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erection physiology: how an erection happens

by director Chris Steidle, MD

Erection physiology is far from simple. The creation of an erection is an extremely complicated cascade of events that requires many different steps. There are numerous chemical transmitters involved in creating an erection including epinephrine, norepinephrine, acetylcholine, prostaglandins and nitric oxide.



Diagram of penis anatomy

The exact mechanism by which erection occurs is still unclear but we do know that the neural input from the brain is extremely important. Reflex erections, as seen in people with cord damage such as paraplegics, are often poor erections and not sustainable for prolonged periods of intercourse.

Erections require a rapid increase in blood flow

An erection occurs when the nervous system activates a rapid increase in blood flow. The vascular muscle in the spongy area inside the penis becomes engorged with blood and the outflow of blood is cut off.

An erection can occur as a reflex as we see in spinal cord patients, or can be caused by psychogenic stimulation (originating in the mind). Numerous sexual stimuli are processed by the brain and transmitted to the penis via the nervous system.

Penis cross section

In order to increase the size of an erection, there must be an increase in blood flow to the penis and, at the same time, the blood has to be prevented from leaving the penis.



Steidle, CP. The Impotence Sourcebook. Lowell House. 1998.

Posted January 2002
Last updated November 2011


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