Urine is formed within the kidneys in functional units known as nephrons. The urine formed within these nephrons passes into collecting ducts, which drain into calyces, which, in turn, drain into the renal pelvis. Each renal pelvis gives rise to a ureter. The ureter is a long tube (25 cm) with a wall composed largely of smooth muscle. It connects each kidney to the bladder and functions as a drainage tube for urine.

The urinary bladder is a hollow organ that is situated anteriorly just behind the pubic bone. It acts as a storage reservoir for urine. The walls of the bladder consist largely of smooth muscle called the detrusor muscle. Contraction of this muscle is mainly responsible for emptying the bladder during voiding. The urethra rises from the bladder.

In the male, the urethra runs through the penis and in the female, it opens just above the vagina. A short distance from its origin, the urethra is encircled by a small bundle of muscle fibers that is called the external urinary sphincter. This sphincter is the major site for control of the initiation of urination.