An erect penis contains about six times more blood than a flaccid one.
Blood is an important part of an erection because it gives the penis the firmness needed for intercourse.
How does it happen?
In the penis, there are two arteries that go into the two sections of the corpora cavernosa – the spongy erectile tissue. Once in this tissue, the arteries branch into many smaller blood vessels.
When a man is sexually aroused, nerves in the brain or in the spine trigger the arteries to widen and allow more blood into the penis. When this happens, the smaller arteries in the corpora cavernosa become engorged with blood.
Once the man ejaculates (or if penile stimulation stops), nerves tell veins to widen and the blood flows out of the penis and back into the body.
How does the blood stay in the penis during erection? During that time, the veins that eventually allow the blood to leave are compressed and closed, essentially locking the blood inside.
Sometimes, men have blockages in their penile arteries. In some cases, surgeons perform penile revascularization, a kind of bypass surgery. Another blood vessel is taken from another part of the body, such as the abdomen or the leg, and transferred to the penis. This allows for clear blood flow so a man can have an erection. This surgery, however, is not for everyone and generally has a 50% success rate.