How An Erection Works

The facts about sex
Men’s sexual physiology: How an erection works

The penis is made up of three long spongy tubes of tissue, the cavernous and the spongy bodies.

Each tube is surrounded by flexible and fibrous tissue, and the whole bundle is wrapped up in another layer of tissue called Buck’s fascia.

There’s a rich blood supply to the penis via arteries which run along the whole length of the penis inside the spongy erectile tissues and whose branches empty blood directly into the spongy tissue.

An erection is driven by this blood. The spongy tubes fill up with blood, which results in the tissue expanding and, at full capacity, hardening as the tissues become inflated, much like a hydraulic system in engineering.

However, many different components have to work together in a well coordinated fashion for an erection to be created and maintained.

First, the blood flow to the penis increases. This happens through the helicine arteries, which are small branches of the main arteries bringing blood to the penis. The helicine arteries have walls which are made up of smooth muscles.

When the penis is flaccid, these muscles are contracted, which reduces the diameter of the blood vessels and therefore the amount of blood which can flow through them.

When presented with an erotic stimulus, the brain sets in motion a series of neuro-physiological events, the net result of which is to cause the muscle cells to relax, which allows the blood vessels, the helicine arteries, to expand. This means that more blood flows into the spongy tissues of the penis. 

After an increase in the incoming blood flow, the next step in an erection is a decrease in the outward flow of blood so that a pressure gradient arises and the erection becomes progressively more rigid.

The decrease in outward flowing blood happens passively through the structural design of the penis. While the incoming arteries bring their blood into the spongy bodies, the veins in the penis are close to the Buck’s fascia tissues which surround the erectile bodies.

In fact the veins tend to run between the fascias for some distance, before traversing them. This means that as the internal spongy tissues fill with blood, they expand against the outside fascia, which puts pressure on the veins.

As a result, the veins are squeezed in the middle between the two layers, thereby decreasing their diameter, which means less blood is able to flow back out from the penis.

The whole design is incredibly efficient. The resistance for the out-flowing blood increases a hundred-fold during an erection, compared to the flaccid state.

Once you have an erection as little as 1 to 5 ml of blood flowing in is necessary to keep the penis hard. The downside is that the design is complicated with a lot of different components, all of which can potentially go wrong.

The best way to look after your penis and your erectile capacity is to maintain a healthy overall  lifestyle, especially looking after your heart and arteries.

(Cholesterol deposited on the walls of the penile arteries can block them and prevent you getting a decent erection.) Second, to look after your greatest asset, use it – a lot! Frequent erections will keep the penile tissues healthy and the erectile mechanism in fine working order. Like a lot of the body’s systems, “if you don’t use it, you lose it”!

Source for the facts cited in this page: 

ABC of Sexual Health (2005) Second edition edited by John M Tomlinson, British Medical Journal Books and Blackwell Publishing.

Principles of anatomy and physiology (2000) Ninth edition by Gerard J. Tortora and Sandra Reynolds Grabowski. Publishers: John Wiley and Sons