Physical factors include drugs (alcohol, nicotine, narcotics, stimulants, antihypertensives, antihistamines, and some psychotherapeutic drugs); injuries to the back, problems with an enlarged prostate gland, problems with blood supply, nerve damage (as in spinal cord injuries); or disease (diabetic neuropathy, multiple sclerosis, tumors, and, rarely, tertiary syphilis); failure of various organ systems (such as the heart and lungs); endocrine disorders (thyroid, pituitary, or adrenal gland problems); hormonal deficiencies (low testosterone, estrogen, or androgens); and some birth defects. Sexual arousal disorders were previously known as frigidity in women and impotence in men, though these have now been replaced with less judgmental terms. Impotence is now known as erectile dysfunction, and frigidity has been replaced with a number of terms describing specific problems with, for example, desire or arousal. The creation of an erection is an extremely complicated cascade of events that requires many different things to happen. There are numerous chemical transmitters involved in this including epinephrine, norepinephrine, acetylcholine, prostaglandins and nitric oxide. The exact mechanism by which erection occurs is still unclear but we do know that the neural input from the brain is extremely important. Reflex erections, as seen in people with cord damage such as paraplegics, are often poor erections and not sustainable for prolonged periods of intercourse. In order to increase the size of an erection, there must be an increase in blood flow and, at the same time, the blood has to be prevented from leaving the penis. Abnormal levels of certain hormones, such as testosterone, thyroid hormone and a pituitary hormone known as prolactin, can interfere with erections and sex drive (libido). This is an uncommon cause of erectile dysfunction.
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