Transfusion of red blood cells is often used in critically ill patients with low red blood cell counts to prevent disease progression and death. Recent studies suggest that the use of "aged" versus "fresh" red blood cells are associated with worse clinical outcomes. There is evidence that red blood cells work with the cells lining our blood vessels to produce a variety of substances that normally cause arteries to relax and increase blood supply. Two of these substances are called nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF). The investigators are trying to determine the nature of these substances in human beings when they are transfused "aged" versus "fresh" red blood cells. It is their thought that "aged" red blood cells have less of the substances (NO and EDHF) that naturally relax our arteries and further changes the blood supply. One way to determine this is to transfuse a subject's own "aged" and "fresh" red blood cells and inject substances such as L-NMMA (L-NG monomethyl arginine) and TEA (tetraethylammonium chloride), which block the production of NO and EDHF respectively, and then, study what happens to the blood flow.