The prostate is a small gland approximately the size and shape of a walnut. It sits directly below the bladder, in front of the rectum. The prostate is a part of the male reproductive tract. It produces fluid that combines with sperm to make semen.
The prostate surrounds the tube that carries urine from the bladder (urethra). During puberty, the prostate expands evenly. The enlargement that occurs in the second half of life, however, is concentrated in the part of the gland next to the urethra. As the gland gets bigger, it interferes with the flow of urine out of the bladder. That makes the bladder work harder to empty urine. Over time, the problem worsens, and eventually not all the urine can be emptied. The bladder wall also may become thickened, which can result in muscle spasms.