Interaction of blood cells with vessel walls in microcirculation

Abstract

In microcirculation, blood cannot be treated as a homogeneous fluid. The individual cellular elements have influence on hemodynamics. In this paper, some salient features of microcirculation as influenced by individual cells are demonstrated. 1. (1) Blood Cells at Cross Roads . We consider narrow capillaries whose diameters are about the same as that of the blood cells. Consider a bifurcation point. A blood cell flowing down a capillary would have to decide to which of the two daughter branches it should go in. We can show that on the basis of pressure gradient as well as the shear stress, the blood cell will flow into the faster branch. Thus, in very narrow capillaries, the branch in which the flow is faster gets all the blood cells. The larger is the cell, the better is this control. Thus the larger leucocytes often lead the flow into faster branches. (Fung, Microvas. Res. 5 : 34–48, 1973.) 2. (2) In Pulmonary Alveolar Sheets, the red blood cell distribution reflects the velocity field. Faster channels have more red cells. The cell distribution is modulated by the apparent viscosity of blood which increases with hematocrit. A movie will demonstrate this effect. (Ref: Fung, Yen, Sobin, Fed. Proc. 34 : 437, April, 1975). 3. (3) The Topology of Pulmonary Capillaries in the alveoli, as related to the arterioles and venules, implies a system of branching sheets in which the velocity of blood flow is non-uniform. Thus, the effect mentioned in (1) applies to pulmonary alveolar septa (sheets). This effect is accentuated toward the apex of the lung in standing posture because of the hydrostatic effect of gravitation. (Fung, Yen, Sobin, Fed. Proc., April, 1975). 4. (4) Turning to Arterioles and Venules, it is well known that leucocytes are often seen to stick to the endothelial wall, or rolling slowly on it, while the plasma and red cells whiz by around them. By high-speed cine-photomicrography, we determined the velocity field around the leucocytes, and by model testing, we determined the similarity law and shear force coefficients. It was found that in the venules of the rabbit omentum, a white blood cell sticking to the endothelial wall is subjected to a shear force which lies in the range 4 × 10 −5 dyne to 234 × 10 −5 dyne. The shear stress was found to range between 50 to 1060 dynes/cm 2. The maximum normal stress is about the same order of magnitude. Compare this with Fry's critical shear stress of 420 dynes/cm 2 in dog thoracic aorta. (G. Schmid-Schoenbein, Fung, Zweifach, Circ. Res. , 36:173–184, 1975).