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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