'Regulation' of capillary haematocrit.

The nature of blood as a suspension with characteristic flow properties is the basis of the uneven distribution of total red cell mass and plasma volume in the cardiovascular system. On one hand the fraction of blood volume occupied by red cells is reduced in the small blood vessels due to axial accumulation of erythrocytes in flow. On the other hand, the fraction of blood flow occupied by red cells is affected by hydrodynamic separation effects at microvascular bifurcations. The combination of these phenomena results in inhomogeneous haematocrit distribution even within the network of capillaries. Both effects are, for different reasons, affected by changes in vascular smooth muscle activity; as a consequence, haematocrit distribution is coupled to the mechanisms regulating vascular resistance.