The first of our organs to form, the last to die, the heart is both a simple pump and the symbol of all that makes us human: as long as it continues to beat, we hope.

One summer day, nine-year-old Keira suffered catastrophic injuries in a car accident. Though her brain and the rest of her body began to shut down, her heart continued to beat. In an act of extraordinary generosity, Keira's parents and siblings agreed that she would have wanted to be an organ donor. Meanwhile nine-ye...