The kidneys, ureters, bladder and urethra make up the urinary system. The kidney’s main function is to extract unwanted substances, including water, from the blood. This fluid waste material, called urine, is transported through the ureters to the bladder for storage. During the act of voiding, the bladder contracts and the urine is expelled from the body through the urethra.

The purpose of urine formation is to regulate the water content and electrolyte composition of the body fluids. Over a period of time, the amount of electrolytes and water excreted by the kidneys very nearly approximates the amount that is taken into the body orally. Although fluid and electrolytes can be lost by other means, such as in sweat or feces, it is the kidneys that have to precisely regulate the internal environment of the body. Today, failure of renal function can be treated by the use of dialysis, or by kidney transplantation.

An important feature of the urinary system is its ability to adapt to wide variations in fluid load, based on the habits of the individual. Basically, the kidney must be able to excrete that which is ingested into the diet and not eliminated by other organs (this translates to 1-1.5 litres of water per day).