The penis contains two chambers called the corpora cavernosa, which run the length of the organ (see figure 1). A spongy tissue fills the chambers. The corpora cavernosa are surrounded by a membrane, called the tunica albuginea. The spongy tissue contains smooth muscles, fibrous tissues, spaces, veins, and arteries. The urethra, which is the channel for urine and ejaculate, runs along the underside of the corpora cavernosa and is surrounded by the corpus spongiosum.

Erection begins with sensory or mental stimulation, or both. Impulses from the brain and local nerves cause the muscles of the corpora cavernosa to relax, allowing blood to flow in and fill the spaces. The blood creates pressure in the corpora cavernosa, making the penis expand. The tunica albuginea helps trap the blood in the corpora cavernosa, thereby sustaining erection. When muscles in the penis contract to stop the inflow of blood and open outflow channels, erection is reversed.

Figure 1. Arteries (top) and veins (bottom) penetrate the long, filled cavities running the length of the penis—the corpora cavernosa and the corpus spongiosum. Erection occurs when relaxed muscles allow the corpora cavernosa to fill with excess blood fed by the arteries, while drainage of blood through the veins is blocked.

An erection problem is the inability to get or maintain an erection that is firm enough for a man to have intercourse. You may be unable to get an erection at all, or you may lose the erection during intercourse before you are ready. If the condition persists, the medical term is erectile dysfunction.

Erection problems are common in adult men. In fact, almost all men experience occasional difficulty getting or maintaining an erection. In many cases, it is a temporary condition that will go away with little or no treatment. In other cases, it can be an ongoing problem that can damage a man's self esteem and harm his relationship with his partner, and thus requires treatment.

If you have difficulty having or keeping an erection more than 25% of the time, it is considered a problem.

In the past, erection problems were thought to be "all in the man's mind." Men often were given unhelpful advice such as "don't worry" or "just relax and it will take care of itself." Today, doctors believe that when the problem is not temporary or does not go away on its own, physical factors are often the cause.

One way to know if the cause is physical or psychologic is to determine if you are having nighttime erections. Normally, men have 3 to 5 erections per night, each lasting up to 30 minutes. Your doctor can explain a test to find out if you are having the normal number of nighttime erections.

In most men, erection difficulties do not affect their sex drive.