urine

Urine consists of excess water and waste products that have been filtered from the blood by the kidneys; it is stored in the bladder until it can be expelled from the body via the urethra. Analysing the composition of an individual's urine can reveal a number of medical conditions, such as poorly functioning kidneys, kidney stones, and diabetes.

Amber-coloured fluid filtered out by the kidneys from the blood. It contains excess water, salts, proteins, waste products in the form of urea, a pigment, and some acid.

The kidneys pass it through two fine tubes (ureters) to the bladder, which may act as a reservoir for up to 0.7 l/1.5 pt at a time. In mammals, it then passes into the urethra, which opens to the outside by a sphincter (constricting muscle) under voluntary control. In reptiles and birds, nitrogenous wastes are discharged as an almost solid substance made mostly of uric acid, rather than urea.