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The release of Nitric Oxide is what causes a penis to become erect. This chemical is normally released in the penis as a response to sexual stimulation.
The following shows how Viagra works:
- Nitric oxide activates an enzyme called guanylate cyclase which in turn, results in increased levels of cyclic guanosine monophosphate (cGMP).
- This produces smooth-muscle relaxation in the corpus cavernosum and allows the inflow of blood.
[There are three types of muscles: smooth, cardiac, and striated. Smooth muscles are those that are involuntary in action and are found principally in internal organs.]
- Viagra (sildenafil citrate) has no direct relaxant effect on the human corpus cavernosum.
- Sildenafil enhances the effect of nitric oxide (NO) by inhibiting phosphodiesterase type 5 (PDE5).
- PDE5 is responsible for degradation of cGMP in the corpus cavernosum which causes the penis to become relaxed.
- Sexual stimulation causes local release of NO, Sildenafil inhibits PDE5, which in turn, leads to increased levels of cGMP in the corpus cavernosum. The result is continued smooth-muscle relaxation and this allows for inflow of blood to the corpus cavernosum thus, the penis becomes erect
- In the absence of sexual stimulation Sildenafil has no effect.
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