How does an erection occur?

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  A Dr. Michael Roizen, MD , Internal Medicine, answered

  

  ​During stimulation, the muscles around the arteries in the penis relax. That relaxation happens so that blood can be absorbed by a spongy structure on the top of the penis called the corpus cavernosum. After the blood rushes in like a tailback breaking for the end zone, the veins in the penis clamp down to keep that blood trapped in the penis. And voila! We've got liftoff, Houston.
  
  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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  A Dr. Madeleine M. Castellanos, MD , Psychiatry, answered on behalf of Good In Bed

  Whatever the cause of stimulation for erection, once stimulated a man's nervous system sends a flurry of signals to trigger the release of nitric oxide, which relaxes the smooth muscle lining of the penis. The spongy tissues in the penis, the corpora cavernosa and the corpus spongiosum, expand and fill with blood, as blood flow to the genitals increases with sexual arousal.
  
  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.
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  A Dr. Harris H. McIlwain, MD , Rheumatology, answered

  

  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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  A Dr. Mehmet Oz, MD , Cardiology (Cardiovascular Disease), answered

  

  When a certain chemical reaction is triggered in the brain, it hops on a downtown express bus to start a ticker-tape parade in the loins. How? In a man, the signals from the brain trigger a reaction that causes blood to rush into the penis like Class V whitewater rapids.
  
  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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  A Dr. Marc B. Garnick, MD , Hematology & Oncology, answered

  An erection begins when a touch, look, or even a thought nudges the brain to send signals of arousal down the spinal cord and into the nerves of the penis. The nerves "talk" to one another by releasing nitric oxide and other chemical messengers. These messengers boost the production of other important chemicals. These chemicals initiate the erection by relaxing smooth muscle cells lining the tiny arteries that lead to the corpora cavernosa, side-by-side flexible cylinders that run the length of the penis. As the arteries relax, the tissues swell with blood, causing an erection.
  
  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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  A Dr. Jan L. Shifren, MD , Reproductive Endocrinology, answered

  At its most basic level, an erection is a hydraulic event. Blood fills the penis, causing it to swell and become firm. But getting to that stage requires an extraordinary orchestration of body mechanisms. Most of the time, an erection really starts in the man's brain. A sight, smell, or touch sparks electrical signals of sexual arousal in the brain. These signals travel from the brain to an area in the lower part of the spinal cord. Nerves in this area signal nerves in the pelvis, which instruct arteries to let blood into the penis, thereby causing an erection.
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