Both, it turns out. As we saw earlier, chemists in the late 1700s determined that the lungs take in oxygen and expel carbon dioxide. (As for the color, when oxygen enters red blood cells it latches onto hemoglobin molecules there. Hemoglobin contains iron atoms, which bond readily to oxygen, and the addition of oxygen changes the shape of hemoglobin. This in turn changes its color from bluish to bright red.) In addition to connecting oxygen with breathing, these chemists had already linked oxygen with combustion, burning. So when they realized that blood delivers oxygen to our cells, they declared, QED, that breathing must involve a sort of slow combustion inside us — a constant burning, with our own bodies acting as the fuel.