The urinary bladder is a sac-like structure that stores urine which is produced by the kidneys. Two tubes, called the ureters, transport urine from the kidneys into the bladder. A small funnel at the bottom of the bladder channels urine through a ring of muscle inside the bladder (the internal sphincter) into a tube called the urethra which is surrounded by another band of muscle (the external sphincter) which passes through the prostate and then down into the penis and out of the body. Enlargement of the prostate (benign prostatic hyperplasia – BPH) or cancer of the prostate can squeeze the urethra and cause problems with the flow of urine.
During urination a special muscle in the bladder, the detrusor muscle, contracts and the sphincter surrounding the urethra relaxes allowing urine to be pushed out of the body. Sometimes the sphincter is damaged during prostate surgery or radiation used to treat prostate cancer. This can cause leaking of urine (urinary incontinence). Although it usually heals after a while some men are left with permanent urinary incontinence. An artificial sphincter can be surgically fitted to control this.

References: Marjorie A et al. Anatomy and Physiology. 1977. Macmillan Publishing Co. Inc.