The heart and lung (the body's gas transport system) are neurally, mechanically, humorally and functionally linked. Several clinical-therapeutic consequences involve heart-lung interactions. Three of these conditions are described here: 1) pulsus paradoxus in asthma; 2) survival in chronic cor pulmonale; and 3) Cheyne-Stokes breathing in congestive heart failure. They provide examples of the pathophysiological and clinical complexity that such correlations involve. The most remarkable "effects" of the heart-lung relationship as a metabolic unit are represented by development of cardiorespiratory intensive care units, and efforts to bring about methods of monitoring cardiorespiratory function.