What does the a-vO2 difference measure in physiology?

• A. Arterial oxygen content
• B. Venous oxygen content difference across tissue
• C. Cardiac output
• D. Hemoglobin affinity

Answer: (B)

The key idea is how much oxygen tissues extract from the blood as it passes through the body. The a-vO2 difference is the difference in oxygen content between arterial blood (oxygen-rich going to tissues) and venous blood returning to the heart (after tissues have taken what they need). This reflects tissue oxygen extraction. It’s also used in the Fick principle: VO2 = CO × (a-vO2 difference), linking oxygen uptake to cardiac output and how much oxygen is removed by tissues. At rest, the difference is modest because tissuesneed less O2; during exercise it widens as muscles demand more oxygen. It doesn’t measure arterial oxygen content alone, cardiac output, or hemoglobin’s affinity; it specifically quantifies how much oxygen has been taken up by tissues from the arterial blood.