Blood cell deformability is a major source of the resistance to flow. Increases in bulk viscosity of RBCs in suspension clearly result from reductions in the ability of RBCs to deform under shear (g). However, these abnormalities in h are small compared with the dramatic rise in resistance to flow as individual cells enter the true capillaries. In most species, blood cellular elements (erythrocytes and leukocytes) are similar in size or slightly larger than the true capillaries, and they must be deformed by prevailing pressure gradients to gain entry into a capillary. The pressure gradient required to propel RBCs and WBCs into a capillary of a given diameter is strongly dependent on the ratio of cell diameter (DCELL) to capillary diameter (DCAP), that is, DCELL/DCAP. As DCELL/DCAP is reduced, small increases in cell stiffness may inhibit its entry into a capillary. As a result, RBCs and WBCs tend to traverse the microvascular network through pathways of least resistance, with the larger and less deformable WBCs bypassing the smallest diameter capillaries. As shown in Figure 4, plasma, red cells, and leukocytes traverse the capillary network through different pathways that are dictated by their ability to enter a vessel of a given diameter.
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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.