The Human Body

Urine

Urine, pale yellow fluid produced by the kidneys, composed of dissolved wastes and excess water or chemical substances from the body. It is produced when blood filters through the kidneys, which remove about 110 liters (230 pints) of watery fluid from the blood every day. Most of this fluid is reabsorbed into the blood, but the remainder is passed from the body as urine. Urine leaves the kidneys, passes to the bladder through two slender tubes, the ureters, and exits the body through the urethra. A healthy adult can produce between 0.5 to 2 liters (1 to 4 pints) of urine a day, but the quantity varies considerably, depending on fluid intake and loss of fluid from sweating, vomiting, or diarrhea.

Water accounts for about 96 percent, by volume, of the urine excreted by a healthy person. Urine also contains small amounts of urea, chloride, sodium, potassium, ammonia, and calcium. Other substances, such as sugar, are sometimes excreted in the urine if their concentration in the body becomes too great. The volume, acidity, and salt concentration of urine are controlled by hormones. Measurements of the composition of urine are useful in the diagnosis of a wide variety of conditions, including kidney disease, diabetes, and pregnancy.