How Erections Work
When you are not sexually aroused, your penis is soft and limp. During sexual arousal, nerve messages release chemicals that increase blood flow into the penis. The blood flows into 2 erection chambers made of spongy tissue (the corpus cavernosum) in the penis. The "smooth muscle" in the erection chambers relaxes, which lets blood enter and stay in the chambers. The pressure of the blood in the chambers makes the penis firm, giving you an erection. After you have an orgasm, the blood flows out of the chambers and the erection goes away.