Definition of Urinary System

Urinary system is the body system that removes urea, uric acid, and other metabolic wastes from the blood and eliminates them from the body in the form of urine. This system consists of four main parts: the paired kidneys, the urinary bladder, the ureters, and the urethra.

The kidneys are brownish, bean-shaped organs located on the back wall of the abdominal cavity at about the level of the waist. Each kidney is about 4.5 inches (11 cm) long and 2 inches (5 cm) across. As blood flows through the kidney, about 3 million tiny tubular structures, called nephrons, selectively filter out the metabolic wastes. Water is also removed from the blood and together with the wastes forms the urine. From the nephrons, tiny collecting ducts carry the urine to larger ducts, which then empty into a large funnel-shaped area known as the renal pelvis.

Each kidney's renal pelvis opens into a ureter. The two ureters are nearly 13 inches (32 cm) long and about as thick as a pencil. They carry the urine into the bladder, their muscular walls propelling the urine along by a squeezing action. The ureters enter the bladder at a sharp angle, producing a flaplike valve that prevents any urine from flowing backward into the kidneys. A backflow could cause kidney damage.

The urinary bladder is a hollow, muscular, somewhat pear-shaped organ that, in an adult, holds 10 to 20 fluid ounces (296–592 ml) of urine. It lies behind the pubic bone and rises into the abdomen as it fills. Embedded in the walls of the bladder are nerve endings that send signals to the brain when the bladder needs to be emptied. Other nerves, upon receiving signals from the brain, stimulate the bladder walls to contract and force the urine out. The bladder normally empties completely with each urination.

From the bladder, the urine passes to the outside through the urethra. In men, the urethra measures approximately 7 inches (18 cm) in length and carries semen as well as urine. In women, the urethra is only about 1 or 1.5 inches (3–4 cm) long and carries only urine. In both men and women, the urethra has a slinglike sphincter muscle that allows for the voluntary control of urination.