Tribology of capillary blood flow

Abstract: The capillaries, with diameters typically in the range of 4 to 8 mm, are the terminal branches of the circulatory system. Blood is a concentrated suspension, containing 40–45 per cent volume of red blood cells (erythrocytes) suspended in plasma. These cells must undergo large deformations while passing through capillaries. Even so, the resistance to blood flow measured in capillary-sized glass tubes is less than would be expected based on the viscosity of blood measured in bulk. Theoretical analyses, in which the red blood cell is modelled as an elastic axisymmetric shell and lubrication theory is used to analyse the motion of plasma around the cell, provide a quantitative explanation for this behaviour. The walls of capillaries in vivo have been found to be lined with an endothelial surface layer of macromolecules with thickness of the order 1 mm. This layer interacts mechanically with moving red blood cells, and may protect them from damage as they traverse the irregularities of the microcirculation numerous times during the typical 120-days lifetime of each red blood cell