We do not know why the prostate gland enlarges with advancing age. It is believed that hormonal changes are the culprit. The prostate gland in an adult male is normally the size of walnut. The prostate gland lies beneath the bladder and surrounds the lower part of the urethra (the tube from which urine leaves the body). Over time, the prostate can grow to twice its original size. When the prostate enlarges, it squeezes the urethra. This can give sensations of an urgent need to urinate, while also making it more difficult to urinate. The enlarged prostate not only compresses the urethra, but also narrows the passage the urine has to flow through, making it more difficult to empty the bladder.