There were more than 20,000 kidney transplants in the United States last year, with some cases involving donors providing organs to complete strangers. On Monday, CBSN profiled the psychology behind those who give their organs to ones they don't know.

"It really is an extraordinary form of altruism in a lot of ways," said professor Abigail Marsh of Georgetown University. Marsh is studying the brains of these altruistic donors to find out if they have specific brain functions that make them more generous.

Marsh claims she can see an enlarged portion of the brain that is associated with altruism when examining scanned images.

Get Breaking News Delivered to Your Inbox

"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 empathetic response," she told CBSN. "There is a structure in the brain called the amygdala, that in people who are psychopathic is smaller than average and in altruistic kidney donors it is larger than average by about eight percent."

One of those donors is Jo Kummerle.

Kummerle is one of a few hundred people every year who become altruistic kidney donors. She lives in Washington State and donated her kidney to Tressa Dombroski, a recipient who lives in New Jersey and needed a second kidney after her first transplant started to fail.

"Why would you not? I've always been a helper. I've always loved people," Kummerle told CBSN. Kummerle and Dombroski have struck up a lasting friendship, bringing their families together and making cross-country trips to visit one another.

"She is family," Kummerle said.

Nearly 100-thousand people U-S are currently waiting for a kidney transplant. For more information about donating, you can go to wait list zero dot org slash donate. www.waitlistzero.org/donate