In recent years cardiologists and pulmonologists alike have taken a very narrow view of the heart and lungs. Each specialty has focused its respective attention on either the left ventricle or the alveoli. Ejection fraction and arterial O2 tension have become the order of the day. These narrowly focused viewpoints of the heart and lungs have distracted us from an equally compelling and more global perspective--the cardiopulmonary unit, in which the heart and lungs function as an integrated metabolic unit responsible for the body's gas transport and, as such, serving the metabolic needs of the tissues. A disease involving the cardiovascular or respiratory systems will disrupt the ability of the cardiopulmonary unit to deliver O2 to the tissues. In more subtle expressions of disease, this defect in O2 transport may require the heightened O2 requirements of exercising muscle to become apparent. The integration of the heart and lungs and the right and left sides of the heart is fostered by a variety of physiologic factors, including pleural and airway pressures, the pericardium, the interventricular septum and alignment of muscle fibers between the ventricles and septum, and the thoracic cage itself. Through its functional integration, the cardiopulmonary unit has been able to link the metabolizing cells to the atmosphere.