Describe the Fick principle in relation to oxygen consumption (VO2).

• A. VO2 = cardiac output × (arterial O2 content − venous O2 content)
• B. VO2 = cardiac output ÷ (arterial O2 content − venous O2)
• C. VO2 = heart rate × stroke volume
• D. VO2 = arterial O2 content × venous O2 content

Answer: (A)

The Fick principle says oxygen consumption equals the amount of blood flowing to the tissues times the amount of oxygen extracted from that blood. In the systemic circulation this is VO2 = CO × (CaO2 − CvO2), where CO is cardiac output, CaO2 is arterial oxygen content, and CvO2 is venous oxygen content. The difference CaO2 − CvO2 represents how much O2 is removed from each liter of blood as it passes through the tissues. So, when more blood is delivered (higher CO) or more O2 is extracted (larger CaO2 − CvO2), VO2 increases.

Why the other forms don’t fit: dividing the flow by the oxygen content difference doesn’t give VO2 and doesn’t align with the idea of flow carrying a certain amount of O2 to be extracted. VO2 isn’t equal to heart rate times stroke volume—that gives cardiac output, not oxygen consumption. And multiplying arterial and venous O2 contents doesn’t reflect the consumption process and would give the wrong units and meaning.