Arteries and veins carry blood to and from the penis. These blood vessels play an important role in erections.

During an erection, the arteries expand to increase blood flow to the penis. The blood fills two tubes of spongy tissue in the penis (corpus cavernosa). This causes them to swell, making the penis larger and stiff, so it angles out from the body. The swelling blocks the veins, which traps the blood and maintains the erection.

An erection goes away when the normal blood flow to the penis is restored. The swelling goes down, letting the veins drain the extra blood back to the body. The penis returns to its normal size and becomes soft. This can happen after ejaculation or when someone is no longer sexually aroused.