Biorheology in the analysis of the lung

Abstract

The pulmonary capillary blood vessels have a unique planar sheet-like geometry with thickness about the same as the diameter of the red cells. The hemorheological properties of the blood in such a network is dominated by the interaction between the red cells and the capillary vessel wall. A distinctive feature of the blood flow in pulmonary capillaries is the non-uniformity in the distribution of red blood cells in the alveolar walls. To explain this nonuniformity, we showed that in branching capillaries the hematocrit distribution depends on the velocity distribution. If a vessel bifurcates and if the velocities in the daughter branches are unequal, then the faster side gets more red cells. If the velocity ratio exceeds certain critical value about 2.5 (with exact value depending on cell rigidity, tube diameter and hematocrit), then the slower branch gets no red cell at all, (i.e. hematocrit → 0). Another reason for the nonuniform hematocrit distribution is due to occasional plugging of small blood vessels by leukocytes. The force of interaction between leukocytes and the vascular enothelium is determined. Turning to the blood flow, we show that it is affected critically by the elasticity of the blood vessels. In recent years we measured the distensibility of pulmonary capillaries and arterioles or venules of the cat by the polymer casts method, and arteries and veins of diameter 100 μm and up by the x-ray method. Thus the elasticity of all blood vessels of the cat’s lung is now determined. With this information the nonlinear pressure and flow relationship is determined. Furthermore, we have shown that all pulmonary veins, including the venules, remain patent (not collapsed) under negative transmural pressure as large as −17 cm H₂O. This important fact tells us that in the sluicing condition of zone 2 (where the pulmonary venous pressure is lower than the alveolar gas pressure), the sluicing gate must be located at the exit ends of the alveolar capillary sheets. With this information the flow limitation problem is solved.