WASHINGTON, Oct. 10 (Xinhua) -- Almost immediately after it is donated, human blood begins to lose nitric oxide, a key gas that opens up blood vessels to facilitate the transfer of oxygen from red blood cells to oxygen-starved tissues.

    Thus, millions of patients are apparently receiving transfusions with blood that is impaired in its ability to deliver oxygen, according to Duke University Medical Center researchers, who reported the results of their studies in two separate papers carried in the latest issue of Proceedings of the National Academy of Sciences.

    The key to the findings is that nitric oxide in red blood cells is crucial to the delivery of oxygen to tissues. However, the vital gas in red blood cells begins breaking down almost immediately after red blood cells leave the body.

    "We document clear indications of nitric oxide depletion within the first three hours," reported one research team.

    Nitric oxide is not only needed for red blood cells to off-load oxygen, it may also influence the flexibility of the saucer-shaped cells. As nitric oxide levels decrease, the red blood cells become stiffer, making it more difficult for them to deform their shape in order to squeeze through tiny blood vessels.

    They also found that adding this gas back to stored blood before transfusion appears to restore red blood cells' ability to transfer oxygen to tissues.

    It is estimated that close to 14 million units of red blood cells are administered to about 4.8 million Americans each year. National blood banks require that blood be stored for no more than 42 days after donation. After that time, unused blood must be discarded.

    Blood has the intrinsic capacity to open blood vessels, thereby enabling oxygen to get to tissues. But banked blood cannot do this properly. However, transfusions are still critically important, said Jonathan Stamler, lead author of one of the papers.

    "Blood can be life saving, only it is not helping the way we had hoped and in many cases it may be making things worse. In principle, we now have a solution to the nitric oxide problem -- we put it back using dogs as animal models -- but it needs to be proven in a clinical trial," said Stamler.

    Both research teams believe that a large-scale randomized clinical trial in humans is needed, arguing that blood has both benefits and risks, and therefore should be evaluated in the same manner as medications.