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.
Post a Comment