The prostate is a gland that produces the liquid part of semen. It's about the size of a walnut and lies at the base of your bladder. The front of your prostate surrounds your urethra-the tube that carries urine from your bladder and out through your penis. Any change in the size or shape of your prostate, such as changes that occur if you have prostate cancer, can narrow this tube, making it difficult for you to pass urine.
Attribution: BUPA