Effect of plasma cholesterol on red blood cell oxygen transport

1. Oxygen (O₂) transfer from the blood to tissues is a function of the red blood cell (RBC) O₂ saturation (SO₂), the plasma O₂ content being negligible. Under conditions of increased tissue O₂ demand, the SO₂ of arterial blood does not change appreciably (97%); however, the SO₂ of mixed venous blood, equal to that of the perfused tissues, can go as low as 20%. 2. Tissue O₂ availability is limited by the exposure time to a RBC, which decreases under conditions of maximum stress (< 1 s). If the O₂ unloading time was to increase significantly, because of a decrease in the RBC diffusion constant or an increase in the RBC membrane thickness, the RBC O₂ unloading time would exceed tissue (e.g. cardiac) transit time and O₂ transfer would be impaired. 3. Cholesterol constitutes the non-polar, hydrophobic lipid of the enveloping layer of the RBC membrane. As the cholesterol content of the RBC increases, the fluidity of the membrane decreases and the lipid shell stiffens. 4. Early studies demonstrated that high blood cholesterol concentrations were associated with reduced blood O₂ transport; in essence, the haemoglobin dissociation curve was shifted to the left. 5. Current investigations have shown that the cholesterol RBC membrane barrier to O₂ diffusion delayed O₂ entry into the RBC during saturation and delayed O₂ release from the RBC during desaturation. In an analysis of 93 patients divided by their cholesterol concentration into five groups, the percentage change in blood O₂ diffusion was inversely proportional to the cholesterol concentration. 6. The RBC membrane cholesterol is in equilibrium with the plasma cholesterol concentration. It stands to reason that as the plasma cholesterol increases, the RBC membrane becomes impaired and O₂ transport is reduced. 7. The implications of this new perspective on O₂ transport include the ability to increase tissue oxygenation by lowering plasma cholesterol.