The Urinary System

The urinary system is concerned with the formation and elimination of urine. In an adult, more than 2,500 pints of blood passes through the kidneys each day. Blood enters via the renal arteries and is filtered to remove most of the waste products of metabolism. Seven pints of filtrate are produced every hour. Purified blood returns to the body circulation via the renal veins. The filtering process is carried out by more than two million tiny kidney units, or nephrons, which produce a highly concentrated solution of chemicals known as urine, which is harmful to the body if allowed to remain. Urine flosw from the nephrons, first into the funnel-shpaed renal pelvis and then into the the ureter. Waves of mucular contraction passing down the ureters push the urine into the bladder. With continuous filling, the bladder, a muscular bag, expands until it holds about one pint of fluid. A circular band of muscle around the neck of the bladder, the sphincter, controls the release of urine from the body.