- Increase the amount of nitric oxide produced in the penis
- Increase the amount of cGMP produced in the penis in response to the nitric oxide
- 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 them -- 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.