Until recently doctors believed that the hemoglobin in our red blood cells carried only oxygen and carbon dioxide when we are in good health. However, according to an article published in the April 2015 issue of the Proceedings of the National Academy of Sciences, hemoglobin also carries a molecule called nitric oxide (NO).
It turns out that NO is essential in dilating blood vessels where oxygen is released in order to maximize its delivery to the tissues that need it. These researchers also demonstrated that without the release of NO that very little oxygen would actually be delivered to the tissues. They bred mice that did not have the receptor site on the hemoglobin molecule to bind NO and found that even though their blood was carrying plenty of oxygen, very little was delivered to the tissues!