Impotence is the inability of a man to obtain a firm erection of the penis when sexually stimulated. It is a very common problem, and something that every man experiences at some time, particularly in middle age and older.
The process of erection sometimes mystifies women. A man has no direct control over his erection, as it is a local reflex in the pelvis triggered by sexual excitement. A man even has difficulty in detecting if his penis is erect unless it is touched or seen.
The penis contains two sausage shaped sponge filled tubes (corpora cavernosa) that fill with blood under pressure when a muscular ring closes off the drainage veins behind the base of the penis. This is how an erection occurs.