For example:

• If you eat a big meal, your body needs to send more blood to the stomach and intestines to help with digestion. 
• If you are running in a marathon, your body needs to send more blood to your arm and leg muscles, and it may want to cut most of the blood flowing to the stomach (and other nonessential organs) in order to save oxygen for the legs.

What your body needs, in other words, is a set of valves that it can use to increase and decrease blood flow to certain parts of the body. And your brain needs a way to control those valves so it can turn them on and off when necessary.

The penis is one of the places in the body where the brain needs to be able to turn the blood flow on and off with a valve. To understand how the brain controls this particular valve, let’s start with the basic concept at work: How does the brain control blood flow to different parts of the body?

Turning Valves On and Off
In the human body, the “valves” open and close using muscles in the walls of arteries. When these muscles relax, the arteries open up and blood flow increases. The valves respond to chemical messages that the brain can control.

The mechanism that the body uses to “open a valve” in any part of the body involves four steps:

1. The brain sends a signal down a particular nerve fiber. This nerve fiber ends in an NANC nerve cell in an artery, somewhere near the point where blood flow needs to change. NANC stands for nonadrenergic-noncholinergic, and what it means is that the NANC nerve cell is able to create nitric oxide. 
2. The NANC nerve endings inject nitric oxide into the blood and surrounding cells. 
3. The nitric oxide stimulates an enzyme called guanylate cyclase in nearby cells, and this enzyme starts producing a chemical called cyclic guanosine monophosphate (cGMP). 
4. cGMP tells smooth muscles that line an artery to relax. When they relax, blood flow increases.

This mechanism is a simple little chemical machine, and the brain uses it to increase blood flow in several different parts of the body. But there is one final part to this chemical machine: Another enzyme called phosphodiesterase (PDE) deactivates the cGMP.

The brain sends signals to NANC cells in the artery. The NANC cells release nitric oxide (NO). Nitric oxide acts as a signaling molecule and stimulates an enzyme called guanylate cyclase in nearby cells. The guanylate cyclase converts a chemical called GTP into another chemical called cGMP. cGMP causes muscles in the walls of the arteries to relax. This relaxation increases blood flow. Meanwhile, PDE is decomposing the cGMP and turning it back into GTP. There is a cycle — guanylate cyclase turns GTP into cGMP, and PDE turns cGMP into GTP. Nitric oxide turns the cycle on.

­cGMP is produced as long as the brain is sending messages down the nerve fibers in the artery, which generate nitric oxide and keep the cycle going. When the brain stops sending the signal, all of the cGMP goes away because PDE is deactivating it. This way, the brain can turn valves on and off whenever it wants to.

So how does this relate to an erection?

When the brain gets aroused, it sends a signal to the penis. Nerve cells in the penis’ corpora cavernosa start producing nitric oxide, which leads to the creation of cGMP. The cGMP causes arteries in the corpora cavernosa to dilate, causing lots of blood to flow into the penis. The extra blood flowing in causes the penis to inflate like a balloon. An erection occurs.

When a man suffers from erectile dysfunction, there can be many reasons for the problem. But one of the most common reasons, especially in older men, is that the arteries in the penis aren’t dilating enough when the brain sends the signal. The man is aroused, and the nerves in the penis are producing NO; but the amount of cGMP produced is not enough to maintain an erection.

The way that Viagra goes about solving this problem is quite ingenious, and involves the following question: How can you create a drug that affects only the penile valve?

If you want to create a drug that increases blood flow to the penis, there are at least three ways to do it:

1. Increase the amount of nitric oxide produced in the penis
2. Increase the amount of cGMP produced in the penis in response to the nitric oxide
3. Eliminate the PDE in the penis so that the cGMP builds up instead of getting decomposed by the PDE

 ­Viagra uses method No. 3 — it eliminates the PDE that is decomposing the cGMP, so cGMP builds up in the penis and has a larger effect on the artery walls. The greater the amount of cGMP the greater the blood flow, and the greater the blood flow the greater the degree of the erection.

The reason that Viagra uses this technique is because of an interesting quirk of PDE.

It turns out that the human body has at least 11 different kinds of PDE that it produces. Only one of those kinds of PDE — PDE5 — is found primarily in the penis. Once scientists discovered this fact, the creation of Viagra®was relatively simple. All that Pfizer needed to find was a chemical that would selectively block PDE5 and nothing else. With the PDE5 blocked, cGMP could build up in the penis and increase the blood flow there without affecting other parts of the body.