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Many metabolic reactions take place in cells of living organisms producing waste products. Most of these are toxic. Thus they should be separated and removed from the bodies. This process is called excretion.

During the metabolic processes some useful substances are produced by cells and released outside the cells to be used. These include hormones, enzymes, hormones, mucus and sebum. This process is called secretion.

In the process of digestion of undigested and indigestible materials are removed from the alimentary canal in solid or semi-solid form. This process is called egestion.