Patients with PAH display increased circulating levels of HIF-inducible factors, some produced by pulmonary endothelial cells, that promote hematopoietic stem/progenitor cell proliferation in a bone marrow microenvironment predisposed to myeloproliferation and fibrosis. Local production of HIF-inducible factors by the pulmonary arterial endothelial cells recruit the mobilized hematopoietic subsets into the vessels leading to inappropriate vascular remodeling that eventually obliterates vascular perfusion through the pulmonary arterioles. Copyright 2010. Cleveland Clinic. All rights reserved. See the complete figure by Farha et al on page 3485.