Anything which interrupts or interferes with the process described from page 1.
The most common cause of erectile dysfunction in older age is vascular insufficiency, either because of decreased arterial flow to the penis, caused by obstruction of the pelvic or penile vessels from atherosclerosis (plaques--related to cholesterol), previous trauma or surgery, diabetic vascular disease, or sometimes from smoking!
Atherosclerosis or other vascular disease can also lead to inelasticity of the tunica albuginea, with resultant failure to stretch with erection, and thus failure to stretch and compress the veins of the tunica, with failure to store the blood within the corpra, and thus a loss of rigidity and failure to maintain erection. (This is called "venous leak" or corporal insufficiency)> Less commonly, abnormal veins can be a cause of "Venous leak"; this usually occurs with a life-long history of failure to maintain erections.
Conditions which interfere with the chemical cascade between the nerves controlling erection and the vascular tree involved undoubtedly occur, but are harder to define. Antihypertensive medications and certain diuretics (hyrochlorthiazide, Lasix), and other medications (Cimetidine for example) probably work this way too.
A low testosterone, the male hormone, which probably works primarily centrally to maintain libido, has also been demonstrated to have a permissive effect on the chemical cascade
Conditions which interfere with the nerve transmission or function of the nerves responsible are also common: Surgery or injury to the pelvis, spinal cord, brain, or perineum can interrupt them; dysfunction (neuropathy) can be a feature of diabetes (the most common cause of erectile dysfunction in middle age men), chronic alcohol excess (alcoholic nueropathy), multiple sclerosis, the neurologic dysfunction of HIV, and other neuropathies. Sometimes pure and simple back pain can directly interfere with erection, without peripheral nerve dysfunction. Even acute prostatities (inflammation) can interfere with erection, as the nerves responsible run along the sides of the prostate.
The central stimulation for erection seems to correlate with libido (Sex drive), and thus can be deficient when serum testosterone is low. (This is usually most apparent when libido disappears before erections decrease). Liver disease, with failure to metabolize endogenous (adrenal) estrogen, and subsequent inhibition of testosterone production by the testes, can cause this. Rarely, a tumor of the pituitary producing prolactin, can turn off testosterone production. Finally, certain psycho-tropic medications (tranquilizers and MAO inhibitors) probably work centrally to interfere with erections. Debilitating disease, such as hypo-thyroidism, adrenal insufficiency, inanition, probably work both centrally and peripherally to take away erections.
Psychological causes are not uncommon, and also work centrally by increasing the sympathetic-mediated inhibition of erection. Anxiety, fear of failure, and more complex causes seem to work in this way. Many of these can be temporary.