The pulmonary artery is the main vessel that carries blood from the heart to the lungs. For a person at rest, blood pressure in a normal pulmonary artery is about 15 millimeters of mercury (mm Hg). In an individual with pulmonary hypertension, the average pressure in the pulmonary artery can be more than 25 mm Hg. This increase in pulmonary artery pressure occurs when the small arteries of the lungs become abnormally narrow. This condition can eventually lead to heart failure and death. Pulmonary hypertension can be idiopathic (unknown cause), familial (runs in families and is often linked to a genetic cause), or associated with other medical conditions. The October 7, 2009, issue of JAMA includes an article about the effects of iron supplementation on pulmonary hypertension due to low oxygen levels at high altitudes. This Patient Page is based on one published in the January 23, 2008, issue of JAMA.