In a normal person, it is essential that with a normal hormonal balance, the blood supply to the penis, both the arteries and the veins must function normally. Importantly, the nerve supply to the penis, in this case, the autonomic nerves should be intact and functioning normally. These nerves are the ones most affected in diabetes and the vast majority of all erectile dysfunction seen in a male with diabetes is due to the nonfunctioning of these nerves.
Without going into details about the vascular supply or the nerve distribution to the penis, this can be seen in the following picture which basically tries and explains the stages of erection and flaccidity of the penis, although we shall discuss these in some more detail later.
The mechanisms of erection and flaccidity are shown in the upper and lower inserts, respectively. During erection, relaxation of the trabecular smooth muscle and vasodilatation of the arterioles results in a severalfold increase in blood flow, which expands the sinusoidal spaces to lengthen and enlarge the penis. The expansion of the sinusoids compresses the subtunical venular plexus against the tunica albuginea. In addition, stretching of the tunica compresses the emissary veins, thus reducing the outflow of blood to a minimum. In the flaccid state, inflow through the constricted and tortuous helicine arteries is minimal, and there is free outflow via the subtunical venular plexus.