Because RBCs are more deformable and faster than the leukocytes, they tend to pass the leukocytes at branch points or expansion zones (such as in postcapillary venules). As RBCs maneuver around a leukocyte, they push it even closer to the wall, often causing a collision between the leukocyte and endothelium. Consequently, high levels of adhesion are observed in these regions.
The combined effect of the plasma-rich zone and collisions with passing erythrocytes (predominantly from the radial direction) "traps" the leukocytes near the vessel wall and at the same time moves them along in the axial direction, thus increasing the number of collisions between the leukocyte and the surface.
Because of the difficulty in quantifying rapid leuko-cyte-endothelium collisions, much of the work in this area has relied upon mathematical simulation. These studies conclude that capillary:postcapillary venule diameter ratio, RBC configuration, and RBC shape are critical determinants of the initiation of contact in postcapillary venules.
Similar fluid dynamics can cause "secondary leukocyte capture" in larger vessels (Figure 1). This phenomenon occurs when a leukocyte in the free stream passes another already rolling or adherent on the surface. The low-pressure zone behind the adherent cell, and leukocyte-leukocyte interactions can cause the flowing cell to contact the wall. This can lead to rapid accumulation of adherent cells in the inflamed vessel.
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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.