In some cases, ED can be a warning sign of more serious disease. One study suggests ED is a strong predictor of heart attack, stroke, and death from cardiovascular disease. The researchers say all men diagnosed with ED should be evaluated for cardiovascular disease. This does not mean every man with ED will develop heart disease, or that every man with heart disease has ED, but patients should be aware of the link.
The Massachusetts Male Aging Study (MMAS) documented an inverse correlation between ED risk and high-density lipoprotein (HDL) cholesterol levels but did not identify any effect from elevated total cholesterol levels.  Another study involving male subjects aged 45-54 years found a correlation with abnormal HDL cholesterol levels but also found a correlation with elevated total cholesterol levels. The MMAS included a preponderance of older men.
Inside the cell, NOS catalyzes the oxidation of L-arginine to NO and L-citrulline. Endogenous blockers of this pathway have been identified. The gaseous NO that is produced acts as a neurotransmitter or paracrine messenger. Its biologic half-life is only 5 seconds. NO may act within the cell or diffuse and interact with nearby target cells. In the corpora cavernosa, NO activates guanylate cyclase, which in turn increases cyclic guanosine monophosphate (cGMP). Relaxation of vascular smooth muscles by cGMP leads to vasodilation and increased blood flow.
Stop trying to force the erection - it’s not possible to give yourself an erection just by trying hard enough, if it were then you wouldn’t be reading this. An erection is known as an ‘unconscious response’, meaning you can’t control it by thinking about it. In fact focusing on your erection too much is more likely to block it, so try instead to focus something else, such as:
Fortunately, impotence is usually treatable. A thorough evaluation starting with a history and physical exam is needed to help diagnose the underlying cause. Once the cause of impotence is determined, treatment can be tailored to target that cause and any other contributing factors. Treatments used for impotence may include medications, vacuum devices, surgery, and psychotherapy.
Although erectile dysfunction increases progressively with age, it is not an inevitable consequence of aging. Knowledge of the risk factors can guide prevention strategies. Specific antihypertensive, antidepressant, and antipsychotic drugs can be chosen to lessen the risk of erectile failure. Published lists of prescription drugs that may impair erectile functioning often are based on reports implicating a drug without systematic study. Such studies are needed to confirm the validity of these suggested associations. In the individual patient, the physician can modify the regimen in an effort to resolve the erectile problem.
The primary nerve fibers to the penis are from the dorsal nerve of the penis, a branch of the pudendal nerve. The cavernosal nerves are a part of the autonomic nervous system and incorporate both sympathetic and parasympathetic fibers. They travel posterolaterally along the prostate and enter the corpora cavernosa and corpus spongiosum to regulate blood flow during erection and detumescence. The dorsal somatic nerves are also branches of the pudendal nerves. They are primarily responsible for penile sensation. 
It's unlikely. Testosterone levels fall as you get older but they have to fall really low to cause erectile problems. Thus assessment for erectile dysfunction includes a testosterone blood test but only replacement if it is very low. Giving someone more testosterone than their body would have made, even at a young age, can cause blood clots due to blood thickening and even reversible infertility. So don't be tempted with homespun remedies.
Lifestyle choices that impair blood circulation can contribute to ED. Smoking, excessive drinking, and drug abuse may damage the blood vessels and reduce blood flow to the penis. Smoking makes men with atherosclerosis particularly vulnerable to ED. Being overweight and getting too little exercise also contribute to ED. Studies indicate that men who exercise regularly have a lower risk of ED.