So, in the third of these articles, we looked at thebasic mechanics of how we get an erection.
Just simple plumbing after all.
Butif our bodies are going to get the desired result, there is a precise checklistof steps to follow.

• the brain has to recognise that we are in the sex zone - sexualstimulation is not just a physical thing, the brain has to be in on the act soit can send those vital messages down to the guys operating the valves,
• "Start releasing the nitric oxide!"
• The chemical factory starts to churn and, before you can say, "Geronimo!"or whatever other words come to mind at a time like this,
• guanylate cyclase (GMP) and cyclic guanosine monophosphate are pumping outinto the muscles
• and artery dilation, here we come.
  

This may look simple on paper, but if anything throwsa spanner in the works to disrupt the sequence, the result is nada.
So let's start breaking it down in manageable chunks.
We have to start in the brain.
If anything is wrongwith the initial response to the stimulation, there is no physical response ofany kind.
This can be a problem with the neurotransmitters - the chemicalsthat transfer messages between neurons and cells - or it can be psychologicalwhich can be just as terminal as a physical cause.
If we have messages, they then have to travel down thespinal cord and transfer to the area around the penis.
Any damage that preventsthe nerve messages from arriving at their destination, means there is nophysical response.
If the messages arrive, you then depend on muscles torespond as they should to dilate arteries and compress veins, and fibroustissue must absorb the blood.
So any damage to the central or peripheral nervoussystem, to the musculature, blood vessels or fibrous tissue can be the cause ofthe ED.
This damage is most usually caused by disease.
The most likelycandidates are:

• kidney disease;
• liver diseases including fatty liver, hepatitis and cirrhosis which areassociated with alcoholism;
• multiple sclerosis which affects the central nervous system;
• diabetes mellitus which may lead to cardiovascular disease or nerve damage;
• arteriosclerosis or other vascular diseases - about 40% of men withdiabetes suffer with ED;
• hormonal problems, for example producing low levels of testosterone; and
• neurodegenerative disorders including Alzheimer's, Parkinson's andHuntington's disease, amyotrophic lateral sclerosis, etc.
  

The way we live our lives may also actively contributeto our lack of sexual success, particularly if our choices have begun to causecardiovascular problems.
So heavy smokers, those who are carrying excessiveweight and those who never take exercise are most at risk.
Those who play sportor pick up an injury in an accident can fall prey to ED if there is damage tothe penis itself, or to the spinal cord, pelvis, prostate or bladder.
Then we come to the unintended consequences ofnecessary surgery.
For those who are diagnosed with cancer, sometimes radicalsurgery is required to cut away as much of the affected tissues and organs aspossible.
If these operations are for cancer of the prostate or bladder, therisk of damage to the nervous system and blood vessels near the penis is high.
Similarly, we may already be takingmedication that causes ED as a side-effect.
The most common classes of drugsthat can have this undesirable effect are:

• for the relief of hypo or hypertension;
• antihistamines;
• tranquillisers and antidepressants;
• appetite suppressants; and
• used to treat heartburn and stomach ulcers, e.
  g.
  Tagamet.
  

Finally, there are a range of psychological conditionsincluding:

• stress;
• anxiety;
• low self-esteem;
• guilt; and
• depression.
  

Whether directly or indirectly, all these mentalstates may contribute to performance anxiety where we develop an unreasonablefear of of sexual failure.
It is estimated that about 15% of all ED cases havea psychological component.
The difficulty with this estimation is that men whohave physical illnesses also have the same list of possible feelings, sodistinguishing cause and effect can be problematic.
Well, that's a fair list of causes - probably a fewmore than you thought.
Some of them are on the serious side which should sendyou warning messages that going to the doctor is always a good idea.
ED is justa symptom.
If all you do is to take Viagra, Cialis or Levitra you are leavingthe underlying condition undiagnosed and untreated.
Do you really want to take that risk?