Red Cell Aggregation

The effect of red cell aggregation (RCA) on the resistance of flow in vivo has also been fraught with controversy. Infusing aggregating agents such as high-molecular-weight (500 kDa) dextran into animals results in some studies in systemic effects indicative of increased resistance to flow caused by RCA. However, other studies demonstrate a reduction in regional resistance in isolated tissues, suggestive of reductions in systemic hematocrit attendant to RBC sequestration in other regions. In vitro studies of RCA in vertically positioned small-bore glass tubes have shown that the apparent viscosity of blood decreases with increasing degrees of RCA. However, in vivo measurements in the low-flow state suggest a dramatic rise in apparent viscosity with reductions in g. The sequestration of red cell aggregates in regional networks has been shown to dramatically decrease systemic hematocrit, the extent of which is strongly dependent on the strength the aggregating agents.

Two mechanisms have been implicated in aggregate formation: osmotic depletion, which results in hydrostatic pressures that tend to force apposing cells together in the presence of macromolecules, and molecular cross-bridging, whereby solutes cause the binding of one cell to another. Modeling and experimental studies reveal that with weaker aggregating forces, red cells assume the configuration of rouleaux (Figure 3A), which may be disrupted by shearing forces at branch points in the arteriolar network. With stronger aggregating forces, clumps of cells are formed that appear to be more easily trapped at microvascular branch points (Figure 3B).

Figure 3 Red cell aggregates at the entrance of precapillary vessels in the low-flow state. Two levels of aggregation are shown, corresponding to (A) rouleaux formation in the presence of elevated fibrinogen (0.7 g%) and (B) clumping in response to 3 g% high-molecular-weight dextran (500 kDa). The apparently stronger clumps are formed as apposing RBC membranes are linked by dextran cross-bridging with the result that clumps are readily trapped within the entrance to small microvessels. From Pearson and Lipowsky (2004). Microcirculation 11(3), 295-306.

Figure 3 Red cell aggregates at the entrance of precapillary vessels in the low-flow state. Two levels of aggregation are shown, corresponding to (A) rouleaux formation in the presence of elevated fibrinogen (0.7 g%) and (B) clumping in response to 3 g% high-molecular-weight dextran (500 kDa). The apparently stronger clumps are formed as apposing RBC membranes are linked by dextran cross-bridging with the result that clumps are readily trapped within the entrance to small microvessels. From Pearson and Lipowsky (2004). Microcirculation 11(3), 295-306.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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