Urine

Urine is a liquid waste product filtered from blood by the kidneys, then stored in the bladder until it leaves the body. It consists of water, dissolved substances, and waste products, chiefly urea, which results from the breakdown of protein.

Urine is part of the body's system for maintaining the balance of water, salt, and acid. For this reason the amount and composition of urine vary depending on fluid intake, diet, temperature, and a person's health.

How it is formed

The kidneys contain filtering units called nephrons. First, blood is filtered through a part called the glomerulus, which produces large amounts of fluid. Then the kidney tubules reabsorb most of the water, glucose, and salt the body needs. What remains becomes urine, which passes through the ureters to the bladder.

Composition and colour

Urine is mostly water. It also contains urea, creatinine, uric acid, and electrolytes such as sodium and potassium. The pale yellow colour comes from a substance called urochrome. When a person drinks a lot of fluid, urine becomes colourless; when fluid intake is low, it becomes darker and more concentrated.

Health applications

Urinalysis is a standard test used to detect health conditions. Protein or blood in urine may indicate kidney disease; glucose may indicate diabetes; bacteria and white blood cells may indicate urinary tract infection. Urine is also used to measure pregnancy and certain medications.