The male sex organs comprise the penis and two testicles, or testes, which are located in a pouch, called the scrotum. The testicles (male counterparts of the woman's ovaries) produce spermatozoa, or sperm.
In contrast to the woman, who is born with a life time quota of eggs, the man's testicles generate a new complement of sperm approximately every 70 days. The sperm begin to mature in the testicles and continue to develop as they travel through a long, thin coiled tubal system in the scrotum called the epididymis. The epididymis is connected to a straight thicker tube called the vas deferens. Just before the vas deferens enters the penis it joins the urethra, which originates in the bladder and allows the passage of urine from the bladder through the penis. Sperm are transported through this system by muscular contractions.
Several glands, including the seminal vesical and the prostrate gland, are located along this tract. These glands release a large amount of milky secretions that nurture and promote the survival of the sperm. The combination of sperm and milky fluid that is ejaculated during erotic experiences is known as semen. Urine is prevented from mixing with semen in the urethra because the bladder-urethra opening constricts during ejaculation; similarly, closure of the vas deferens-urethra juncture prevents passage of semen during urination. In certain cases, removal of a diseased prostrate gland may compromise this separation effect and cause the man to ejaculate backward into the bladder rather that outward through the penis; this is known as retrograde ejaculation. This condition may cause infertility, but it can be treated by inseminating the woman with sperm separated from urine, the man would pass immediately following orgasm.
Microscopically, sperm resemble tadpoles. Each sperm consists of a head, whose nucleus contains the hereditary or genetic material arranged on chromosomes, a midsection that provides energy, and a tail that propels the sperm along the male reproductive system and through the woman's reproductive tract. The top of the head is covered by the acrosome, a protective structure containing enzymes that enable the sperm to penetrate the egg; and the surface of the acrosome is enveloped by the plasma membrane.