Elevated haematocrit - when too much of a good thing wreaks havoc on the endothelial surface layer.

摘要

The convective supply of oxygen through the microcirculation to the tissues is the product of blood flow and oxygen content of the blood and depends on red blood cells (RBCs) in two ways. The RBCs contain the oxygen-binding protein haemoglobin so that the oxygen content is proportional to the concentration of RBCs or haematocrit (Hct). Thus increasing Hct leads to an increase in the capacity for the blood to carry oxygen. However, the rheological properties of the blood depend critically on Hct and the viscosity of the blood increases exponentially with Hct. Since blood flow depends inversely on viscosity, increasing Hct leads to a reduction in blood flow. The balance between these two opposing effects of Hct on oxygen supply leads to the concept of optimal haematocrit at which oxygen supply is maximum, usually near a Hct of about 50% (Birchard, 1997).