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The prostate gland, which is normally about the size and shape of a walnut, wraps around the urethra between the pubic bone and the rectum, below the bladder. In the early stage of prostate enlargement, the bladder muscle becomes thicker and forces urine through the narrowed urethra by contracting more powerfully. As a result, the bladder muscle may become more sensitive, causing a need to urinate more often and more suddenly.

The prostate grows larger due to an increase in the number of cells (hyperplasia). However, the precise reason for this increase is unknown. A variety of factors may be involved, including androgens (male hormones), estrogens, growth factors and other cell signaling pathways.

As the prostate grows larger and the urethra is squeezed more tightly, the bladder might not be able to fully compensate for the problem and completely empty. In some cases, blockage from prostate enlargement may cause repeated urinary tract infections and gradually result in bladder or kidney damage. It may also cause a sudden inability to urinate (acute urinary retention) -- a medical emergency.

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