EXCRETION

   Metabolism involves two major processes - (i) synthesis of compounds, known as anabolism or (ii) breakdown of compounds called as catabolism. In a biological system, breakdown of compounds produces products which are either utilised by the body for various purposes or consumed in a reaction or removed from the body. The breakdown of proteins,  in particular and other nitrogenous compounds, which are not utilised by the body for repair, growth or synthesis, produce the so called " waste materials". These waste materials are actually the breakdown products of proteins forming various kinds of nitrogenous compounds viz. urea, ammonia, uric acid and creatinine. These are called as nitrogenous waste products. They cannot remain in the body for long because they are highly toxic and therefore must be removed from the body. This process of removal of nitrogenous wastes from the body is known as excretion. Further, the removal of these wastes requires water. The amount of water required varies with the type of nitrogenous compound excretion. Further, the removal of these wastes requires water. The amount of water required varies with the type of nitrogenous compound excreted, e.g., removal of ammonia requires lot of water. Therefore, organisms living in water or aquatic organisms produce this as a waste product and are known as ammontelic organisms. The animals living on land, the terrestrial animals who  need to conserve water, however, excrete uric acid or urea and are therefore called as uricotelic or ureotelic organisms respectively, depending upon the nature of excretory product. Uric acid required very little water during excretion while urea requires moderate amount. Thus, fishes are ammonotelic whereas reptiles snakes and lizards) and birds are uricotelic and mammals including man are ureotelic in nature.


Excretory Structures in different animals
1. Diffusion trough cell membrane amoeba, sponge, etc.
2. Nephridia - earthworm 
3. Flame cells  -Flat worms 

4. Malapighian tubules - lnsects
5.  Kidneys -Man

In the animal world, there are many different types of organs of excretion ivz, flame cells, or malpighian tubules i lower animals and kidneys in human beings. The nitrogenous wastes are removed from body in man,  in the form  urine, effected by kidneys.