Why can untreated septal defects lead to pulmonary hypertension?

Untreated septal defects, such as ventricular septal defects or atrial septal defects, lead to pulmonary hypertension primarily through the abnormal shunting of blood.12

In these conditions, a hole in the heart's partition allows blood to flow from the left side of the heart to the right side. This creates a persistent increase in both the volume and flow of blood entering the pulmonary circulation. Over time, the pulmonary vessels are subjected to chronic stress from this high-volume blood flow, which elevates the blood pressure within the lungs. If left uncorrected, this sustained pressure can cause irreversible damage to the pulmonary vessels and may eventually lead to right-sided heart failure.123