What is an erection, and how does it work?

The penis. If you have one, you’re probably very familiar with it, yet its mechanics can still seem pretty exotic. An erection is a pretty amazing process, requiring several body systems to collaborate simultaneously. But it's not just a sign that you're ready for sex; if this process isn’t working properly, it can be an early indicator of larger health issues. Here's a quick look at how erections work. 

Erections are surprisingly complicated. Each one takes your hormones, nerves, blood vessels, and even your mood to work together seamlessly in a complex sequence of events. When everything goes right, an erect penis can contain up to eight times the normal amount of blood (Van Driel, 2015). But one missed reaction in that chain can lead to erectile dysfunction.

The penis consists of the shaft and the head (or glans), which houses the meatus (or the opening of the urethra). Two tubes called the corpora cavernosa run down the length of the penis—they're filled with spongy tissue and many tiny blood vessels. So is the corpus spongiosum, which runs down the underside of the penis and houses the urethra.

An erection is perhaps the most obvious sign of sexual arousal. It's a physical response to a chemical reaction in the body. During sexual arousal, those tiny blood vessels in the penis dilate. The spongy tissue fills with blood, and an erection occurs. After ejaculation, or when a man isn’t sexually excited anymore, that blood drains away, and the penis reverts to its flaccid size and appearance. 

Every erection is the result of a relay race that starts in the brain with arousal and culminates in the penis with an erection:

1. Men become aroused either by thinking about, feeling, or seeing someone or something that sexually excites them.

2. The brain passes that signal to the body through the spinal cord and nerves.

3. cGMP causes the corpora cavernosa in the penis to relax, allowing blood to rush into the cavernosal arteries

4. While blood rushes into the penis, the veins that drain blood from the penis get compressed, causing the increased blood to be trapped in the penis, resulting in an erection.

5. An enzyme called phosphodiesterase-5 breaks down cGMP, causing the penis to return to its flaccid state.

Erectile dysfunction happens when you can't get or maintain an erection sufficient to have satisfying sexual intercourse. ED can be the first warning sign of more serious medical conditions like diabetes, heart disease, high blood pressure, or low testosterone (McMahon 2019). If you're struggling with erectile dysfunction, seek medical advice to find out what might be causing your ED and the treatments that are available. Your sex life—and your overall health—can only benefit.

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Current version

January 18, 2022

Written by

Michael Martin

Fact checked by

Felix Gussone, MD

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About the medical reviewer

Felix Gussone, MD, Ro

Felix Gussone is a physician, health journalist and a Manager, Medical Content & Education at Ro.