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The erection is a vascular or blood flow event. The erection results when the penis fills with blood. During the erection process, blood flow to the penis increases, venous drainage from the penis decreases, and blood becomes “trapped” in the penis. The process is a result of a cascade of events involving the brain, the nerves, the hormone system, and the vascular system.
The erection process starts in the brain. For example, when you see or sense something that is sexually appealing, an impulse arises in the part of your brain called the hypothalamus. The nerve impulse then travels down the spinal cord to nerve centers located in the pelvis near the penis.
The nerve impulses in the pelvis trigger the release of a chemical messenger called nitrous oxide. Nitrous oxide is the most important chemical messenger for the erection. This substance leads to dilation of the arteries to the penis. This vasodilation causes an increase in blood flow to the penis.
Because the erection is a vascular event, the arteries to the penis must be in good shape for the erection to occur. The arteries must be relatively free of cholesterol plaque that can block the flow of blood to the penis.
In order to understand the erection, it is important to understand some basic anatomy of the penis. The penis contains two long two long cylinders that run the full length of the penis. These cylinders are called the corpora cavernosa. These corpora fill with blood during the erection.
The outside of the corpora cavernosa is made of a tough lining called the tunica albuginea. The inside of the corpora contain vascular spaces called sinusoidal spaces.
As blood enters the corpora cavernosa, the blood fills the sinusoidal spaces. These spaces expand and they compress the veins that exit the penis. Blood flow through the veins comes to a stop.
So, blood rushes into the penis through the arteries, but blood cannot exit the penis through the veins. Blood becomes trapped in the penis and this produces the erection.
At this point, the penis becomes hard and the pressure inside the penis is about equal to the systolic blood pressure. This is about 120-140 mm of Hg (mercury).
But the penis will become much harder than this. There are two muscles that surround the corpora cavernosa. They are called the ichiocavernosus and the bulbospongiosus muscles. These muscles begin to contract. As a result, pressure inside the penile corpora cavernosa can reach 700 mm of Hg. This is several times higher than the systolic blood pressure. At this point, the penis is extremely rigid.
When the man becomes sexually aroused a nerve impulse is generated in the brain. This impulse travels to the penis and generates a chemical series of events. These chemical events lead to dilation of the artery to the penis.
The most important chemical messenger is nitrous oxide. This substance is the chemical key to the erection. Nitrous oxide leads to an increase in another chemical called C (cyclic) – GMP. C –GMP directly causes the penile arteries to dilate. Viagra works by increasing the levels of C-GMP in the penis .
The role of testosterone in the erection is controversial. Testosterone is definitely responsible for male libido or sex drive. In addition, most of the evidence suggests testosterone is responsible for maintaining the healthy anatomy of the penis. Finally testosterone plays an important role in the release of nitrous oxide. This substance is critically important for the erection.
 Christ, G, Lue, T, Physiology and Biochemistry of Erections, Endocrine, Volume 23, Nos. 2-3, 93-100, March-April, 2004
 Guliano, F, Neurophysiology of Erection and Ejaculation, Journal of Sexual Medicine, 2011, 8, 310-315
 Meldrum, D, Burnett, A, Dorey, G, Esposito, D, Ignarro, L, Erectile Hydraulics: Maximizing Inflow While Minimizing Outflow, Journal of Sexual Medicine, 2014, 11, 1208-1220
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