The alveoli are the parts of the lung in which oxygen transfers from air to blood. When the amount of oxygen in air is reduced, changes occur in the lungs and heart to compensate for the low oxygen at the alveoli. These changes, which include constriction of the blood vessels in the lung, decrease a person's ability to exercise. High altitude is one situation where the amount of oxygen in air is low. The low oxygen and resulting changes in the heart and lungs account for many of the symptoms that people develop when they go from low to high altitudes. Sildenafil, a drug that has become a popular treatment for erectile dysfunction, causes the blood vessels of the lung to dilate (become wider). Previous studies have shown this effect of sildenafil in patients breathing air mixtures with low oxygen content at sea level. These observations have led experts to speculate that sildenafil might help prevent some of the problems with exercise ability caused by other conditions of low oxygen. One such condition is high altitude.