Red blood cell transfusion is performed to correct insufficient oxygen carrying capacity or oxygen delivery, but no robust measure of tissue oxygen delivery is currently available. Specific aim 2 will address this gap in knowledge by performing absolute quantification of citric acid cycle intermediates using mass spectrometry that are markers of mitochondrial respiration. Of particular interest, increased plasma levels of succinate have been shown to be associated with trauma associated mortality and the accumulation of succinate is likely due to reversal of flow of the succinate dehydrogenase complex in the absence of oxygen (unpublished data). Measurement of these markers before and after transfusion will determine the resuscitative capacity of allogenic packed red blood cells compared to fresh autologous whole blood.