Renal System

The body's excretory system is made up of a pair of kidneys and ureters, urinary bladder, and urethra. Kidneys do most of the work; the other structures just transport and store urine.

The kidneys are bean-shaped organs, about four inches long and weighing only five ounces. They function as extraordinarily efficient chemical treatment plants, cleansing the blood of urea and other toxic wastes while maintaining the proper balance of fluid, salts, and other blood components. They are also instrumental in maintaining blood pressure.

The renal arteries branch off the abdominal aorta and carry a prodigious amount of blood. Each day, up to 500 quarts of fluid circulate through the kidneys. After it is cleansed, most of this fluid is returned to the bloodstream; only two to four pints are excreted as urine. This waste material collects in the central portion of the kidney -- the renal pelvis -- and from there it passes into the ureter, a long, narrow tube that carries the urine to the bladder.

A normal adult bladder can hold about one pint of liquid, but when it is about half full, it begins to send nerve signals of an urge to urinate. Voluntary muscles in the pelvic floor control bladder function; when these muscles drop, the sphincter that controls the bladder opening relaxes and urine flows into the urethra. The female urethra is about 1.5 inches long and carries only urine; the 8-inch male urethra transports both semen and urine.