During maximal exercise, a subject has a cardiac output of 20 L/min and an oxygen consumption of 2.5 L/min. Using the Fick principle, the average arteriovenous oxygen difference is:

• A 5 mL O2/dL of blood
• B 12.5 mL O2/dL of blood ✓
• C 20 mL O2/dL of blood
• D 50 mL O2/dL of blood

Explanation

The Fick equation states VO2 = cardiac output x arteriovenous O2 difference. Rearranging: 2500 mL/min divided by 20,000 mL/min gives 0.125 mL O2 per mL of blood, which equals 12.5 mL O2 per dL. This is close to the maximal attainable extraction, since arterial content is about 20 mL/dL and mixed venous content can fall to about 5 to 7 mL/dL. Option C equals arterial content alone, ignoring venous reserve.

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

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