If the following steps in the aetiology and progression of cor pulmonale were placed in order, which would be 1st?
1) Adaptive hypertrophy of the pulmonary artery, leading to pulmonary hypertension.
2) Dilatation of the right ventricle of the heart.
3) Shunting of blood flow away from alveoli that are not participating in gas exchange.
4) Decreased exchange of O2 and CO₂ within alveoli.
5) Elevated blood pressure in the pulmonary artery.