Researchers have found that over-generous blood transfusions can do more harm than good. Ironically, the measure which is supposed to improve oxygen provision to the patient's organs, can have the opposite effect.
Now there is an explanation to this paradox. It appears that the oxygen transport depends on a signalling process by which the red blood cells help to widen the blood vessels, and this involves nitric oxide bound to hemoglobin. Donor blood for transfusion can be legally stored for 42 days, but the nitric oxide disappears within the first few hours of storage, so virtually all stored blood samples are deficient in that respect.
While this sounds quite scary, there is a silver lining in that the NO content can be restored.
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Blood transfusion risk explained