All animals possess some mechanism of getting rid of the waste substances produced in their body during metabolic activities. These waste substances include CO2, water, urea, uric acid and ammonia. Such substances can be harmful if retained in the body.

Besides metabolic wastes, excess salt (like NaCl taken in food), H2O and even excess of some vitamins needs to be eliminated. Certain medicines (antibiotics) too are removed from the blood in the urine. Removal of all harmful, unwanted products (specially nitrogenous wastes) from the body is called excretion. Excretory system is primarily associated with removal of nitrogenous wastes.

Urea is the main nitrogenous waste in our body. It is formed by the breakdown of surplus amino acids and nucleic acids in the liver. Blood transports urea to the kidneys for filtration and removal in the form of urine.

Modes of Removal of Nitrogenous Wastes

Depending upon the nitrogenous wastes excreted, animals can be classified as ammonotelic, ureotelic and uricotelic.

Importance of Excretion

  1. Excretion is necessary for the elimination of nitrogenous wastes formed during metabolism of proteins (amino acids) and nucleic acids.
  2. Elimination of excess salts like NaCl, vitamins, bile pigments (from the breakdown of old RBCs) and certain medicines and drugs.
  3. Removal of excess of water or its retention in case of shortage of water. This is to maintain the required quantity of water (osmoregulation) in the body.