IL6 and pulmonary arterial hypertension: BMP, bone morphogenetic protein; BMPR2, bone morphogenetic protein receptor type 2; EndMT, endothelial-to-mesenchymal transition; ET-1, endothelin-1; IL-6, interleukin-6; NO, nitric oxide; PAH, pulmonary arterial hypertension; PASMCs, pulmonary artery smooth muscle cells; PVR, pulmonary vascular resistance; RV, right ventricle; TGF-β, transforming growth factor beta; TNF-α, tumour necrosis factor alpha.