Another structure, the tunica albuginea, surrounds the spongy tissue that creates tension on the veins as an erection increases and prevents blood from flowing out of the penis. As the tissues grow and continue filling with blood, so does the penis, swelling and becoming hard (thanks to the pressure created by a good deal of blood flowing into the penis, and very little flowing out of the penis) -- thus becoming what we call an erection.
And that's not the only body part that gets erect: His nipples, earlobes, lips, and even nostrils also swell and darken, and his testicles draw up closer to his body. Plus, his heart rate quickens and his blood pressure rises.
During stimulation, the muscles around the penis relax to let the blood in and be absorbed by a spongy structure on the top side of the penis called the corpus cavernosum. After the blood rushes in, the veins in the penis clamp down like a dam so the blood stays absorbed in the corpus cavernosum. All the while, a soft structure on the underside of the penis called the corpus spongiosum remains relaxed to allow semen to be released at ejaculation.
Without proper blood flow-meaning that arteries are inflamed or clogged-men can't get an erection because they can't get blood in or keep it there. And that's one of the most common causes of erectile dysfunction-a condition that affects some 30 million American men.

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If those arteries are inflamed and/or clogged, then you don't have proper blood flow. And that means you don't have enough nitric oxide to open up your arteries, so you can't get an erection because you can't get blood into your penis. Plus, without an engorged penis, the veins that drain the blood don't kink off, so whatever meager blood enters the organ quickly spills back out.
That's why erectile problems, for the most part, aren't manhood problems or mental problems. They're plumbing problems that require you not only to be turned on but to have the right biological faucets turned on as well.

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When you are sexually aroused, your brain sends signals to the nerves in the penis. These penile nerves can also be simulated by touch or direct sensory contact. The nerve impulses go to the two corpora cavernosa, causing the penile arteries to expand, and blood flow increases to the penis. As the two “erection chambers” fill with blood, this causes pressure on the smaller veins which take blood away from the penis. These spaces change from low volume of blood normally present to a high volume of blood that makes the penis hard and erect. As a result of more blood flowing in from the arteries and less blood flowing out through the veins, the erection occurs.
Researchers have found that the penile nerves produce nitric oxide, a “messenger” chemical that allows erections. Nitric oxide causes the blood vessels to relax in the corpus cavernosa. When the spaces containing blood relax, they become filled with blood and the erection occurs.

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Chemical signals from the brain cause arteries in the penis to widen, allowing more blood to enter the erectile bodies known as the corpora cavernosa. These tissues swell with blood, causing an erection. At the same time, blood-engorged tissues compress the veins, keeping blood in the penis and maintaining the erection.
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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.