Blood expires primarily because its ability to effectively deliver oxygen to the body's tissues diminishes over time. Specifically, the levels of nitric oxide, a crucial gas for oxygen delivery, decrease significantly during storage.
Here's a more detailed breakdown:
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Nitric Oxide's Role: Nitric oxide helps blood vessels relax and widen (vasodilation), which allows red blood cells to efficiently release oxygen to tissues.
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Nitric Oxide Degradation: After blood is drawn, nitric oxide levels begin to decline. After the standard 42-day storage period for donated blood, nitric oxide is almost non-existent.
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Impaired Oxygen Delivery: The reduction in nitric oxide compromises the blood's ability to deliver oxygen effectively. While the blood still contains red blood cells and hemoglobin (the oxygen-carrying protein), the constricted blood vessels hinder oxygen release where it's needed.
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Clinical Implications: Giving a patient "expired" blood may not deliver the expected oxygen boost and could even potentially cause adverse effects due to the changes in blood chemistry during storage.
In summary, while stored blood still contains red blood cells, the critical loss of nitric oxide during storage impairs its oxygen-carrying capabilities, making it "expired" and less effective for transfusion.
