Excretion
the process in animal and plant physiology which separates from the essential substance or animal body waste matter of no further use in nutrition. The organs so employed partake of the characters of strainers, which retain substances soluble in the blood necessary to health and extrude the harmful. The primary excretory organs in vertebrates are the kidneys.
The function of these is to separate from the blood the waste materials produced by the decomposition of nitrogenous substances and expel them from the body. The lungs and the skin likewise perform some of the duties of excretory organs. In its simplest form the excretory organ is found in the Protozoa, in which the contractile vacuole by means EXECUTION of delicate infusoria drains the waste material and then discharges the contents.