Kidneys are the most popular — bought and sold on the global black market at a rate of at least 20,000 per year. Blood, tissue, skin, corneas and eggs are also highly valued. Human bones are a centuries-old mainstay.

The demand outstrips the supply, and so millions of variations on that old urban legend — some unsuspecting victim waking up in a bathtub in Vegas, missing a kidney — actually exist: People snatched off the street in India and China, held for years as chained-up blood donors.

Prisoners in China forced to donate body parts, plucked apart, sometimes alive, sometimes without anesthesia. Entire villages, like the Baseco slum in the Philippines, where the bulk of inhabitants have only one kidney — having sold the other off for a few hundred dollars to pay rent or buy food or medicine for a sick relative.

Or, as in the very recent case of a young Chinese man, to buy an iPad 2.

CLICK HERE TO SEE THE GOING RATES FOR BODY PARTS ON THE BLACK MARKET

Such is the state of worldwide organ transplantation in the modern age. Its commerce is controlled through sophisticated networks composed of kidnappers, smugglers, morticians and brokers, all aided by the porous borders of international law and advances in medicine, which have made transplantation exponentially less of a risk. The poorest countries supply; the richest countries buy.

And that includes the United States, where patients in need most typically hire an organ broker to find a foreign donor, run him or her through the testing process, arrange for entry, and find a hospital where the transplant will be performed, few-to-no questions asked.

"The fundamental problem in America is that we look at body parts as commodities," says Scott Carney, author of the new book "The Red Market: On the Trail of the World's Organ Brokers, Bone Thieves, Blood Farmers and Child Traffickers" (William Morrow). "It's not unlike going to the grocery store and buying veal — you think, 'Oh, this looks good.' You tend not to think of the unpleasant life that veal had before it wound up there."

It's for this reason, largely, that blood, tissue and organ transfers in the United States are done through voluntary donation: It allows for all involved to find reassurance in the sense that altruism is the prime motivator. The only body parts and processes we feel comfortable paying for, it seems, are reproductive ones: sperm and eggs are bought and sold, embryos implanted, wombs rented, and maybe because these involve the creation of life, rather than the extension of a dying one, commerce feels OK. Maybe there's simply too much money in it. Yet as Carney points out, we still use the "language of altruism" to describe such transactions, with the donor in these cases often described as "giving a gift."

The trafficking of organs, however, remains the dominant exchange, with the largest and most profitable rings operating in and among the US, Israel, Japan, Egypt, South Africa and Brazil — the latter three primarily suppliers, with the highest concentration of consumers in the others.

"One transplant can involve four different countries and all these different characters," says Nancy Scheper-Hughes, professor of anthropology at UC- Berkeley and one of the world's foremost experts in organ trafficking. "And you don't have to be trained as a kidney surgeon to do a transplant — most transplant surgeons regard that surgery as the equivalent of tying your shoelaces.

"It's never been busted," she says of the trade. "You can break up parts, but you can't bust it."

CHINA

Parts: Corneas, hearts, livers, kidneys, skin

Despite a nationwide ban on organ selling, China harvests the bulk of its supply from prisoners, living or dead. According to a 2006 report, co-authored by UN delegate David Kilgour, the state most often targeted imprisoned Falun Gong practitioners as unwilling donors; one witness told Kilgour that her husband, a surgeon, had extracted over 2,000 corneas from prisoners who were alive. Others are shot through the head or given lethal injections before their bodies are pillaged for parts. The government maintains that only executed prisoners who have been on death row are used as donors, and then only if their families don't claim the remains in time.

Yet in 2001, Carney writes, a surgeon named Guoqi Wang testified before the US House of Representatives that he'd witnessed hundreds of operations on prisoners who were executed solely to be sold off as body parts, and that he'd removed skin from prisoners while other surgeons extracted organs. The skin went for $1.20 per square centimeter; Wang was paid, at most, $60 per operation. He also testified that, at times, the prisoners were alive as their organs were being plucked out.

"If someone needs a kidney," Carney says, "a prisoner's getting shot in the head."

As recently as 2006, China advertised its cheap and plentiful supply of organs online, under the aegis of an organization called the China International Transplantation Network Assistance. Costs were considerably low, about one-fifth of an American organ: $30,000 for a cornea, $62,000 for a kidney, $98,000-$130,000 for a liver, $150,000-$170,000 for a lung, $130,000-$160,000 for a heart.

In the late 1990s, Carney writes, Dr. Thomas Diflo, the director of renal transplantation at NYU Medical Center, began seeing an uptick in the number of dialysis patients who suddenly showed up one day with new kidneys once belonging to Chinese prisoners — and yet another ethical dilemma is born. 'What do you do to these people," asks Carney, "who are trying to save their lives, and, in the process, may have killed someone?"

PHILIPPINES

Parts: Kidney, livers

The number of on-the-books kidney transplants here averages between 3,000-5,000 annually, yet the real number is likely far higher. One village, a slum in Manila called Baseco, is more commonly known as "One Kidney Island" after the number of residents who've sold their organs. In 2007, a 43-year-old Welshman and former surfing champion named Mark Schofield told The Post that he shopped for a donor online and was spending $81,000 of his life savings for a new kidney after waiting four years on a transplant list. "I'm not prepared to lie down and play dead," he said. "I know the moral argument, but I also know I have two young children whom I want to see grow up."

PAKISTAN

Parts: Kidneys, renal transplants

Since the passage of a 2010 law banning organ selling, researchers in Pakistan have recorded over 450 cases of trafficking, most often kidneys. Again, this is likely a very low number. As in India and China, many of those organs come from men and women who have been kidnapped. In April of this year, the president of Pakistan's Transplantation Society held a press conference. "In recent months, thousands of poor people have sold their kidneys and commercial transplantations are openly being carried out at hospitals in the large cities of the country, all under the nose of health officials, government functionaries and law enforcers," he said.

RUSSIA

Part: Eggs

Young women from Russia (as well as Ukraine) are routinely flown to Cyprus by fertility clinics looking to up their supply of fresh eggs. (Cyprus has the most fertility clinics per capita in the world, and many engage in sex-selection, a practice most other clinics will not.) The women are typically given an overdose of hormone treatments in their native countries so that they'll produce far more eggs than usual, a practice so risky it can prove fatal. They're then flown to Cyprus for a few days, paid $500 for the entire extraction, and sent home with no follow-up.

UNITED STATES

Parts: Kidneys, cadavers

Much of the black-market trade is conducted here: Morticians sell body parts, their families often none the wiser. Citizens travel for transplants, but more often work with brokers who will arrange for a seller's entry into the United States and for a doctor and a hospital that will perform the surgery, no questions asked. "One of the hospitals in Philly said, 'Oh, we took care of our problem — we fired the two surgeons involved,' " says Professor Nancy Scheper-Hughes, who investigates the trade. "And those surgeons went to work at the hospital next door. It's like the way the Catholic Church deals with abusive priests."

INDIA

Parts: Kidneys, blood, skeletons

The world's leading supplier of human bones till 1985, when the nation outlawed such transactions after one man was caught selling 1,500 child skeletons. But skeletons for medical study are in short supply — in the US, most bodies are either buried or burned — and so a black market flourishes. As Carney says, "There are a lot of laws in India, but no enforcement." Grave robbing is the main form of procurement, and it pays about $1.25 a day. Entire skeletons go for about $45 wholesale. (First-year medical students in the West are required to buy a box of bones along with their textbooks.) Most of the skeletons belong to Muslims, who bury their dead; Hindus practice cremation. In the US, importing skeletons isn't a crime: "Here," Carney says, "a skeleton is just a commodity."

Within the country, blood is in short supply. India outlawed its sale in 1998 and, since then, the price has spiked from $5 a pint to $25 per. (The United States, which has a donor-only supply, sells its excess blood to India as well as other nations, grossing billions of dollars annually.) Yet the demand is such that, as in China, "blood farms" have sprung up, kidnapped men and women chained to beds in small rooms and sheds, sometimes for years, drained of their blood to the point of death. "I know people in India," says Carney, "who were kidnapped right off the street and held for three years as blood cows."

In the 1990s, Carney says, India was a major destination for Western organ tourism, with hundreds of US citizens a year flying to India for liver transplants. (Having a transplant performed at a hospital in India costs a sliver of what it would in the US.) Yet here, as globally, kidneys are the most in-demand organ, and this is in large part due to Western consumers who don't want to spend years hooked up to a dialysis machine, willing to ignore the risks of HIV or other infectious diseases.

After the 2004 tsunami, a refugee camp for displaced Indians soon became known as "Kidneyvakkam," because so many of its inhabitants sold off their kidneys. In 2008, authorities busted a kidney-selling ring operating out of an apartment complex in a rich suburb of Delhi, where, over the past decade, at least 500 migrant workers had been operated on, selling their kidneys for about $1,250. The kidneys were then resold on the black market, to wealthy Indians and Westerners alike, for $50,000, with some of those workers reporting they had been kidnapped, their kidneys forcibly extracted.

There is, as Carney writes, another force animating the black market: People whose organ comes from a live donor rather than a brain-dead one live longer.