The kidneys are the most important excretory organs within the human body. The word excretion means the removal of waste substances from the body. Metabolism produces a large variety of substances in addition to energy. This "metabolic waste" is the large number of chemical reactions that occur in the cells, tissues and organs. Of these substances some of the by-products and end-products of metabolism are toxic. Therefore they will have to be removed, before they will poison the body tissues. The wastes that are eliminated are "excretory" products. Kidneys play a major role in this task.