Viagra is one of the best-known drugs of all time:

• Billions of spam e-mail messages advertise Viagra every day. There is so much Viagra spam, in fact, that Pfizer (the maker of Viagra) has a page addressing the problem, called Avoid Fake Viagra.

• Pfizer has spent untold millions of dollars advertising Viagra, so you see advertisements for the drug constantly on TV.

• Pfizer claims on its Web site that nine Viagra pills are dispensed every second -- nearly 300 million tablets per year.

The name recognition of Viagra is so good that nearly every adult in America has heard of the drug and can tell you what it does.

What Viagra does is simple: When it works as intended, Viagra causes a man who is sexually stimulated to get an erection.

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­How does Viagra do that? And why does Viagra work only if the man is sexually stimulated? For that matter, what causes an erection in the first place? In this article, we'll answer all of those questions and more.

This is actually a fascinating story -- it involves the technology of the human body and the techniques that scientists use to control different parts of the body with drugs. And in the case of Viagra, the story starts with the technology of the penis.

The Male Anatomy

For many people, talking about the penis is tough. This area of the body is considered private and is usually not discussed publicly. However, the penis is simply a part of the male anatomy designed to accomplish a task, and we'll treat it that way here.

In the case of the penis, there are actually two tasks that it handles:

1. releasing urine from the bladder, known as urination
2. releasing sperm and seminal fluid from the prostate gland, known as ejaculation

Viagra helps with the second task: ejaculation.

Dreams and Erections The average male has four to eight spontaneous erections every night while he sleeps. They usually occur during the REM stage, when dreaming is most common. When a doctor wants to know whether a patient's difficulty achieving an erection is due to physical or mental reasons, one way to find out is to fit the patient's penis with a sensor and see whether or not the patient's dream erections are working properly. If not, the problem is probably physical.

When things are working properly, ejaculation is a three-step process:

1. The man becomes sexually aroused.
2. The penis responds by becoming erect.
3. Stimulation of the penis causes ejaculation.

That sounds simple enough, but in many cases, step two doesn't happen, making step three difficult or impossible. Although the man is stimulated, the penis doesn't become erect. To understand why, you need to understand the technology of an erection.

When you want to move nearly any part of your body, you do it using muscles. Whether you are moving your fingers, toes, arms or legs, muscles do the work. Even when you stick your tongue out, you do it using muscles:

• You think about moving some part of your body
• The appropriate muscles contract
• That part of the body moves

Muscles let you move your body voluntarily with precise control.

The penis, on the other hand, is completely different. There are no muscle contractions involved in making the penis erect. To become erect, the penis instead uses pressure.

Probably the easiest way to understand how the penis becomes erect is to think about a balloon. If a balloon has no air in it, it's limp. As you inflate a limp balloon with just a little air, it becomes elongated and rigid.

The penis uses a similar mechanism, but instead of using pressurized air to become rigid, the penis uses pressurized blood. The penis contains two cigar-shaped structures, called corpora cavernosa (singular: corpus cavernosum), that it uses to become erect.

Think of the corpora cavernosa as balloonlike tubes. Arteries­ bring blood into these two tubes, and veins carry blood away from them. The penis can be either limp or erect, depending on the flow of blood:

• In a non-erect state, the arteries that bring b­lood into the corpora cavernosa are somewhat constricted, while the veins that drain the blood from the penis are open. There is no way for pressure to build inside the penis. In this state, the penis is limp.

• When a man becomes aroused, the arteries leading into the penis open up so that pressurized blood can enter the penis quickly. The veins leaving the penis constrict. Pressurized blood is trapped in the corpora cavernosa, and this blood causes the penis to elongate and stiffen. The penis is erect.

If the arteries leading to the penis don't open up properly, it is difficult or impossible for a man to become erect. This problem is the leading cause of erectile dysfunction (ED).

To solve an erection problem when the cause is poor blood flow, you need to open the arteries. Let's take a look at how this can be done -- and how it was done before Viagra.