I go running around a lake and brambles scratch me. The wounds should heal quickly on my legs, but they don’t, because I scratch the scratches, and I scratch and scratch. I have always been this brutal with healing injuries, but usually my skin healed them fine. Now that I am menopausal, and my collagen is affected by my fluctuating hormones, the injuries stay as scars and reminders. I can plot every fell run, every fall, from the white lines. I know this, yet still I scratch. And I ask myself, why do I like to see the blood?

A man I talk to at a party begins to look green after he asks what I’m working on. I don’t like to see blood on screen, but I don’t understand haemophobes and fainters. I love my blood. How could I not? This is what it does for me: it carries oxygen to my organs and tissues; it gives me the strength to run up hills and carry shopping; it removes carbon dioxide and waste products so I won’t die; it carries the white blood cells that rush to repel invaders and infection, and usually defeats them; it travels around my body along a circulation of veins, arteries and capillaries that, stretched out, would measure 60,000 miles, twice round the earth and more. All those scars and scratches: each time, my blood rushes to the injury, performs what is called, beautifully, a clotting cascade, yet doesn’t clot anywhere else.

Blood has always been thought magical. Homer wrote of a dead woman who came alive by drinking the blood of a sheep. Outside poetry, the sight of blood meant death and injury. The only people who bled and survived were women, so they must be witches: many men have believed that women exude powers when they menstruate. We could scare the leaves off trees or strip ears from corn. We could reduce the power of dough rising by 22% (an actual study), and make roses wilt. This is not an ancient belief. Throughout the world, the bleeding woman is still thought to pollute temples and kitchens or to curdle milk. I’ve met smart young women who are convinced they can rot nail polish by menstruating. Don’t dismiss these taboos as barbaric; they are everywhere. It took until last year for the feminine hygiene industry to dare to show a liquid the colour of blood in a sanitary pad advert, rather than the usual blue windscreen fluid.

For most of history, blood went on a one-way journey: it was spilled, or it was removed. Bloodletting was used to regulate the body’s humours, but also to fix everything, including severe haemorrhage. It was not until the 17th century that any serious attempts were made at putting blood back into an ailing body. Any blood: dog, sheep, cow. Calves and lambs were popular, as they were thought to transmit their sweet natures along with their proteins and usefulness. Samuel Pepys wrote of a madman, “cracked a little in his head”, who was given 12 ounces of sheep’s blood. This was transformed into a satirical play whose hero, given ovine blood, began chewing the cud, “had Wool growing on him in great quantities, and a Northamptonshire Sheep’s Tail did soon emerge or arise from his Anus or Human Fundament”.

It was like putting a half-dead flower in water on a hot day

But the survival rate was patchy. Transfusion, by now with human blood, only thrived after Karl Landsteiner understood in 1901 that there were blood groups, and that mixing them could be fatal. In the First World War, military surgeons reported dramatic results in the hopeless and near lifeless. It was, wrote one, “like putting a half-dead flower in water on a hot day”. Best of all, this new astonishing medicine was walking around in the shape of human beings. Even better: the people who gave blood could clearly spare it. Chop off an arm and you can’t replace it. Give away a pint and your plasma will regenerate within 24 hours and your haemoglobin within weeks.

So I do. Because I can, mostly. I have no particular reason to give blood. Neither I nor anyone in my family has ever needed a blood transfusion. But we might have. Every three seconds, worldwide, someone receives blood, usually from a stranger. But whole blood is rarely used any more: instead, my donation will be fractionated into components. That bag of red stuff hanging by the bed on TV soaps is red blood cells. The yellow stuff is plasma. There may also be platelets, or cryoprecipitate, or other parts of blood that have been separated so they can be more useful, more specialised.

Almost three-quarters of blood now is used to treat chronic disease; 6% is used to treat blood loss in childbirth and the rest is used in surgery, emergency and elective. In the resus room of a London hospital, I watched a catastrophically injured cyclist be given blood. By the end of her treatment, she had received 60 units. In the UK, there are 2m blood transfusions a year.

The NHS Blood and Transplant service (NHSBT) needs 6,000 donors a day (Scotland and Northern Ireland have their own blood services). Stocks run low during football matches, the Olympics, Christmas and royal weddings. Also, the majority of donors are over 45, like me, and they need young donors to replace us. Every year, they need 200,000 new donors to replace the retired, deferred or deceased.

So I make my appointment, and I turn up with the nervousness of a pupil summoned to the head’s office. I dread the deferral. For most of my adult years, I’ve been to too many strange places to be a desirable donor. “Have you travelled outside the UK in the last six months?” Yes. Iraq, Afghanistan, Kosovo, Liberia. No, thank you.

Nowhere does a mint biscuit taste as good as at a donor centre

Other things that would get me deferred: a recent tattoo or piercing, a pregnancy or being a male homosexual who has had sex in the past three months. Once, I was rejected for having small veins and felt wounded, as if I’d been found wanting. The refusal was medical, not personal: making blood safe is an ongoing challenge when viruses don’t announce themselves before arriving to infect us; and when people are still dying from the contaminated blood scandal of the 1980s, with haemophiliacs and others given blood products along with HIV and hepatitis C. Deferrals and refusals – along with the scientific wizardry of processing – are our best defence.

Even now, lying in the plastic donating chair, clenching my buttocks to keep the blood flowing as advised, I watch my blood anxiously, checking the flow, counting the numbers on the monitor: 50ml, 120, 350, up to the maximum 470ml. Almost a pint. About 13% of my blood supply. I know I will suffer for this, because I run, and because my haemoglobin, which delivers oxygen to my muscles, will not recover for a few weeks, making hills harder as my body tries to make its usual effort with less fuel. Still, I give my blood away because it feels good and because there is nowhere else that a mint biscuit tastes as fine as in the hot drinks area of a blood-donating centre.

But why do I feel good? I will never meet the people my donation goes to. Although I am giving away something that could not be more intimate or personal, the process is profoundly anonymous. This wasn’t always the case: during the Second World War, Russian blood donations included the donor’s name and address. As the donors were usually women, and the recipients usually soldiers, this had an obvious outcome of bloody romances. Anonymity is thought to be the foundation of a safe blood supply, along with the blood suppliers being unpaid volunteers: paying people for blood is thought to encourage people to lie about their health.

I don’t need to know who has got my blood, but I’d like to know it has gone somewhere. For a year or so, NHSBT has texted donors to tell them their blood has been issued to a particular hospital: this has been wildly popular. My latest armful went to Leeds General Infirmary, about half a mile from the blood donor centre where I gave it away (actually it travelled to Manchester to be processed and separated before coming back again). I hope it will help someone who has anaemia or thalassaemia or sickle cell; who has cancer or heart surgery; who is bleeding after childbirth or an accident. Despite millions of dollars and decades of research, nobody has yet figured out how to make artificial blood as good as the real thing. If we want blood, it has to come out of someone’s arm. It may as well be mine.

Nine Pints by Rose George, is published 25 October (Granta, £14.99). Order it for £12.89 at guardianbookshop.com