If the arterial-venous oxygen content difference increases (as a percentage), what happens to cardiac output?

• A. Increases
• B. Decreases
• C. Remains unchanged
• D. Varies with activity

Answer: (B)

The main concept is the Fick principle that relates oxygen consumption to cardiac output and the amount of oxygen extracted by tissues. VO2 equals cardiac output times the arterial-venous O2 content difference (VO2 = CO × (CaO2 − CvO2)).

If the arterial-venous O2 difference increases while oxygen consumption stays the same, the cardiac output must decrease to keep VO2 the same. In other words, more oxygen is being extracted from each liter of blood, so less blood flow is needed to meet the overall oxygen demand. Keep in mind that during exercise, VO2 rises and both CO and the a-vO2 difference typically increase, but for a constant VO2, a larger a-vO2 difference lowers cardiac output.