Causes

The penis contains two cylindrical, sponge-like structures that run along its length, parallel to the tube that carries semen and urine (urethra). When a man becomes sexually aroused, nerve impulses cause the blood flow to the cylinders to increase about seven times the normal amount. This sudden influx of blood expands the sponge-like structures and produces an erection by straightening and stiffening the penis. Continued sexual arousal or excitation maintains the higher rate of blood flow, keeping the erection firm. After ejaculation, or when the sexual excitation passes, the excess blood drains out of the spongy tissue, and the penis returns to its nonerect size and shape.

Specific steps take place to produce and sustain an erection:

Arousal. The first step is sexual arousal, which men obtain from the senses of sight, touch, hearing and smell, and from thoughts.
Nervous system response. The brain communicates the sexual excitation to the body's nervous system, which activates increased blood flow to the penis.
Blood vessel response. A relaxing action occurs in the blood vessels that supply the penis, allowing more blood to flow into the shafts that produce the erection.
If something affects any of these factors or the delicate balance among them, erectile dysfunction can result.

Nonphysical causes
Nonphysical causes may account for impotence. They may include:

Psychological problems. The most common nonphysical causes are stress, anxiety and fatigue. Impotence is also an occasional side effect of psychological problems such as depression.
Negative feelings. Feelings that you express toward your sexual partner — or that are expressed by your sexual partner — such as resentment, hostility or lack of interest also can be a factor in erectile dysfunction.

Physical causes
Physical causes account for many cases of erectile dysfunction and may include:

• Nerve damage from longstanding diabetes (diabetic neuropathy)
• Cardiovascular disorders affecting the blood supply to the pelvis
• Certain prescription medications
• Operations for cancer of the prostate
• Fractures that injure the spinal cord
• Multiple sclerosis
• Hormonal disorders
• Alcoholism and other forms of drug abuse

In fact, erectile dysfunction may be one of the first signs of an underlying medical problem.

The physical and nonphysical causes of erectile dysfunction commonly interact. For instance, a minor physical problem that slows sexual response may cause anxiety about attaining an erection. Then the anxiety can worsen your erectile dysfunction.