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The erection nerves end in the walls of the arteries which supply lood to the penis, and the impulses cause the arteries to become wider, so that an increased amount of blood flows into the penis. Normally, blood flows in and out of the penis at a steady rate, but if the penis is sexually stimulated, the blood flows in faster than it flows out, and an erection results. You can appreciate this better if you can imagine a hollow plastic cylinder shaped like a penis, closed at one end and filled with sponge. If you dip the open end of the model in water it will rapidly become heavier and stiffer. This state will continue as long as the liquid remains in the cylinder.
In some diseases, such as diabetes, the blood-vessels which supply the penile cylinders may become narrowed (or atherosclerotic). These damaged blood-vessels are unable to dilate in spite of parasympathetic nerve stimulation and the man is unable to achieve an erection – in other words he has erectile impotence.
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