Upon sexual arousal, there is the transmission of signals between the brain and the structure of the penis. This causes release of certain chemicals and enzymes that cause smooth muscles contained in the penis to relax. Under normal circumstances, these smooth muscles remain constricted and blood flow is limited. Upon relaxation of these muscles, more blood is able to get into the penis resulting in an erection. The erection is lost when an enzyme called PDE5 (phosphodiesterase type5) is released which causes the reversal of the process by causing the breakdown of certain enzymes released during sexual arousal and erection. This reversal brings the smooth muscles to its original constricted state causing the exit of blood from the penis and loss of erection.