During maximal exercise, oxygen consumption (VO2max) is most accurately described as being limited by which factor in healthy untrained individuals?

• A Pulmonary diffusion capacity, which becomes limiting during intense exercise
• B Cardiac output (maximal heart rate × stroke volume), limiting O2 delivery to exercising muscle ✓
• C Mitochondrial oxidative capacity of skeletal muscle
• D Haemoglobin concentration and O2 carrying capacity of blood

Explanation

VO2max is primarily limited by convective O2 delivery — i.e., cardiac output × arterial O2 content. In untrained healthy individuals, the Fick equation demonstrates that maximal cardiac output (which plateaus in untrained subjects at ~20–22 L/min) is the main constraint; pulmonary diffusion is adequate at rest and moderate exercise. In highly trained athletes, peripheral O2 extraction approaches near-maximum, and pulmonary diffusion can become limiting. Skeletal muscle mitochondrial density limits performance below VO2max but adapts with training; Hb concentration matters in anaemia or altitude but is not the primary limit in healthy individuals.

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

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