Excretion : What is Excretion? Example of Excretory Organs and Product

 Excretion : What is Excretion? Example of Excretory Organs and Product

Excretion is defined as the removal of metabolic waste products from the body of living organisms. The common excretory product formed in the bodies of animals are: water, carbon (IV) oxide, mineral salts, bile pigments and nitrogenous waste products; such as urea, uric acid and ammonium compounds.

In plants, metabolic waste products include: water, carbon (IV) oxide, Oxygen, salts crystals, resins, latex alkaloids, anthocyanin and gums. The importance of excretion is to remove substances that could be toxic or poisonous from the body of living organisms. If waste substances are allowed to accummlate in the body, they could prevent the maintenance of constant internal environment and this could lead to death of the organism.

 The maintenance of a constant internal environment, irrespective of changes in the external environment, is known as homeostasis. Processes which contribute to homeostatsis, such as osomoregulation and excretaion, are referred to as homeostatic mechanisms.

 Secretion and egestion should not be with excretion. Secretion is the release of useful substances or products, including some products of metabolism from a cell. Examples include: secretion of saliva by the salivary glands and gastric juice by gastric glands. Egestion, on the other hand, is the removal of undigested food materials, i.e. faeces, from the alimentary canal. The egested materials have not been involved in any metabolic process, since they have not been absorbed into the cells.

Lower organisms, such as, Amoeba and Paramecuim, can get rid of excretory products simply by diffusion through their entire body surface, due to their large surface to volume ratios and the small quantity of metabolic wastes, they produce. They also have contractile vacuoles to assist in both excretion and osmoregulation. This method of excretion is, however, insufficient to eliminate metabolic wastes from larger and more complex organisms, such as, worms, insects, amphibians, fishes, reptiles and mammals.

This is due to their smaller surface to volume ratios and the large amounts of metabolic wastes they produce. These organisms therefore, need special tissues or structures to help them eliminate their metabolic wastes. These structures, concerned with excretion. are known as excretory organs.

In mammals, the four excretory organs are: kidneys, for eliminating water, salts and large quantities of urea; skin, for eliminating water, salts and small quantities of urea; lungs, for eliminating carbon (IV) and alcohol and liver, for eliminating bile pigments.