Men reach their sexual peak between the ages of 16 and 18, shortly after puberty. Since they are able to produce sperm continuously from puberty onwards, men remain fertile for a longer period than women; some men can become fathers at the age of 70 or older. Sperm are produced in the testes, and, during sexual intercourse, the penis delivers them into the female reproductive system. The male reproductive system also manufactures sex hormones that are essential for sperm production and for sexual development during puberty.
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The male reproductive system begins to develop before a male baby is even born. The only visible parts of the male reproductive system are the penis and the scrotum, but inside the body there is a complicated network of ducts, glands, and other tissues that work together to make the production and transport of sperm possible.
Once puberty is reached, sperm are manufactured continuously in the two testes at a rate of about 125 million each day. Since sperm production is not efficient at body temperature, the testes are kept cool by being suspended outside the body in a sac of skin called the scrotum. Mature sperm leave each testis through an epididymis, a long coiled tube that lies above and behind each testis. The sperm are stored in the epididymis and mature there before going to the vas deferens, the tube that connects an epididymis to an ejaculatory duct. During sexual activity, each vas deferens contracts and pushes sperm towards the urethra (the tube that connects the bladder to the outside of the body). The sperm are ejaculated during sexual activity or are reabsorbed into the body. Some dribble through the upper end of the vas deferens into the urethra and are later washed away in the urine.
The sperm are carried in a fluid consisting of secretions from various glands. Most of these secretions are produced by glands called seminal vesicles as the sperm leave the vas deferens. Fluid is also added by the prostate gland. In addition to acting as a vehicle for the sperm to help them to swim, the fluid provides nutrients that keep the sperm healthy. Together, these secretions and sperm form semen, containing about 50 million sperm per millilitre.
In order for reproduction to take place, sperm must enter the female reproductive system (see Sex and reproduction). During arousal, the penis becomes enlarged and firm. Muscular contractions at the base of the penis then force sperm through the male urethra and into the vagina during male orgasm.
The principal male sex hormone, testosterone, is produced throughout life. Testosterone plays an important part in the development of the genitals and other male sexual characteristics. During puberty, there is a dramatic increase in the level of testosterone. This increase triggers the growth of the genitals and the development of secondary male sexual characteristics, such as body and facial hair, deepening of the voice, and muscle development.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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