The prostate is a gland in the male reproductive system. The prostate makes and stores a component of semen and is located in the pelvis, under the bladder and in front of the rectum. The prostate surrounds part of the urethra, the tube that empties urine from the bladder. A healthy prostate is about the size of a walnut. Because of the prostate’s location, the flow of urine can be slowed or stopped if the prostate grows too large.
Source: the National Cancer Institute