As discussed in Chapter 4, this decrease occurs passively by recruitment and distention of pulmonary vessels. This indicates a decrease in pulmonary vascular resistance. Mean pulmonary artery and mean left atrial pressures increase, but the increase is not as great as the increase in pulmonary blood flow. An increased venous return, due to deeper inspiratory efforts (in addition to extravascular compression by the exercising muscles and by a decrease in venous capacitance), also contributes to the increase in cardiac output. This normally occurs more as a result of an autonomically mediated increase in heart rate than from an increase in stroke volume. As already mentioned, the cardiac output increases linearly with oxygen consumption during exercise.
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