During the teenage years and afterward, the testes (testicles) produce a steady supply of hormones -- mostly testosterone. The testes also make millions of sperm each day. It takes about 74 days for the sperm to grow and mature. As part of this process, the newly-made sperm must travel through a 20-foot long tube called the epididymus to ripen. This tube forms a coiled structure that sits on top of and behind each testicle.

After the sperm mature, another tube called the vas deferens takes them from the epididymus into the body toward the prostate gland. There the sperm is mixed with special fluids from the prostate and the seminal vesicles, which sit on either side of the prostate. These whitish, protein-rich fluids help to support and nourish the sperm so that they can live for some time after ejaculation. During orgasm this mixture of fluid and sperm, called semen, is moved through the urethra and out of the tip of the penis. The drawing below shows the male sex organs.

The role of testosterone

Testosterone is the main male hormone. It causes the reproductive organs to develop, and promotes erections and sexual behavior. Testosterone also causes secondary sexual characteristics at puberty, such as a deeper voice and beard growth. The testes make most of this hormone. The adrenal glands, which sit on top of the kidneys, also make small amounts in both men and women.

The hypothalamus region of the brain controls the amount of hormone the body makes. When the testosterone level gets low, the hypothalamus signals the pituitary gland at the base of the brain. The pituitary sends a hormone messenger through the bloodstream to tell the testicles to speed up production.

Men's hormone levels vary widely, but most men have more testosterone in the bloodstream than they need. A man with a low level of testosterone may have trouble getting or keeping erections and may lose his desire for sex. In the healthy younger man, hormone problems are rare and anxiety is the main cause of erection problems. Common medical causes for erection problems include problems with the blood vessels or nerves in the pelvic area.

The normal pattern of arousal and erection

An erection begins when the brain sends a signal down the spinal cord and through the nerves that sweep down into the pelvis. Some of these important nerves run along both sides of the prostate gland.

When this signal is received, the spongy tissue inside the shaft of the penis relaxes and the arteries (blood vessels) that carry blood into the penis expand. As the walls of these blood vessels stretch, blood races into the penis at up to 50 times its usual speed. The blood fills 2 spongy tubes of tissue inside the shaft of the penis. The veins in the penis, which normally drain blood out of the penis, squeeze shut so that more blood stays inside. This causes a great increase in blood pressure inside the penis, which produces a firm erection.

The nerves that allow a man to feel pleasure when the penis is touched run in a different path from the nerves that control blood flow. Even if nerve damage or blocked blood vessels keep a man from getting erections, he can almost always feel pleasure from being touched. He can also still reach orgasm.

A third set of nerves, which run higher up in a man's body, controls ejaculation of semen.

How male orgasm happens

A man's orgasm has 2 stages. In the first stage, called emission, the prostate, seminal vesicles, and vas deferens (tubes joining the testicles with the seminal vesicles) contract.

During emission, the semen is deposited near the top of the urethra (the tube running through the penis), ready to be ejaculated. At this time, a small valve at the top of the tube shuts to keep the semen from going upward and into the bladder. A man feels emission as "the point of no return," when he knows he is about to have an orgasm. Emission is controlled by the sympathetic or involuntary nervous system.

Ejaculation is the second stage of orgasm. It is controlled by the same nerves that carry pleasure signals when the genital area is caressed. Those nerves cause the muscles around the base of the penis to squeeze in rhythm, pushing the semen through the urethra and out of the penis. At the same time, messages of pleasure are sent to the man's brain. This sensation is known as orgasm or climax.

Last Medical Review: 02/02/2009
Last Revised: 02/02/2009