Urine is formed in human beings in a manner substantially similar to the way urine is produced in similar mammal species. Essentially, it is a liquid manufactured in the kidneys in order to store and then remove from the body various waste by-products made by cells in the body, which would otherwise be harmful if they built up in the blood. This process, known as urination or micturition, begins with urine forming in the kidneys before ending with the removal of urine from the body via the urinary bladder and then the urethra. Renal physiology is the area of medical science which studies the kidney and urine production.
Urine is one of the byproducts of the functioning of the kidneys, a pair of specialized organs (although only one is absolutely necessary for sustained health, having two kidneys means that a person can afford to lose one, either to medical problems or through donation to a loved one) which essentially act as filters for the blood in the circulatory system. A wide variety of chemicals are filtered from the blood by the kidneys in order to regulate their levels in the blood. Once this filtering has been accomplished, by sets of specialized cells called nephrons, the cleansed blood is then returned to the general circulatory system.
In the meantime, however, the filtering produces a small amount of waste material. The most important component is urea (from which the term "urine" is drawn), a waste product which results from the body's method of processing protein in our diet. However, the kidney also filters out a variety of other chemicals, including a pigment called urochrome which colours urine yellow. The largest component of urine, by volume, is simply water, which the kidneys remove in order to hold all of the other chemicals. Removing water from the blood for use in urine is also an important way of regulating and removing excess water in the body after too much has been drunk in a short time period.
Once produced, urine is moved through ureters, or tubes, to a special organ, the bladder, where it is stored until the individual relieves himself or herself. The bladder normally holds a certain amount of urine, but eventually fills to the point where pressure becomes uncomfortable and a person feels the need to "go to the bathroom." Normally, an adult human should produce about 1.5 quarts (1.4 litres) of urine daily.