Using the flow model as well as the hypoxia-induced pulmonary hypertension neonatal calf model characterized by proximal arterial stiffening [17], we have shown that stiffening-induced HPF caused pro-inflammatory responses and upregulated expression of TLRs (TLR2 and TLR4) in “downstream” ECs. This evidence concerns the gene TLR4 and pulmonary arterial hypertension.