In a general way excretion can be defined as the separation and ejection of the metabolic wastes from the body, usually in aqueous solution. Metabolism produces a number of byproducts. Some of these byproducts are compulsorily excreted while others are used in different metabolic pathways. The excretory materials are removed from the body with the help of specialised excretory organs.
The excretory organs have a number of functions. All these functions have the aim to maintain a constant internal environment of the organism. To maintain a constant internal composition there is always present a basic requirement.
Any material that an organism takes in must be balanced by an equal amount removed. The removal of materials from the body, after the separation of essential metabolic end products, involves a very complicated process.
However, in general the functions of excretory system can be listed as follows:
(i) Maintenance of proper concentrations of individual ions (Na+, K+, CI–, Ca++, H+ etc.).
(ii) Maintenance of proper body volume (water content).
(iii) Maintenance of osmotic concentration.
(iv) Removal of metabolic end products.
(v) Removal of foreign substances and/or their metabolic products.
In an animal body all the above mentioned functions are performed synchronously by a variety of ways. The first three functions are performed by skin, glands, kidneys and various other organs in different organisms. One of the major metabolic end product, CO2, is removed by the respiratory organs. Other metabolic end products along with a variety of foreign substances, are removed by excretory organs.
There are different varieties of excretory organs present in animal kingdom. However, in principle, only two basic processes are responsible for the formation of the excretory fluid – ultrafiltration and active transport.
This is a process of filtration where pressure forces a fluid through a semipermeable membrane that withholds proteins and similar large molecules but allows water and small molecular solutes, such as salts, sugars and amino acids, to pass. This type of filtration occurs in glomerulus of kidney.
This is a type of biological trans-membrane movement of solute against its electrochemical gradient by processes requiring the expenditure of metabolic energy. This is also found in kidney to reabsorb the necessary solutes from the ultra-filtered solution.