During exercise, which of the following mechanisms is primarily responsible for increasing oxygen delivery to working skeletal muscle?
- A Increased cardiac output and local arteriolar vasodilation (via CO2, H+, O2, adenosine) markedly increasing blood flow to exercising muscle ✓
- B Increased hemoglobin synthesis to carry more O2 per unit blood volume
- C Hyperventilation alone, by increasing alveolar O2, raises hemoglobin saturation from 98% to 100%
- D Redistribution of blood from pulmonary to systemic circulation via opening of pulmonary arteriovenous shunts
Correct answer: A. Increased cardiac output and local arteriolar vasodilation (via CO2, H+, O2, adenosine) markedly increasing blood flow to exercising muscle
Explanation
During exercise, O2 delivery increases primarily through a massive increase in cardiac output (can increase from 5 L/min at rest to 25 L/min in trained athletes) and local metabolic vasodilation in exercising muscle (mediated by CO2, H+, adenosine, K+, falling PO2, and nitric oxide). Hemoglobin remains ~98% saturated at rest; increasing ventilation adds little additional saturation. The Bohr effect (right-shift of ODC from rising temperature and H+) enhances O2 unloading at tissues. Local arteriolar dilation can increase muscle blood flow 20–100 fold.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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