 It doesn't. It sticks. Why would it jump off? The oxygen's happy on the iron. The hemoglobin moves the oxygen because it's got an atom of iron in the center. And oxygenated iron is called rust. You just made a microscopic amount of rust. And so why would it unrustify? It won't. It doesn't. Except for the way the hemoglobin is set up, the hemoglobin will warp and twist and pop that oxygen off as long as carbon dioxide attaches to the bottom.