How does your blood grab oxygen in the lungs and let it go in your muscles?

Your blood grabs oxygen in the lungs and drops it in your muscles thanks to hemoglobin, a protein that changes shape as it fills up. Each hemoglobin molecule has four parts, and each part holds an iron atom that can carry one oxygen molecule. The trick is teamwork: once one part grabs an oxygen, the whole molecule shifts shape and the other parts grab oxygen more easily. So in the lungs, where oxygen is plentiful, hemoglobin fills right up. In working muscles, where oxygen is scarce, the same effect runs in reverse and it lets go readily.

This protein is the reason blood can carry so much oxygen: about seventy times more than if the oxygen were simply dissolved in blood plasma. It makes up about 96 percent of a red blood cell's dry weight, and it also carries away some of the carbon dioxide your body produces.

Its weak spot is carbon monoxide, which sticks to the same iron site about 250 times more strongly than oxygen, and is colourless and odourless. And a few animals manage without hemoglobin entirely: Antarctic icefish have colourless blood. Working out hemoglobin's shape took the Vienna-born scientist Max Perutz most of his career, and earned him a Nobel Prize in 1962.