The prostate is part of a man's reproductive system. It is located in front of the rectum and under the bladder. It surrounds the urethra, the tube through which urine flows. A healthy prostate is about the size of a walnut.
The prostate makes part of seminal fluid. During ejaculation, seminal fluid helps carry sperm out of the man's body as part of semen.
Male hormones (androgens) make the prostate grow. The testicles are the main source of male hormones, including testosterone. The adrenal gland also makes testosterone, but in small amounts.
If the prostate grows too large, it squeezes the urethra. This may slow or stop the flow of urine from the bladder to the penis.