How does the erection occur? It is a physiological-phenomenon, but I know that not many people know its mechanism. When saying it plainly, the erection is a kind of “hyperemia”. The erection occurs when the penile vessels become loose and dilated, and blood flows into the cavernous body. It is the nitric-monoxide (Nitric oxide: NO) which plays the role of dilatation of the blood vessels as I previously explained.
NO is produced in the nerves and blood vessels by the synathase which is called NO synthase.
When receiving sexual excitement, NO is secreted from the nerves and it causes dilatation of blood vessels. After that, NO comes out of the blood vessels and erection is maintained.
In other words, NO is required not only for getting an erection but also for maintaining it.
The ED curatives such as Viagra, Levitra, Cialis and so on are so-called PDE5 inhibitors, which improve production of NO and so they dramatically improve ED. Moreover, they do not only increase the NO, but also make NO work more effectively.