The corpus cavernosa are the two spongy chambers along each side of
the shaft of the penis. When you have an erection, the smooth muscle in these
chambers relaxes, blood flows in through the penile arteries, and the chambers
fill with blood and expand. The expanded chambers press against the veins that
would normally drain blood from the penis, trapping the blood inside and making
the penis erect.
After ejaculation or when you are no longer sexually aroused, the
arteries narrow, the veins expand, and the blood drains out. The penis becomes
Problems with the arteries and veins that supply blood to or return
blood from the penis can cause erection problems. Vascular problems may be
Hardening of the arteries (atherosclerosis) in the penis. This may block blood
flow into the arteries that fill the penis.