How is erection sustained?
In the expanded penis there is a pressure which affects the veins in tunica albuginea (the ones that allow the blood flow to leave the penis). The blood is trapped in the penis so that an erection can be sustained. Erection stops when the level of cyclic GMP in corpora cavernosa decreases. The smooth muscles of the corpora cavernosa become smaller and less blood can get into the penis. The veins which drain the blood away fro penis open. The decrease of the cGMP level is caused by an enzyme: phosphodiesterase type 5 (PDE5), which destroys the cyclic GMP.