Discuss the major causes of pulmonary vasoconstriction

If you check the internet for pulmonary vasoconstriction, every hit names the condition as hypoxic pulmonary vasoconstriction. That should clue you in to its major cause: hypoxia, or a lack of oxygen in the body. 
Hypoxic pulmonary vasoconstriction is a body's way of channeling deoxygenated blood to the most effective areas of the lung for oxygenation. When alveoli are blocked, damaged, or diseased, specialized membranes react and trigger a tightening of smooth muscles around small arteries leading to those parts of the lungs. This funnels blood to the working parts of the lung, preventing damaged lobes from being overloaded and returning blood to the body without enough oxygen.
Unfortunately, certain circumstances can trigger pulmonary vasoconstriction across 100% of the lungs' capacity. This reduces the body's ability to process air and increases the load on the heart, sometimes leading to bleeds and clots in the lungs. One of the main causes of this detrimental form of pulmonary vasoconstriction is high altitude, where thinner air and lower oxygen can trick the body into harming itself.