The paired cylinders, called the corpora cavernosa, are capable of enlarging as engorgement with arterial blood occurs. The penile artery arises from the pudendal artery and splits into the dorsal, spongiosal, and cavernosal arteries. The deep, superficial, and intermediate veins drain the penis. As the erectile cylinders are engorged they press up against the veins to prevent leakage of blood back into the body. It is by this mechanism that an erection is maintained.
Some normal changes occur in the aging male.
As a man matures, his sperm count and the amount of semen will decrease. The ejaculate will no longer be propelled with the same force. Men may experience a delay in reaching climax and an increased refractory period between erections. The rigidity of erections may be slightly decreased along with a decrease in the time an erection can be maintained. However, men should still have sufficient rigidity for penetration.