Well, you can call it what you like; from woody to hard-on, the horn, a babies arm; whatever – they are all erections. But how do they happen (mechanically at least)? The penis has three long tubes of cylindrical erectile tissue which , are connected together by fibrous tissue. Called corpora cavernosa these two identical sections run parallel along the sides of the penis with the third tube (corpus spongiosum) laying underneath. These three also surround the urethra the tube which transports either sperm or urine. All three masses are like sponges in that they contain large spaces between loose networks of tissue. When the penis is limp (doctors call this ‘flaccid’ or ‘resting’), then the spaces collapse and the tissue is reduced (that’s why it’s smaller). However when you start to get an erection, blood flows into these spaces causing the penis to enlarge. Now in theory this happens because of physical or psychological stimulation, but try telling that to a teenage boy who gets one while slumbering through algebra! Anyway as blood enters there is also a temporary reduction in the rate and volume of blood leaving the penis. So as the arteries carrying blood to the penis dilate the veins leading away use funnel-shaped valves to restrict the outflow of blood. Then as the erectile tissue begins to enlarge additional pressure happens as the veins to be compressed against the surrounding tissue, which in turn further restricts the outflow of blood. Now during all this process the three tubes don’t swell up by the same pressure. The underneath tube (remember it’s called the corpus spongiosum) doesn’t become as hard as the two main sections (corpora cavernosa), if it did this would crush the urethra making it impossible to ejaculate (no thanks)! When this process is over your penis is becoming soft again as the arteries relaxed and contract.
And that’s the end of the story until the process starts again; for teenagers this will probably be in five minutes – but for the rest of us it takes a little longer, but that’s another story!