Breathing during Exercise: Respiratory and Circulatory Interactions

Under conditions of exercise the heart and lungs are linked in important ways. First, both the heart and lungs are linked in series as pumps for O₂ and CO₂ transport. Second, they are mechanically linked owing to their location within the thorax. This means that changes in lung volume (and intrathoracic pressure) will influence cardiac filling and ejection. The respiratory muscles cyclically contract during exercise to allow for gas exchange. This ensures that the partial pressures of O₂ and CO₂ in the blood are largely kept within narrow limits, while the heart and circulatory system ensures gas transport to and from the tissues. The increase in metabolic rate during exercise requires substantial increases in both cardiac output and ventilation, meaning that cardiopulmonary interactions become more pronounced. In this review, we explore the physiological interactions between the respiratory system and circulatory systems by asking two interrelated questions. First, what are the mechanical interactions between the respiratory and circulatory systems? Second, what are the respiratory influences on sympathetic vasomotor outflow and blood flow distribution during exercise? We end with a summary of recent studies that have addressed questions of respiratory-circulatory interactions with respect to sex differences and cardiopulmonary disease.