Thirteen patients with healthy new kidneys from what's believed to be the world's largest kidney exchange met the donors who made it happen Tuesday — including three who are sure to face the question, "Why?"

A hospice nurse who handed homemade cookies to her operating team. A retired stockbroker who had volunteered with the National Kidney Foundation and decided to walk the talk. And a woman inspired by President Barack Obama's call to volunteer. They all donated a kidney with nothing to gain — they didn't have a friend or loved one in the marathon chain of transplants that they helped make possible.

"It feels wonderful," Sylvia Glaser, 69, the hospice nurse, said Tuesday at a news conference where most of the donors and recipients met for the first time. "You are giving someone a life, and there is no substitute for that."

"It's not like I'm doing anything courageous," Bill Singleton, 62, the kidney foundation volunteer, told The Associated Press before his surgery. "If I don't volunteer, who will?"

Kidney exchanges widen the pool of potential donors for the hardest-to-transplant patients — minorities as well as people whose immune systems have become abnormally primed to attack a donated kidney. What happens: Patients find a friend or relative who isn't compatible with them but will donate on their behalf, and the pairs are mixed to find the most matches.

But a donor whose kidney isn't directed to a particular patient — a so-called altruistic or non-directed donor — multiplies the number of operations that can be done in a kidney swap. And Dr. Keith Melancon at Georgetown University Hospital had three such donors, people he calls "pieces of gold."

"People keep wanting to know why, why, why," Glaser, the Gaithersburg, Md., nurse said before her surgery. "It sounds very trite but you pass through this world, and what do you ever do that makes a difference?"

The AP documented weeks of the complex logistics as Melancon's team initially planned for a 16-way exchange, juggled donors and recipients for the best matches — and emerged with a record-setting exchange: Twenty-six operations over six days at Georgetown and nearby Washington Hospital Center.

Ten of the 13 recipients were black, Asian or Hispanic. And five were patients who never would have received a kidney under the traditional system, because they needed an extra blood-cleansing treatment to remove those hyperactive immune cells, treatment that only a handful of hospitals in the country offer.

"I cannot explain in words. I can raise my children now. He gave me life," said Solomon Weldeghebriel, 42, a Washington cabdriver. Two of his three children wiggled on his lap as he met Singleton, his donor.

The exchange started with a 45-year-old Maryland woman inspired by Obama. She asked to remain anonymous but told The AP: "I just wanted to help someone out that needed my help, to give them a better life."

Then Glaser, the Gaithersburg, Md., nurse, made her offer. She'd wanted to donate years earlier but had been dissuaded by her family, who worried that she could be harmed.

"I told her it's a very good kidney," said her recipient, Washington IT worker Gertrude Ding, as the two exchanged addresses and phone numbers on Tuesday.

Singleton, the stockbroker and kidney foundation volunteer, is going to use his experience to argue that giving a kidney is good for all of society because of the huge toll of kidney disease.

More than 335,000 Americans depend on dialysis for survival. Dialysis costs more than $70,000 a year, mostly paid by Medicare.

Some 88,000 people are on the national waiting list for a kidney transplant. Fewer than 17,000 a year are performed. A transplant can cost nearly $200,000, or not quite three years' worth of dialysis.

"We could end the wait," Singleton said. "You help yourself by helping them."