How does MS cause pulmonary hypertension?

Passive backward transmission of elevated LA pressure, pulmonary arteriolar constriction (second stenosis) triggered by LA and pulmonary venous hypertension, interstitial edema in walls of small pulmonary vessels, organic obliterative changes in pulmonary vascular bed in end stages. Severe pulmonary HTN leads to RV enlargement , secondary tricuspid regurgitation and pulmonary regurgitation and right sided heart failure