the blood either too acidic or too alkaline.
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They are also chief organs for regulation of the osmotic pressures of the blood and other body fluids and for the maintenance of the comparative constancy of the internal environment of the living cells.