Think of this as you approach orgasm with a tender 'Urrggh!', or even a triumphant 'Aaarghhh!'. Think of what's going on down there, the extraordinary complexity of this simple act that will change your life forever, and start someone else's. Think back to school science lessons, and those gruesome diagrams of a human penis in cross-section .....
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It has, of course, been designed for this purpose (writers in Sunday newspapers will sooner or later call it a "design classic'). The penis has a rich blood supply from the internal pudendal artery. An erection comes about when the penis's cavernous spaces - a technical term, as it happens - are filled with this blood, and the veins compress to stop it draining away (I think we all know what this feels like). Meanwhile, the testes are gearing up for action. Sperm have been growing there for several days. They are as ready now as they will ever be. There are 300 to 400 million of them in every ejaculation. They are not large.
Each one has two distinguishable parts, a head and a tail. The head of a sperm varies in shape for each animal species. In man, it's flattened and almond-shaped. Each one is never more than five microns long and three microns wide (there are 25,000 microns to every inch). Inside are the chromosomes, which carry some of the characteristics that define you: the colour of your hair, the shape of your nose, your knobbly knees. It might help, then, to imagine each sperm as a tiny swimming plankton-type thing that just happens to have your face.
Covering the head of.the sperm is a cap known as the acresome. This is the bit that contains the chemical substances the sperm needs to enter an egg - the sperm's equivalent of a warrant. You only need one sperm to fertilise each egg. He may be little, but my, is he potent. ......
So you cry 'Aaarghhh!' (or maybe 'Urrggh!') and the sperm start swimming. Their tails are about 50 microns long, and at the tip, barely half a micron wide. They whip and undulate, and the sperm move forward. Up the urethra, out of the glans and, because you have forgotten to use a condom, or have refused to use one, or have pretended to use one and whipped it away at the last minute (this requires practice and a plausible manner), straight into the waiting vagina and off on their long and certainly doomed quest.
Here we should pause a moment, as it's possible that you now have in your mind one of several images from Woody Allen's seminal film Everything You Always Wanted To Know About Sex (But Were Afraid To Ask). Indeed, you might be unable to shake off the suspicion that your sperm have not your face but Woody Allen's. This would be terrible bad luck, as well as being unlikely. Each of the 400 million sperm in a good day's load will be slightly different. And chances are that all of them will be slightly different to Woody Allen.
No, the Hollywood films you should be thinking about tend to star Arnold Schwarzenegger, Bruce Willis or, if the video shop is about to close and all the best ones have already been taken out, Sylvester Stallone. Films starring these men tend to conform to certain narrative rules. Whatever Arnold, Bruce or Sly has to do to reach the end credits, the odds are against him. The chances of his reaching his goal are negligible. His enemies are many and powerful. All he has in his favor are determination, an enormous gun and a flak jacket/dirty vest/half-chewed cigar. But we know, and he knows, that Arnold, Bruce or Sly will make it to his destination or die trying. Not unlike 400,000000 of your sperm, as they splash hopefully past the G-spot.
For you, as issuer of these sperm, the job is done. Indeed, you have already rolled over and started to snore. But the sperm swim, swim and swim some more. They can live in your partner's body for two or three days if they are lucky. Of the 400,000,000, approximately 24,177,489 will swim in the wrong direction and be disqualified. A further 156,436,002 will be eaten by ravening antibodies with sharp teeth, while 219,386,508 will eventually give up and have a lie-down. This leaves Arnold/ Bruce/Sly and his lovable best mate who gets all the best lines but will be killed at the very last moment, just when you were hoping he'd pull through. These two will have swum from the vagina through the cervix, then a sharp left turn into the uterus and over to the fallopian tube, where a vast egg, or ovum, will be waiting.
Did she tell you she was ovulating? Did you ask? But never mind about that now, for our two sperm are still swimming as hard as they can. And then, disaster. The Best Mate sperm is chewed up by an antibody. Arnold/Bruce/Sly is alone. Gritting his teeth, and firing his Uzi at any hormone that strays out of line, our hero makes straight for the egg. To him, it's huge - between 120 and 150 microns in diameter, slightly bigger than the width of a human hair - and full of chromosomes which, over succeeding months, will fight tooth and nail with Spermy's chromosomes for biological supremacy. That's for the future. For now, as Bruce/Arnie/Sly would say, there's an egg to fertilize. It swims ever closer, sure of its destiny, never pausing for a moment to wonder whether this is a good idea, until ...
POP!
Congratulations. You're going to be a father. To find out more, you can check out Welcome To Fatherhood.