Flow of blood through narrow capillaries: Rheological mechanisms determining capillary hematocrit and apparent viscosity 1

Abstract

The dynamic volume concentration of red cells in a narrow: capillary tube is a resultant of several mechanisms which take place either at the capillary orifice or within the tube. Both types of mechanisms have been studied in experiments in vitro. Human red cell suspensions were perfused through glass capillaries (I.D. 3.3 – 12μm) branching from a larger feed channel. In addition, relative apparent viscosity was measured as a function of tube hematocrit. The results indicate that the red cell flow fraction is determined by the flow rate distribution at the branching point, and may even be reduced to zero if the fraction of flow diverted into the capillary orifice is decreased. At any discharge hematocrit the hematocrit within the tube is further reduced due to the velocity difference between cells and plasma. At equal tube hematocrit, the relative apparent viscosity showed a minimum at a tube diameter of approx. 5μm, whereas the dynamic hematocrit reduction was maximal at approx. 15μm. These results were correlated with observations of the dynamic flow behaviour of the red cells in a “travelling capillary” which indicate that red cells tend to travel in groups rather than equally spaced, show a hematocrit dependent transition from single-file to multi-file arrangement in tubes larger than 6μm, and, even in the smallest capillaries, exhibit a distinctly non-symmetric shape, regardless of flow rate and hematocrit.