The heart is a beating muscle that pumps oxygenated blood to the entire body. After leaving the heart, blood must first pass through the lungs to receive oxygen before it can go to the body's cells. In order for blood to move from the heart to the lungs, blood must travel through the pulmonary artery and then into smaller and smaller vessels of the lungs. The diameter, or tone, of these blood vessels is regulated by a substance called endothelin, which circulates in the blood stream. A condition called pulmonary hypertension is marked by a large increase in the amount of circulating endothelin causing the small arteries of the lungs to narrow. When the arteries of the lungs become narrowed, increased resistance to blood flow develops, thereby necessitating the heart to work harder to pump blood to the lungs. At the same time, the lungs work harder to bring more oxygen into the body. This condition causes the blood pressure in the arteries of the lungs to become abnormally high. Because the heart has to work harder to pump blood, the right side of the heart may become abnormally enlarged and heart failure can develop.