The red cells pick their payload—oxygen—from the capillaries of the lung and ferry it around. And when the cells reach oxygen-poor environments in the body—with the heart muscle pumping and pushing them around minute after minute—hemoglobin, literally, twists and unclasps the oxygen that the iron atoms have bound. Hemoglobin is blood’s hidden secret—a complex of proteins so vital to our existence as organisms that we have evolved a cell whose principal job is to act as a suitcase to carry it around.
