THE PHYSIOLOGY OF AN ERECTION
Contrary to popular belief, there aren’t any bones in the penis. The penis is composed of three different regions: a pair of parallel spongy columns called the corpora cavernosa and the central area called corpora spongiosa. The central area contains the urethra (the tube that urine comes out of). All three regions are made up of tissue that becomes erect or hard.
Erectile tissue, or the part of the penis that gets hard, has lots of tiny pool-shaped blood vessels called cavernous sinuses. These pools are connected to small arteries. When the penis is limp, the small arteries that lead to the cavernous sinuses are small and there is only a small amount of blood flowing to the pool-shaped vessels.
When arousal occurs, the nervous system activates an increase in blood flow to the penis. The pool-shaped vessels fill up with blood.
An important reminder from biology 101, is that arteries bring blood to organs, and veins move blood away from organs. When the penis becomes erect, the fullness of the penis pushes against the veins and closes them off. The blood is trapped in the arteries and the veins. This is what makes it hard and firm.
In order to maintain an erection, there must be an increase in blood flow and at the same time, the blood has to be trapped in the penis. When the erection goes down, the valves or the doorways to the veins open up, and the blood flows away and the penis becomes limp again.