GSTM1 and ischemia: Hypoxia is not synonymous to ischemia and, as described by Siggard-Andersen et al. [18], many factors apart from blood flow may influence tissue oxygen availability, including the total concentration of O2 in blood (which depends on the PaO2 and the concentration of effective Hemoglobin (Hb)), the affinity of the Hb (which also depends on other factors such as temperature, pH, and the concentration of 2,3-Diphosphoglycerate (DPG)), the degree of arterio-venous shunt, the diffusion of O2 from the capillary, and the metabolic rate of oxygen consumption, among others.