In the back room of a fire station, Jill Ridenhour opens a package. Printed on the side, in all caps, it reads “Human Blood.” Ridenhour is a field supervisor for emergency medical services at Summit Fire and EMS in Colorado’s ski country. She pulls out a clear pouch filled with dark liquid and examines it. “The color of the blood is good,” she said. “It's kind of dark red, purplish. There are no bubbles at the top.” She puts the pouch in a special cooler. This blood is called “whole blood.” It’s the centerpiece of a new idea: to let emergency responders give transfusions at the scene of an accident, instead of having to wait until patients get to the hospital.

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