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Function: Sexual Intercourse

When a couple is sexually aroused (see Sexual response), the man’s penis becomes erect and the woman’s vagina is lubricated. During sexual intercourse, also called coitus, the penis is inserted into the vagina and the man begins thrusting pelvic movements. At orgasm, which the partners may experience simultaneously or at different times, intense, pleasurable sensations spread throughout the body. The woman’s vaginal walls contract rhythmically and the man ejaculates, releasing sperm.

Egg and sperm

The egg is about 0.1 mm ( 1 / 250 in) in diameter. The sperm (not shown to scale) is 20 times smaller – about 0.005 mm ( 1 / 5000 in) long, including the tail.

Sperm journey

On ejaculation, about 250 million sperm may enter the vagina. Only about 200 survive to reach the fallopian tubes, where one sperm may be successful in fertilizing an egg.

Function: Fertilization

Fertilization takes place in one of the fallopian tubes. Many sperm reach the egg and try to penetrate its outer covering. If a sperm succeeds, its head enters the egg and its tail is then shed. A membrane then rapidly forms around the egg, creating a barrier to prevent other sperm from entering. Fertilization occurs when the head of the sperm fuses with the nucleus of the egg.

Sperm penetrating egg

The head of the sperm pushes through the egg’s outer coating in order to reach the nucleus.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

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