How much would you pay for a kidney? It's a question most of us wouldn't care to answer and hope we'll never need to ask. We assume that if our kidneys fail, either someone else's death would give us the gift of life or that someone genetically or emotionally related to us would bravely come to the rescue.

The inconvenient truth is that across the world the gap between the numbers waiting for transplants and the supply of organs is getting bigger. In the rich world, rising living standards and medical advances are extending life expectancies, producing an ever-expanding population of sick, elderly people in need of further repair. While the illnesses of old age cause more cases of kidney failure, advances in transplant surgery make spare-part surgery almost as routine as changing the oil filter on the family jalopy. So where are all these extra organs to come from?

As usual, global market economics has come up with a neat answer, at least it has for kidneys, because in his infinite wisdom God has given us two, though strictly speaking we need only one. There are a billion people in the world trying to live on 50p a day and billions more up to their ears in debt. Sooner or later it was inevitable that unscrupulous entrepreneurs operating globally would work out that a lot of money could be made by linking the desperately ill with the desperately poor.

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A conference in Istanbul this week is looking at "transplant tourism", a growing industry worth an estimated £1bn a year. At least 10,000 people annually are being paid to donate kidneys. Organ trafficking probably accounts for about 10% of transplants worldwide, according to the World Health Organisation. Here's a hypothetical scenario: a poor young Brazilian labourer, mired in debt, is befriended by an agent who offers him £3000 if he will sell a kidney. He agrees and travels to South Africa, where he waits until the patient, a rich elderly American, arrives at a nearby private clinic. The Brazilian signs forms, including one that says he's related to the recipient, though they have never met. Then a top surgeon performs the transplant. The American pays £30,000. The deal is brokered by an international organ trader.

There is a school of thought that maintains this is a perfectly legitimate transaction, a simple example of the economics of supply and demand. The impoverished donor has his debts wiped out. The rich recipient has a new lease of life. Win, win. People sell their labour, sex, sperm, ova. What's so special about kidneys? In certain cases - soldiers on active service and surrogate mothers, for instance - the entire body is essentially a commodity, so why can't a kidney be one, too?

Some use the same argument for legalising organ trafficking that is applied to abortions: that those involved are more at risk when it is underground and legalisation hands to the state control over standards, payments, aftercare and the like. In one scenario, the state would buy kidneys and allocate them on whatever basis it chose.

There's so much wrong with these arguments that it's hard to know where to begin. For a start, it is often not a win-win situation at all. Anthropology professor Nancy Scheper-Hughes, director of the pressure group Organs Watch, has found that within five years of selling a kidney, sellers are just as poor as before, often because they are unable to work for a time and lose their jobs. (The surgery is more invasive and debilitating than many are led to believe.) Some sellers are so desperate for money that they will agree to sell for as little as £250. And some are promised a small fortune, then cheated by the broker.

Also, you don't need to be either religious or superstitious to believe that the commodification of body parts is wrong and that organ trafficking simply exploits the desperation of both buyer and seller and encourages what could be called a modern form of cannibalism. Japanese sociologist Tsuyoshi Awaya describes it like this: "We are now eyeing each others' bodies greedily, as a potential source of detachable spare parts with which to extend our lives."

Of course, we should be concerned that even in a country like Scotland, there's a huge gap in health and life expectancy between rich and poor. Even more fundamentally immoral, surely, is a trade where organs move only one way: from the poor to the rich. Predominantly, it is also from black, brown and yellow people to white ones and from women to men. Will the sellers ever get transplants if they need them? Unlikely. And we can't leave it to hospitals to decide if a donor is acting under duress or because of extreme need because donors would lie out of desperation.

Perhaps the worst aspect of all this is the way organ trafficking threatens legitimate donation by disrupting what is known as "the gift relationship". This is where members of society voluntarily make selfless gestures for the good of society as a whole, without accruing any direct benefit. Carrying an organ donor card is one of the most obvious examples. This week Euro MPs called for an EU-wide donor card scheme to tackle the increasing shortage of organs. One of their fears is that the mounting commercial trade in kidneys, especially from the poor East European accession states, could undermine legitimate donation in countries where it is well established. Once a monetary value is put on a kidney, people would be less willing to consider donating them freely.

A UN conference in Vienna last month named Australia, Canada, Israel, Japan, Oman, Saudi Arabia and the US as "major organ-importing countries". The big exporters of kidneys from live donors include Brazil and India, where 43-year old Dr Amit Kumur allegedly made millions by persuading hundreds of the poor to give up their kidneys, which were sold to rich Greek, American and British patients. He was arrested in February after a police raid. And though it's illegal to buy or sell human organs in China, an undercover BBC investigation last year suggested that death row prisoners (officially around 1700 a year but believed to be many more) "volunteered" to give their organs as a "present to society". Some end up on the international market.

All this should bring into sharp focus the debate in Britain over presumed consent. The last time the issue came before MPs, very soon after the Alder Hey scandal (about dead children's body parts being stored without consent), understandably it received short shrift. Now the climate is more favourable and both Gordon Brown and Nicola Sturgeon seem receptive.

Of course, the very idea provoked a charge of the right-wing ideological cavalry, ranting about "state-sponsored body-snatchers" and spreading scare stories about people losing their eyes and livers before they were "really dead". Yet globally, a properly regulated system of presumed consent would save the lives of thousands who die, often in terrible pain, while waiting for transplants. And it would undermine the cruel exploitation of poor people in the name of a disgusting form of medical apartheid.

By the way, anyone who wants my kidneys ("one careless owner"), after I've finished with them, is more than welcome.