This project deals with the acute cardiovascular response to exercise stress. The particular areas of interest are the role and effects of changes in systemic and pulmonary vascular resistance and of changes in intravascular blood volume and blood content on cardiovascular performance. We plan to test the hypothesis that the increase in sympathetic activity which occurs with exercise is dependent upon systemic vasodilation. We will study the effects on cardiac pump performance and oxygen delivery of increases in red cell mass. Finally, we will assess whether the relatively modest fall in pulmonary vascular resistance during exercise is due to regional vasoconstriction in certain portions of the pulmonary bed occurring simultaneously with recruitment in other portions.