Pulmonary arterial hypertension (PAH) is high blood pressure in the arteries of the lungs. These are the vessels that carry blood from the heart into the lungs, where it picks up oxygen to deliver to the rest of the body. In a person with PAH, the pulmonary arteries constrict and become narrow, making it difficult for the heart to pump blood through them. This deprives the body of much needed oxygen and puts extra strain on the heart as it works harder to do its job. Over time, this can cause the right side of the heart to become larger and the walls of the heart thicker. These effects can lead to symptoms of PAH and, if left untreated, can cause right heart failure.3,4 Learn more About PAH.