Chen et al. (2003) demonstrated that S. miltiorrhiza in pulmonary hypertensive rats could reduce thickening in the SPAs, could increase the levels of HO-1 and iNOS and decrease the expression of eNOS. In high altitude conditions, HO-CO pathway is up-regulated and the expression of HO-1 protein and the CO production increased. In other words, HO-CO is a pathway related to pulmonary arterial vasodilation and protective from pathological remodeling (Llanos et al., 2012). Therefore, according to the available evidence, S. miltiorrhiza could ameliorate PH. Here, HMOX1 is linked to pulmonary arterial hypertension.