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.
Understanding Blood Flow
To understand how to make a penis-specific drug, think about the way blood flows in your body. Your body has just one pump -- the heart. But different parts of the body need different amounts of blood at different times. For example, if you're running 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 them.
The mechanism that the body uses to open a valve in any part of the body is a simple little chemical machine:
- The brain sends a signal down a particular nerve fiber. This nerve fiber ends in a nerve cell in an artery, somewhere near the point where blood flow needs to change.
- These nerve cells -- called nonadrenergic-noncholinergic, or NANC -- produce nitric oxide and inject it into the blood and surrounding cells.
- The nitric oxide stimulates an enzyme (guanylate cyclase) in nearby cells that starts producing a chemical called cyclic guanosine monophosphate (cGMP).
- cGMP tells smooth muscles that line an artery to relax. When they relax, blood flow increases.
There is one final part to this chemical machine: Another enzyme, called phosphodiesterase (PDE), is deactivating the cGMP all the while.
cGMP is produced as long as the brain is sending messages down the nerve fibers in the artery. When the brain stops sending the signal, all of the cGMP goes away because PDE is deactivating it. This is how the brain turns valves on and off whenever it wants to.
Later, we'll learn how Viagra works its magic, discuss some of the side effects and even new competitors on the market. But first, let's delve deeper into the chemistry.