WASHINGTON (CBS) – Researchers are studying the brains of altruistic kidney donors, i.e., people who donate one of their kidneys to a complete stranger.

CBS News got a look at the science and were introduced to two women connected by the ultimate gift.

As a mother, firefighter and paramedic… putting others first is a way of life for Jo Kummerle. So, it wasn’t a difficult choice when she decided to donate one of her kidneys to a total stranger.

“I guess I just keep thinking ‘why not?'” she said. “Why would you not? I’ve always been a helper, I always love people. Why wouldn’t I?

Jo is one of just a few hundred people every year who become altruistic kidney donors.

Professor Abigail Marsh at Georgetown University is studying the brains of those donors, including Jo, to understand why some people are simply more altruistic than others. And she says the images are telling.

Through scans, Marsh says you’re able to see an enlarged area of the brain that is associated with altruism. “They seem to have just a little extra matter – a little extra material in this region of the brain that we know is really important to producing an empathic response,” she explained. “There is a structure in the brain called the amygdala and people that are psychopathic have smaller than average. And in altruistic kidney donors, it’s larger than average by about eight percent.”

Jo – who lives in Washington state – donated her kidney to Tressa Dombroksi in New Jersey. When asked how often she thinks about the generosity of her donor Tressa said, “I think about it all the time, and I give thanks.”

Tressa, a married mother of two, has an autoimmune disorder and needed a second kidney transplant, after her first started to fail. “When you go through an illness, a serious illness where your organs are failing, you don’t ever take that for granted again. It gives you a whole new perspective on life,” she said.

Tressa was able to thank Jo in person when they met for the first time three years ago. It was instant friendship.

Nearly 100,000 people in the U.S. are currently waiting for a kidney transplant. Click here to learn more about some of the criteria for becoming a donor and how to make a living donation.