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Blood is the body’s best friend. It is also the body’s CIA, its FBI, its MI5 – it hoards a wealth of secrets. Blood is a database. Blood tests can show what kind of creature you are, how and where you live, how healthy you are. What you do. What you eat. If you have a fry-up for breakfast, your blood will quickly fill with white globules of fat. A blood bank technician need only glance at his bags of donated blood to discover which donors have recently had a heavy meal – their donations will have a thick coating of ‘cream’ on top. These fat globules are soon dissipated. As the blood flows through the liver, the fat is efficiently skimmed off. A couple of hours after the meal is clear again.”

Even your state of mind can be detected in your blood. At times of physical stress, we speed up our secretion of ‘acute phase proteins’ – chemicals which circulate in the blood and help it to clot faster. This is useful if we are in danger of being wounded at any minute. Our bodies are well supplied with mechanisms for protecting us from bleeding to death. Blood clotting is an exquisitely complex and finely-choreographed dance performed by cells and molecules in harmony. The cells (the blood platelets) can sense when a blood-vessel is damaged, and immediately release chemical messengers. These chemicals make the vessel contract – to slow or stop the bleeding. Further chemical reactions, involving a score or more of clotting factors, turn the blood within minutes from a freely-flowing liquid into a sticky, rapidly hardening jelly. It’s a mechanism that unobtrusively saves our lives day after day.

Yet it can let us down badly. A recent case was reported of a young woman found dead in a blood-drenched flat. She had cut a small vein in her foot. As the wound bled – slowly but steadily – she wandered distractedly around the house, trying to mop up the mess. As long as she stood upright, her veins could not contract to stop the bleeding. Gradually, she bled to death. If she had just put her feet up (or been lucky enough to faint), she would have been all right. As it was, the pressure exerted by five feet of blood vertically above her injury kept the blood flowing out of her.

The taller you are, the worse this particular problem. For the giraffe, upwards of twelve feet high, the pressure exerted by its blood on the tissues of its legs and feet is immense. Consequently, it has developed nature’s version of the ‘anti-gravity suit’ – the suit supersonic fighter pilots use to stop all their blood flooding into their legs as they turn upwards at speed out of a steep dive. The giraffe’s legs are clothed in a tight, leathery casing which is stretch-resistant. It has a sophisticated system of valves in the veins to ensure that the slightest movement of the legs pumps blood back uphill towards the heart. But a giraffe with a lacerated leg vein would be in serious trouble. And as for an injured artery – the giraffe’s heart generates a blood pressure twice as high as a human being’s. A severed artery could shoot a jet of blood ten yards or more. Even in human surgery, a spray of arterial blood on the operating theatre wall is no uncommon occurrence. It only takes a surgeon to divide a small artery by accident, without tying it off first.

The human penis is another organ equipped with an unyielding fibrous coat. And the purpose is similar – to prevent the organ from blowing up out of all proportion, even rupturing, under the pressure of blood. In this case, it’s all to do with the elegant and intricate mechanism of penile erection.

What saves the penis from bursting? Firstly, its tough covering. Secondly the presence of an enzyme which continuously counteracts the effects of nitric oxide – so that NO must be continuously replenished for the erection to be maintainee.

This is where Viagra comes in. By suppressing the enzyme that opposes NO, it allows NO levels to build up – keeping the arteries relaxed and dilated, filling the penis with blood, and restoring or improving erections in otherwise impotent men. (In terms of units of happiness per gram of drug, it must be among the star performers).

The problem – there’s always a problem – is that the erection doesn’t go away when its work is done. It may persist for an hour or even longer, and this may be somewhat disturbing. A state of continuous erection is known as priapism, after the god Priapus whose statues – unless defaced with a chisel, as they often were – maintained a spectacular, rock-hard erection.

Some people are afflicted with this condition even without the help of Viagra. Reputedly, Henri IV of France was such a case, relying on a constant stream of mistresses to service his permanent erection.

In the real world, however, priapism is not a pleasant condition. It is a common enough symptom in people with blood diseases where the blood tends to clot too easily. Young men with sickle cell disease can suffer this way, for example. In them, an attack of priapism is a common (and painful) medical emergency. It’s vital to get blood flowing through the organ (and out of it) again, if the choked-up blood spaces are not to clot irrevocably. If they do, the penis becomes internally scarred and permanently impotent. Viagra is a forbidden drug for sickle cell patients and others with disordered clotting systems; and even in people with normal clotting, there have been fears that Viagra could trigger prolonged priapism. In the event this has only happened very rarely, and the effect has eventually worn off.

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