Two days before Christmas in 1954, at the Peter Bent Brigham Hospital in Boston, the surgeon Joseph Murray removed a healthy kidney from a man named Ronald Herrick and placed it in his identical twin, Richard, who was dying of kidney failure. Doctors had tried kidney transplants before, and the immune system had always destroyed the new organ. This time it did not. The first successful kidney transplant worked because donor and recipient were identical twins, so Richard's immune system saw nothing foreign. He lived another eight years, and Murray later shared a Nobel Prize. To see why a kidney is worth such effort, look at what it does.

Two filters

The kidneys are two fist-sized organs against the back wall of the abdomen, just below the ribs. Though small, they receive about a fifth of all the blood the heart pumps. Each contains about a million tiny filtering units called nephrons.

Filter everything, then take back

Each nephron begins with a knot of capillaries, the glomerulus, where blood pressure pushes water and small molecules out of the blood into a thin tube. Blood cells and large proteins stay behind. Together the kidneys filter about 180 liters of fluid a day, yet a person makes only about 1.5 liters of urine. About 99 percent of the filtered fluid is reabsorbed as it flows along the tube, which reclaims water, glucose, salts and other useful substances, and actively adds some wastes and drugs.

Filtering everything and then taking back what is wanted can look wasteful, but it has an advantage: anything the body does not specifically reclaim, including unfamiliar toxins, is removed. The main waste is urea, a nitrogen waste made by the liver from broken-down protein. Because blood cells and proteins should not pass the filter, finding them in urine is an early sign of kidney damage.

Water balance

When the blood grows too concentrated, after sweating or a salty meal, the hypothalamus senses it and the pituitary releases antidiuretic hormone, or ADH. ADH makes the kidneys return more water to the blood, so urine becomes smaller in volume and darker. When you drink a lot, ADH falls and the kidneys release more dilute urine. Alcohol blocks the release of ADH, which is one reason a night of drinking leads to many trips to the bathroom and a dry mouth the next morning.

More than plumbing

The kidneys do several other jobs. By adjusting how much salt and water they keep, and by releasing an enzyme called renin, they help control blood pressure. They release EPO, the hormone that Level 3 showed drives red blood cell production. They convert vitamin D into its active form, which the gut needs to absorb calcium. And they fine-tune the acidity of the blood.

When kidneys fail

Chronic kidney disease is common, and diabetes and high blood pressure are its leading causes, since both damage the delicate capillaries of the glomerulus. A person can lose much of their kidney function before noticing anything, and can live a normal life with a single healthy kidney. When both fail, waste and water build up and the person will die without treatment.

The first practical treatment was invented in wartime. In the occupied Netherlands in 1943, the young Dutch physician Willem Kolff built the first practical dialysis machine, passing a patient's blood through sausage casing wrapped around a rotating drum in a bath of fluid. Most early patients died, but in 1945 a woman survived, and modern dialysis grew from his design. Today a transplant is the best treatment for many patients, with drugs that hold back rejection between unrelated people.

There are far fewer donated kidneys than people who need them, which raises a hard question about human dignity. Almost every country bans buying and selling organs. Defenders of the ban argue that a market would pressure the poor to sell parts of their bodies and would treat people as spare parts. Critics, including some economists and doctors, reply that a regulated, paid system could save thousands of lives and that the ban leaves living donors to bear risks without compensation. Iran has allowed paid kidney donation since the late 1980s, and both sides cite its record.

Worked example: a hot day's hike

A hiker walks for six hours in summer heat, sweating out around three liters and drinking only one. Her blood grows more concentrated. Her hypothalamus notices and makes her thirsty, while her pituitary releases ADH, so her kidneys reclaim water and her urine turns dark and scanty. As her blood volume drops, her kidneys release renin, which helps hold her blood pressure up. Back at the car she drinks two liters. Her blood dilutes, ADH falls, and within an hour or two she is producing plenty of pale urine. Two lessons follow. Urine color is a rough guide to hydration. And the system can be overwhelmed in the other direction too: endurance athletes who drink huge amounts of plain water can dilute the sodium in their blood dangerously, a condition called hyponatremia.