The primary role of the reproductive system – whether male or female – is to create a new generation and further propagate human life. The male reproductive system is required to produce and supply the sperm to fertilize the egg (ova). The sperm and egg contain the genetic material of the parents, which combine to form the genetic blueprint of the fetus.

The male reproductive system, along with the pituitary gland in the brain, is responsible for the production of male sex hormones (androgens), including testosterone. These hormones are a crucial part of puberty. It is not until after puberty that reproduction is possible. Though the age that puberty begins varies widely between individuals, it generally starts at some point between the ages of 10 and 14 years. However, some children experience puberty earlier than their peers (precocious puberty). During puberty, the reproductive organs mature and increase in size and pubic hair begins to grow in the groin region.

It takes about 72 days for a sperm cell to develop into sperm. After puberty, millions of sperm are produced in the seminiferous tubules every day. They continue to develop and mature in the epididymis, in a process that takes about 15 to 25 days. Sperm are about 1/600 of an inch (about 0.05 millimeters) long. They are composed of a spherical head, which contains genetic information, and a slender tail used to move the sperm through the male and female reproductive systems. If enough time passes and the sperm are not ejaculated, they die and are reabsorbed by the body.

Most of the time, the penis is flaccid (limp). However, during an erection, the sponge-like tissue inside the penis fills with blood. This causes the penis to become stiff and erect, making insertion during sexual intercourse easier. Erections go away after ejaculation or on their own. They are frequently caused by sexual arousal but may occur for no particular reason, especially during puberty. They may occur many times during REM (rapid eye movement) sleep. Sometimes, ejaculation may also occur during sleep. This is called nocturnal emissions, or “wet dreams.”

During ejaculation, mature sperm are pushed from the epididymis through the vas deferens and urethra. At the same time, the accessory glands secrete the fluids that make up the bulk of semen. These fluids and the sperm mix and are expelled from the body through the penis. Between 0.05 and 0.2 fluid ounces (1.5 to 6.0 milliliters) of semen, including between 75 million and 900 million sperm, are released from the penis during ejaculation.

When ejaculation occurs inside the vagina, the sperm travel through the cervix and uterus and into the fallopian tubes. If intercourse is not protected using contraception, then when a sperm encounters an egg in a fallopian tube it may fertilize the egg, producing a zygote. A sperm can survive in the uterus for up to two days while waiting for an egg.