Urine

Urine, waste product consisting of a dilute solution of excess salts and nitrogenous material, such as urea and deaminated protein, excreted by many animals. The wastes are filtered from the blood by the kidneys or equivalent structures and stored in the bladder until excreted. The passage of urine not only serves to eliminate wastes, but also provides a mechanism for maintaining the water and salt concentrations and pH of the blood. While all mammals excrete nitrogenous wastes in urine, other animals—birds, insects, and fish—excrete them as ammonia or in solid crystals as uric acid.