Sperm, the male gametes or reproductive cells, are produced in the testes and stored in the epididymus (both enclosed in the scrotum). When ejaculation occurs, smooth muscles contract and propel mature sperm from the end portions of the epididymis through the vas deferens, which is just beneath the bladder. From there, the sperm get mixed with nutrient-rich fluids from the seminal vesicles and a milky secretion from the prostate gland. (The prostate gland is situated just below the bladder and encircles the urinary urethra.) This combination of sperm and fluids is called semen, which provides a watery environment in which the sperm cells can swim once outside the body, nutrients for the sperm cells, and protection for the sperm cells by neutralizing acids in the female’s sexual tract.