On the outside of the body, the male reproductive system consists of the penis and the scrotum. Inside the scrotum are the testicles, which produce sperm cells and male hormones such as testosterone. Each testicle is surrounded by several thin layers of tissue and muscles that allow the scrotum to contract in response to cold and other stimuli.
Each testicle produces millions of sperm cells daily in a healthy adult male. These cells mature as they move through a mass of tightly coiled tubes at the back of each testicle called the epididymis. When they leave the epididymis, sperm cells migrate up two thin tubes called the vas deferens, which widen as they approach the prostate gland.
The prostate gland is about the size of a walnut. It sits just below the bladder, which is where urine is stored. The tube that lets urine flow from the bladder out through the penis is called the urethra. The urethra passes through the prostate.
The prostate gland makes a fluid that is part of semen. During orgasm, sperm in the vas deferens is propelled into the prostate gland where the sperm cells are mixed with prostate fluid. The seminal vesicles on the back of the prostate also provide liquid during orgasm. The resulting mixture is called semen, and it passes through the urethra and comes out at the tip of the penis during ejaculation.