What is the Fick Equation?

• A. VO2 = Q x (venous O2 content difference)
• B. VO2 = Q x (a- vO2)
• C. VO2 = HR x stroke volume
• D. VO2 = Q x (arterial O2 content)

Answer: (B)

The Fick equation expresses oxygen uptake as a product of blood flow and how much oxygen is extracted by the tissues. Specifically, VO2 equals cardiac output (the amount of blood the heart pumps per minute) times the arteriovenous oxygen content difference, which is the arterial O2 content minus the venous O2 content. In other words, VO2 = Q × (CaO2 − CvO2), and the a-vO2 difference represents how much O2 each liter of blood delivers to the tissues.

This is why the option using the arteriovenous O2 content difference is the best one: it accounts for both the delivery of oxygen via blood flow and the amount of oxygen returned deoxygenated, reflecting tissue extraction. The other forms either omit the extraction component, mix up the sources of O2 content, or equate VO2 with cardiac output alone, which does not describe oxygen uptake.