Male erectile dysfunction is defined as the inability to obtain erection sufficient for vaginal penetration(or intercourse) persistently for a period of at least 3 months.
The causes of such dysfunction can best be approached with an understanding of the process of normal erection. The arteries supplying the erectile bodies (Corpora Cavernosa) initially dilate, increasing blood flow into the corpora, and finally stretching the hard covering around the bodies (tunica albuginea), and thus stretching and compressing the veins which normally allow egress of blood from the corpora. The resultant pooling of blood, at higher pressure, within the erectile body, and with further compression of the corpora by muscles at the base of the penis (bulbar muscles, against the elastic but firm tunica albuginea(when stretched to its limit), produces the rigidity required for vaginal penetration.
The dilation of the (deep cavernosal) arteries is produced by a chemical nitrous oxide, which is produced as a result of a chemical cascade, started by the stimulation of the nerves required for erection. These are the pelvic nerves, some of which are parasympathetic, and which bring relevant impulses from the brain and spinal cord. Erection is reversed (detumescence) by contraction of the arteries involved, and that in turn occurs with stimulation by other nerves (sympathetic), either with ejaculation, anxiety or other causes of inhibition.