Pulmonary hypertension is characterized by the narrowing or blockage of small arteries (pulmonary arterioles) and capillaries in the lungs. This condition makes it difficult for blood to flow through the lungs, significantly increasing blood pressure in the pulmonary arteries. The heart is forced to exert excessive effort to pump blood against this high pressure, which over time leads to the weakening of the heart muscle. Consequently, this can result in heart failure and, in advanced cases, severe complications leading to death.