The prostate is a small walnut-sized gland that lies just under the bladder. It is composed primarily of glandular tissue contained within a firm vascular outer capsule. The prostate surrounds the urethra and contributes one of the fluids that make up semen.
Also contained in the region of the prostatic urethra are two sphincter muscles that control urine flow. The external sphincter which is on the “down-stream” end of the prostate is primarily responsible for controlling urine flow. The internal sphincter muscle which lays right at the bladder neck also assists in urine flow control but is also important as a “valve” that closes during ejaculation thereby deflecting the seminal fluid down the urethra rather than allowing it to simply enter the bladder.