How an erection occurs

How does an erection happen? An erection is a complex process within the male reproductive anatomy which is based upon the blood supply to and from the penis.

Basically, blood flows into the penis which causes it to increase in size and thicken. This enlarged penis is then ready for penetrative sex.

Once orgasm has been reached, blood then flows out of the penis which causes it to become soft or flaccid.

An erection involves changes within the blood vessels, nerves and muscles of the penis which enable it to become hard or soft depending upon external factors.

Anatomy of the penis

The penis consists of the root (base), shaft (body) and the glans (head). It also contains the urethra “ a thin tube which runs from the bladder to an external opening which excretes urine and semen from the male body.

There are two cylindrical shaped tissues on either side of the penis called the ‘ corpora cavernosa’. These tissues are very similar to sponges in that they absorb liquid, in this case blood, which causes the penis to stiffen and enlarge in size.

Muscle fibres within the corpora cavernosa contract when the penis is relaxed.

The corpora cavernosa are surrounded by a tough outer coating or sheath called the ‘ tunica’. This hard exterior restricts the amount of expansion by the corpora cavernosa. As blood flows into the penis it causes the tunica to become tight which also raises the pressure within it. The penis becomes hard and increases in size.

There is a third cylindrical shaped area of tissue between the two corpora cavernosa. This is called the ‘ corpus spongeosum’ which also contains the urethra.

This is thicker towards the head of the penis which then forms the characteristic shaped glans. The glans is covered by the foreskin except in men who have been circumcised.

To reiterate: an erection is a complex process which is as follows:

• The man becomes sexually aroused “ usually by external triggers (e.g. erotic images)
• This causes a reaction in a part of the brain called the para-ventricular nucleus which increases its signal output.
• These signals travel through autonomic nerves within the spinal cord, pelvic nerves and nerves along the prostate gland to the corpora cavernosa and arteries that supply them with blood.
• Muscle fibres within the corpora relax “ in response to these signals which allow blood to flow into the spaces between these muscle fibres.
• Muscle fibres in those arteries which supply the penis also relax which causes a major increase in blood flow to the penis. This expands the corpora cavernosa and stretches the tunica.
• The stretched tunica then blocks veins which normally take blood away from the corpora cavernosa. Blood is trapped within the penis which raises the pressure even higher. This causes the penis to become erect.
• During this, pressure within the penis is at least twice that of blood in the main circulation system. This is due to muscles of the pelvic floor contracting around the base of the corpora.
• During an orgasm there is a dramatic change in signal output from the brain. This is accompanied by an increase in the production of noradrenaline from nerves within the male genitals. This triggers an orgasm as well as causing the muscles fibres in the corpora and their arteries to contract.
• Once sex is over, pressure within the corpora drops which relaxes the tunica and causes blood to flow out of the penis. The penis decreases in size and becomes flaccid.

A failure in one or more of these processes is responsible for the inability to get or sustain an erection.

This is known as erectile dysfunction (or impotence).

This can be caused by tiredness, excess alcohol consumption, anxiety or side effects of certain types of medicines, e.g. medication for high blood pressure.

Note: If you have high blood pressure and are taking medication for this then check with your GP about the risk of impotence.

Note: it is more difficult to sustain an erection in your forties than your twenties. This becomes even more difficult in your sixties.

Find out more about high blood pressure medication and impotency in our complete guide to high blood pressure.

Erectile dysfunction is discussed in more detail in the following section.