A healthy adult walks on a treadmill at a steady submaximal workload well below her ventilatory threshold. Compared with rest, which change is expected in her arterial blood gases?

• A PaCO2 rises because metabolically produced CO2 accumulates faster than it is exhaled
• B PaCO2 falls because hyperventilation outpaces CO2 production
• C PaO2 rises substantially because alveolar ventilation exceeds perfusion
• D PaCO2 remains close to 40 mmHg because ventilation increases in proportion to CO2 production ✓

Explanation

During mild to moderate steady-state exercise, minute ventilation rises linearly with carbon dioxide production, so PaCO2, PaO2, and arterial pH are held almost constant near resting values despite a many-fold rise in metabolism. This tight matching implies feed-forward control from central command and feedback from exercising muscle rather than a rise in arterial chemoreceptor drive. PaCO2 falls only after the anaerobic threshold, when lactic acidosis stimulates a disproportionate ventilatory rise.

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

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