Supreme Vigor The reason for male erection is related to the brain and the nerves. The sexual arousal instigates the brain cells in the center for control of sex organs to send signals to the male reproductory organs through the nerves. As a result of this, the blood flow in the penis gets increased. This causes the muscles and soft sponge like connective tissues to get activated and become taut. This tightening is also assisted by retention of the excess body fluid in these tissues. As a result, hardening of the penis occurs, and this condition stays for the full length of sexual act till ejaculation.