Page 9, CV Review, Essentials, Develop. Physiol., Dr. D. Penney

Misconception 10. The heart increases its oxygen uptake by increasing both oxygen extraction from the blood and blood flow.

The difference in O2 content between arterial blood entering an organ and venous blood leaving is the A-V O2 difference (Figure 1.05). This is quite different for the heart (14-15 ml/dl) compared to the kidney (2-3 ml/dl).

A-V O2 difference can increase considerably for some tissues during periods of maximal O2 utilization. This is true, for example, for skeletal muscle. The heart however has only a very limited ability to increase A-V O2 difference, mainly because the extraction ratio is already so high; to increase it further would necessitate areas of the myocardium becoming hypoxic.

Thus, to increase oxygen delivery the heart must rely primarily on increased flow. An increased A-V O2 difference at rest usually indicates ischemia. Ischemia is relieved by increasing blood flow, or decreasing O2 consumption, or both. Increased A-V lactate also indicates inadequate flow. The heart usually uses lactate, but in ischemia or hypoxia it produces lactate.

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