VO2 max is considered the gold standard measure of cardiorespiratory fitness. Which physiological variable is the PRIMARY determinant of VO2 max, according to the Fick principle?

• A Pulmonary diffusing capacity — the ability of the lungs to transfer O2 to blood
• B Peripheral O2 extraction (a-vO2 difference) — the ability of muscles to extract O2
• C Cardiac output (specifically maximal heart rate × stroke volume) — the ability to deliver oxygenated blood to muscles ✓
• D Hemoglobin concentration — the O2 carrying capacity of blood

Explanation

By Fick's principle: VO2 = Cardiac Output × (CaO2 − CvO2). In healthy individuals, the primary limitation to VO2 max is maximal cardiac output (CO = HR × SV), not pulmonary diffusion or peripheral extraction. Elite athletes can achieve CO of 35–40 L/min at maximal exercise (vs. ~25 L/min untrained). Training improves SV (increased cardiac chamber size, eccentric hypertrophy, improved plasma volume). Peripheral adaptations (increased mitochondrial density, capillary:fiber ratio, myoglobin) increase the a-vO2 difference but are secondary. In highly trained athletes at altitude, pulmonary diffusion may also limit VO2 max.

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

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