A man’s ability to achieve and sustain an erection depends on the healthy operation of a network of systems that cause vascular tissue in the penis to fill with blood. For an erection to take place, the nerves to the penis must function properly, an adequate blood supply must circulate to the penis, and the penis must receive a stimulus from the brain. Erectile dysfunction, or ED, is the term used to describe a malfunction in one of these systems. Estimates of the number of American men who suffer from erectile dysfunction range from 15 million to 30 million, according to the National Institutes of Health.

Premature ejaculation, a low sperm count that results in infertility, or a low sex drive is not the same as erectile dysfunction, though one or more of these conditions may accompany ED. Occasional failure to achieve an erection (less than 20 percent of the time) can occur for a variety of reasons–drinking too much alcohol, for example, or extreme fatigue–and is not considered unusual. But failure to achieve an erection more than 50 percent of the time usually indicates a problem that requires treatment. Though erectile dysfunction is much more common in older men (researchers estimate that roughly half of men over age 60 have ED), the condition is not considered normal at any age.

Erectile dysfunction is associated with a wide variety of physical and psychological conditions including diabetes, excess weight, atherosclerosis, prostate surgery, depression, stress, performance anxiety, kidney disease, and hormonal imbalances. Over 200 prescription medications can cause erectile dysfunction as a side effect, as can smoking, drinking alcohol, and using illegal drugs. A number of therapies exist to treat the condition.

 Need-to-know anatomy

Two spongelike chambers, called the corpora cavernosa, run the length of the penis above the corpus spongiosum, which contains the urethra, the channel for urine and semen. To initiate an erection, the brain and body send sensory signals to the nerves of the penis, causing the muscles of the corpora cavernosa to relax and the blood vessels to open up. Blood flows in, and the penis expands. A membrane surrounding the corpora cavernosa (known as the tunica albuginea) traps the blood, sustaining the erection.

Ejaculation occurs as a result of signals that travel along the sensory nerves of the penis into the spinal cord and the brain, which control the release of semen from the urethra. Ejaculation consists of two phases. In the first phase, the vas deferens (the tubes that transport and store sperm from the testes) contract to squeeze the sperm toward the base of the penis while the prostate gland and seminal vesicles release secretions to make semen. In the second phase, muscles at the base of the penis contract every 0.8 seconds and force the semen out of the penis in up to five spurts.

Causes

Achieving an erection involves a complex interaction of sensory information, nerves, blood vessels, hormones, and emotions. Because of this complexity, erectile dysfunction can involve physical factors, psychological factors, or a combination of the two. Essentially, erectile dysfunction occurs when the body experiences a breakdown in the sequence of events leading to an erection. This disruption can occur in nerve impulses to and from the brain, spine, and penis; or in the muscles, fibrous tissues, veins, and arteries in and near the corpora cavernosa that respond to those impulses.

A number of diseases that affect the tissues near the penis cause most cases of erectile dysfunction. Vascular diseases, such as atherosclerosis (hardening of the arteries), hypertension, and high cholesterol, cause up to 70 percent of erectile dysfunction in men with physical causes. Atherosclerosis alone accounts for between 50 and 60 percent of erectile dysfunction cases in men over age 60.

Between 35 and 50 percent of men with diabetes experience erectile dysfunction because the disease can damage nerves and arteries, making an erection difficult. Kidney disease can affect hormones, circulation, nerve function, and energy level. These changes can lower libido (sex drive) or sexual ability. Drugs used to treat kidney disease can also cause erectile dysfunction.

Since the nervous system plays a vital part in achieving and maintaining an erection, it is common for men with diseases such as stroke, multiple sclerosis, Alzheimer’s disease, Parkinson’s disease, and spinal cord injuries to experience erectile dysfunction.

Surgery and injury can also cause ED. Though prostate and bladder cancer don’t cause erectile dysfunction on their own, the surgery to remove the cancer can lead to erectile problems. To remove these cancers, doctors must also remove nerves and tissues around the affected areas. Some of these surgeries result in only temporary ED (lasting six to 18 months), but other procedures result in permanent damage to the nerves and tissue responsible for causing an erection. Radiation therapy, also used to treat bladder and prostate cancer, may also cause ED. Injuries to the pelvis, bladder, spinal cord, and penis that require surgery also commonly cause ED. Injury or disease can also cause venous leak, a condition in which the veins cannot prevent blood from leaving the penis, which can cause men difficulty in maintaining erections.

Though hormonal imbalances account for only 5 percent of ED cases, problems with the pituitary gland, which regulates hormones such as prolactin and testosterone, can affect a man’s response to sexual stimulation. Hormonal imbalances that affect erection can also result from kidney disease, liver disease, or a tumor on the pituitary gland.

Tobacco, alcohol, and drug use all can damage a person’s blood vessels and restrict blood flow to the penis. Smoking, in particular, plays a large role in causing ED in people with arteriosclerosis.