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Excretion is the removal of toxic (nitrogenous) waste products of metabolism from the body.  The process of maintenance of osmotic and ionic concentration of body fluids is called osmoregulation.

In excretion, the waste end products of catabolism containing carbon, hydrogen, oxygen and nitrogen are eliminated from the body. In osmoregulation, the concentration of internal salts and water is maintained.

Osmoregulation and excretion are intimately related; these processes together maintain homeostasis (i.e. staying the same), and are performed by the same set of organs. The kidney is the major organ of osmoregulation and excretion in vertebrates.