A Answers (4)
Jan Shifren, MD, Obstetrics & Gynecology, answeredAt 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.
Marc Garnick, Oncology, answeredAn 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.
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.
Harris McIlwain, Pain Medicine, 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.