An endurance-trained athlete has a resting cardiac output of 5 L/min and a maximum cardiac output of 35 L/min. An untrained individual has comparable cardiac outputs of 5 L/min and 20 L/min, respectively. The trained athlete's superior maximal cardiac output is primarily due to which adaptation?

• A Increased stroke volume at maximum exercise — the athlete has cardiac hypertrophy (eccentric LV enlargement) increasing end-diastolic volume (EDV) and ejection fraction; maximal SV of ~180 mL vs untrained ~100 mL accounts for the difference, while maximal HR is similar (~200 bpm) in both ✓
• B Significantly higher maximal heart rate (220 bpm in athlete vs 180 bpm untrained) because endurance training increases sino-atrial node automaticity
• C Reduced total peripheral resistance at rest in trained athletes means the heart pumps against lower afterload, directly raising cardiac output
• D Increased haemoglobin concentration in athletes increases O2 delivery per unit cardiac output, allowing lower cardiac output to achieve the same VO2max

Explanation

Endurance training causes cardiac remodelling: physiological eccentric hypertrophy increases LV chamber diameter (higher EDV) and wall thickness, allowing greater SV via the Frank-Starling mechanism and increased ejection fraction. At maximum exercise, trained athletes achieve SV of 160–200 mL vs ~100 mL in untrained individuals, while maximal HR is not significantly different between trained and untrained (approximately 190–200 bpm, determined by age: ~220 − age). Therefore VO2max and maximal CO differences are primarily SV-driven. Resting bradycardia in athletes reflects enhanced vagal tone, not elevated maximal HR. Hb concentration is modestly increased in endurance athletes (sports anaemia can actually occur) but this does not raise cardiac output.

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

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