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 veins narrow, which traps the blood and maintains the erection.

After the man ejaculates or is no longer sexually aroused, the veins expand and the trapped blood flows back to the body. The penis returns to its normal size and becomes soft (flaccid).