How the erection system works and what causes it to fail

What happens during an erection

The penis contains three long cylinders of erectile tissue. Two of the cylinders lie side by side, while the third lies beneath them. The urethra, which is the urine and sperm channel, runs through this lower cylinder. Sexual excitement causes the cylinders to fill with about 100–140 mL of blood and, as they swell, so the penis becomes erect. And as the erectile tissue swells, it squeezes the veins in the penis. These veins normally drain blood away from the penis, so the squeezing action prevents blood flowing away and keeps the penis erect. (Imagine a bath-tub filling with the tap on and a plug in the drain.) After orgasm and ejaculation these events go into reverse, and the penis becomes limp again.

How the erection system fails (impotence)

A few years ago it was assumed that most cases of impotence had psychological causes. Now more is known about the blood supply to the penis, and it is recognized that physical problems are often responsible. For example, nerves to the spongy tissue can be damaged by diabetes, blood flow can be impaired by atherosclerosis (clogged arteries) and many medications interfere with erections.

Medications and drugs. Many medications and drugs can interfere with erections. Here are some of the most common culprits:

cimetidine (for duodenal ulcer)
some drugs for high blood pressure (for example, thiazide diuretics especially in high doses, methyldopa, most beta-blockers); erection problems affect about 1 in 5 men taking beta-blockers (Journal of the American Medical Association 2002;288:351–7)
finasteride (for prostate enlargement or baldness)
phenothiazines (for some psychiatric conditions)
alcohol and marijuana (cannabis)
drugs used for prostate cancer (for example, some GnRH analogues and anti-androgens)
some antidepressants
cholesterol-lowering drugs (probably)
non-steroidal anti-inflammatory drugs, also known as NSAIDs (possibly).

Medical conditions. Many medical conditions can cause difficulty with erections by damaging the blood vessels and nerves. Of course, if you have one of these conditions, it does not mean that you will necessarily be impotent. Some of the most important medical conditions that can cause impotence are:

diabetes
hypertension (high blood pressure)
vascular disease (clogged arteries) – linked with smoking
severe liver disease
thyroid disease
neurological conditions (for example, spinal injury, multiple sclerosis)
Peyronie’s disease
tight foreskin
kidney failure.

As cardiovascular disease is one of the most common causes of erection failure, erection problems can be a warning that your arteries are unhealthy. So it is not surprising that men with impotence have an increased risk of angina, heart attacks and stroke (Journal of the American Medical Association 2005;294:2996–3002).

Some prostate operations (especially radical prostatectomy) can also cause impotence.

Ageing. The likelihood of erection problems increases with age. The US Massachusetts Male Aging Study reported erection problems in:

25% of 65-year-olds
55% of 75-year-olds
65% of 80-year-olds.

This is because the older you are, the more likely that you have one of the medical conditions that damage blood vessels and nerves causing impotence (see also the section on sex and ageing.)

Smoking. Smoking doubles your likelihood of erection problems. This is because if you are a smoker your arteries are likely to become clogged (atherosclerosis). During an erection the penis swells because it fills with blood. If your arteries are clogged, the blood cannot flow in efficiently and your erection will not be as good.

A study of 4462 Vietnam war veterans, aged between 31 and 49, showed that smokers had a 50-80% increase in the risk of impotence compared with non-smokers. Another study has shown that for every year you smoke 20 a day, you increase your risk of impotence by 2–3%.

According to a British Medical Association report, about 120 000 men in the UK in their 30s and 40s are impotent as a result of smoking.

Unhealthy lifestyle. A sluggish lifestyle, in which you spend a lot of time watching TV and are too fat, makes impotence more likely. This was shown in a US study of 43000 men (Annals of Internal Medicine 2003;139:161–8). Those who took regular exercise were less likely to have erection problems. And according to Harvard Men?s Health Watch, a man with a 42-inch waist is 50% more likely to be impotent than a man with a 32-inch waist.

Distance cycling and motorcycling. If you decide to take more exercise, distance cycling may not be the best choice. About 1 in 5 distance cyclists notices some numbness of the genital area after a ride, and 13% experience problems with erections for the following week or longer. The cause is pressure of the saddle on nerves and arteries. For the same reasons, motorcycling may also increase the risk of erectile dysfunction. Choose a wide saddle with plenty of padding that makes you sit back firmly, and dismount if you start to experience numbness.

Psychological factors can, of course, cause impotence. These include:

guilt
depression
losing interest in your partner
a partner who finds intercourse painful
low self-esteem
fear of not performing well.

Often both physical and psychological factors are involved. A physical problem impairs erections, and you then become so preoccupied with the question ‘Can I maintain my erection this time?’ that sexual arousal becomes impossible. Anxiety actually has the physical effect of contracting the muscles of the erectile tissue, preventing blood entering the penis and allowing the blood to drain away.

Written by: Dr Margaret Stearn
Edited by: Dr Margaret Stearn
Last updated: Saturday, September 3rd 2011

In part 3 of his penis trilogy, Dr Phil Hammond discusses penis size, shape and function, and what you can do about erectile dysfunction. Click on the video below for more information.

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Comments on this article

Posted by Optional on 08/01/2012 at 06:15

I hate the look in my wifes eyes when having sex, it takes her much lomger than me to reach an orgasm. My problem is, when I ejaculate, I lose my erection, so she is not being satisfied. I have to finsh her with my fingers or sex toy, but she doen'ts like it as much. I smoke, high cholestorol, diabetes, high blood pressure. I take viagra, but it doesn't help me keep an erection, only helps me get one

Posted by tequero on 06/09/2011 at 06:40

ihave aweak base of erected penis.the penis falls down in erected state.ihave been using size genetics extender,but to no use ,can we use hot water therapy on erect penisor acold watered therapy canwe use aultra sound massager onthe area infected by peyorenes disease to improve blood circulation

Posted by sak on 20/08/2011 at 06:28

I am 42 yrs.old & having problem with errection since 3yrs back.Iam diabetic & fasting sugar around 200 dl.is this problem with me due to diabetis? if so how to get rid from it?

Posted by Optional on 11/08/2011 at 02:44

Having trouble maintaining an erection, My Girlfriend can get it up but when she goes to put it in, It goes soft. I also take stimulation pills, They worked before but now there useless.... Help..:(

Posted by Optional on 11/07/2011 at 06:05

I havd a funny experience, whenever I have my girlfriend, my erection is perfect but this worsen when i meet new girl and wants to make love to her. I fail to erect totaly. What could be the problem.

Posted by sexually frustrated on 30/06/2010 at 11:34

How much does alcohol play a part in lack of sex drive and in premature ejactulations? or not being able to keep an erection?

Posted by Optional on 31/03/2010 at 09:29

I am only 18 and i am having erection problem GOD help me. Yes I do ride motorcycles for a long distance. Must stop to see if i get full erection.

Posted by jason on 09/02/2010 at 05:46

i am having problems with girlfriend she is very frustrated cus i cant get an erection and when i do i goes down on me very quickly have tried alot of stimulation bathing in cold water masterbation and oral i do go up but come down very quick had depression over a year ago and of meds need to find ways that is gonna help me maintain and satisfy my lover

Posted by Rebecca on 09/01/2010 at 04:22

I am wondering how can I handle this problem with my husband I have told that I need intimacy to be quite honest I need sex My body is craving it ,I'm getting sexually frustrated.What should I do

Posted by Derren on 23/12/2009 at 04:33

I am taking some anti inflamatories for some leg pain that I am having and it prevents me from having an erection. Today when the meds were not in my system anymore, the erection was totally there.

Posted by leo on 30/11/2009 at 05:21

have ed for years now about 5 could my arteries be blocked and need surgery

Posted by clarence saunders on 16/07/2009 at 12:40

I have a burging disc in my lower back. Does this cause leg problems??? I am having problem with walking.