During maximal exercise, cardiac output can increase 5-fold. The most important mechanism by which venous return increases proportionally to maintain cardiac output via the Frank-Starling mechanism is:

• A Increased sympathetic tone directly increases sinoatrial node firing rate as the primary mechanism increasing cardiac output
• B Peripheral arterial vasodilation reduces afterload, allowing more ventricular emptying and secondarily increasing venous return via the Bernoulli effect
• C Skeletal muscle venous pump, respiratory pump (increased intrathoracic negative pressure during forced inspiration), and venoconstriction all increase mean systemic filling pressure and reduce venous capacitance ✓
• D Epinephrine-mediated increase in cardiac contractility (positive inotropy) increases stroke volume as the primary mechanism of increased cardiac output during exercise

Explanation

Venous return is the rate-limiting determinant of cardiac output during exercise (Guyton's venous return model). Three mechanisms increase mean systemic filling pressure (MSFP) and decrease venous resistance: (1) the skeletal muscle pump — contracting muscles compress veins and propel blood centrally, with venous valves preventing backflow; (2) the respiratory pump — increased inspiratory effort lowers intrathoracic pressure, expanding the right atrium and augmenting preload; (3) sympathetic venoconstriction reduces venous compliance, compressing blood volume centrally. Although heart rate and contractility increase through sympathetic activation, they increase cardiac output only insofar as venous return is matched — the heart cannot sustain output without adequate preload.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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