The purpose of this study is to better understand S-nitrosohemeglobin (SNO-Hb) in transfused blood of extremely preterm infants. The long term goal of the project is to identify variation in the SNO-Hb between packed red blood cell units, and between and among individual preterm infants pre and post-transfusion. Duke investigators are developing methods to replenish SNO-Hb, which, if successful, would improve RBC deformation in addition to providing a vasodilatory stimulation to hypoxic tissue, and lead to a randomized clinical trial testing treated vs. untreated RBC transfusions in extremely premature infants.