The removal of toxic, waste metabolic products from the body is known as excretion. All plants and animals produce harmful substances due to a number of metabolic activities occurring in their bodies. Carbon dioxide produced during respiration is removed by lungs. The chief nitrogenous waste produced as a result of metabolism of proteins and amino acids is ammonia. Ammonia is the immediate break down product of amino acids.
It is highly toxic to animals and also highly soluble in water. In many aquatic animals, particularly in animals without backbones, it is rapidly excreted to the surrounding water. So, it does not build up to toxic concentrations. Animals that live on land are not surrounded by water all the time. In such animals ammonia is rapidly synthesized into a more complex but less toxic compound, urea, which is excreted from the body.
The other nitrogenous waste is uric acid, which is excreted in large amounts in reptiles and birds. Complex food materials ingested by man and other organisms get digested in the alimentary canal. The undigested part is evacuated from the body as faces. Starch and fat in the food pose no problem of excretion as their end products are water and carbon dioxide. But end products of protein are water, carbon dioxide, and nitrogenous compound. In other words, proteins are never completely metabolized to yield free nitrogen.
The commonest end products that contain nitrogen in chemical combination are ammonia, urea and uric acid. Urea is produced in the liver and discharged in the blood. When blood circulates, it goes to the kidney as well. Kidney plays a major role in eliminating a host of dissolved substances in blood which are generally harmful to the body. Even those compounds which are not used for growth and which are in far excess in blood, have to be eliminated by the kidney such as excess water, salts, urea, etc. excretion in unicellular plants and animals takes place through diffusion.
Excretion in Higher Plants
Green plants utilize carbon dioxide for photosynthesis which is a metabolic waste product of respiration. Some of the methods of excretion in plants are As follows.
1. Resins, latex, rubber and gums are exuded from various parts of the plant body.
2. In some deciduous plants, the excretory matter is thrown out when the leaves fall.
3. In some plants, tannin is stored in the bark and woody part of the trunk. Due to this the wood appears dark.
4. In the different parts of the plant body, crystals of some chemical substances are set aside, for example calcium carbonate crystals in the leaf of fig, calcium oxalate crystals in the leaf of colocasia. These excretory materials do not harm the plant. Saprophytic plants such as Mucor, Rhizopus and Penictllium excrete their wastes through diffusion.
Organ of Excretion in Animals
In higher animals excretory organs are skin, lungs, liver, and kidney.
Temperature regulation linked with excretion by skin: Skin offers a very large surface for losing body heat. It is supplied with a large number of body vessels (capillaries) along with sweat glands. Sweat glands become activated with the rise in temperature. Sweating leads to cooling of the surface of the skin with the evaporation of sweat. The body conserves and produces extra heat in cold weather. Sweat glands are less active during winter. Heat is produced by rapid involuntary muscular activity when a person is in cold. This is called shivering.
Elimination of Metabolic Wastes through Perspiration Small amounts of urea, salts and other metabolic wastes of most body cells are eliminated through sweat. These waste products are collected from all over the body and sent to the sweat glands through the blood stream.
Liver as Organ of Excretion: Liver also performs excretory functions which are as follows:
(a) Bile pigments arte produced in liver which are discharged in the alimentary canal. These are egested with the faeces.
(b) Metabolic activities of disease-causing organisms in the body result in the production of poisonous (toxic) substances. Theses are carried to the liver and converted into harmless substances. There are some toxic substances which are returned to the blood stream for filtration by the kidneys.
Lungs help in the elimination or release of carbon dioxide from the body. In this way they also act as organs of excretion.
It is the chief excretory organ in vertebrates. In order to study the structure and function of vertebrate kidney, we shall consider the human kidney.