Blood leaves the heart through a large blood vessel called the pulmonary artery. It flows through the lungs, absorbing oxygen, and then returns to the heart to be pumped through another large vessel, the aorta, to the body. Just as a person has a normal blood pressure for the body, he or she also has a normal blood pressure for the blood passing through the lungs. In a person with pulmonary hypertension, the pressure becomes abnormally elevated. This happens because the small arteries in the lung become abnormally narrow. As a consequence, blood cannot pass easily through the lungs to absorb oxygen, compromising the delivery of oxygen-rich blood to the tissues of the body. Because the heart is working harder against the high pressure, heart failure can develop. Pulmonary hypertension can be idiopathic (the cause is unknown) or inherited (runs in families and is often linked to a specific gene) or may be associated with other medical issues. The October 3, 2012, issue of JAMA contains an article about pulmonary hypertension. This Patient Page is based on one previously published in the October 7, 2009, issue of JAMA.