Excretion is the process of eliminating waste products of metabolism and other non-useful materials.It is an essential process in all forms of life. It contrasts secretion, where the substance may have specific tasks after leaving the cell.

In single-celled organisms, waste products are discharged directly through the surface of the cell. Multicellular organisms utilize more complex excretory methods. Higher plants eliminate gases through the stomata, or pores, on the surface of leaves. Animals have special excretory organs.

Urine is an aqueous solution of electrolytes and metabolites excreted by mammals, birds, reptiles, fish and amphibians. Although urine is excreted as a paste (uric acid) by most birds, it is commonly excreted as a fluid varying in color from clear, when dilute, to a dark amber, when concentrated. Urine is produced by the kidneys, and plays a vital role in maintaining homeostasis by removing excess water, electrolytes such as sodium, chloride, potassium, and calcium ions, urea and other metabolites from the blood. Urine excreted by healthy kidneys is sterile. The production of urine is called diuresis. Urine samples can be taken by your doctor to check for symptoms of disease.