Postcapillary venules start where two capillaries meet and discharge their blood into a microvessel of peculiar morphology that rapidly increases its diameter as more capillaries enter. At the beginning, postcapillary venules have a diameter of less than 10 mm, and further downstream, postcapillary venules can be as large as 80 mm. Wall shear rate, the rate of change in velocity near the wall, and wall shear stress, the force exerted per unit area of venule parallel to the vessel axis, are lower in postcapillary venules than in precapillary arterioles, and this contributes to, but does not entirely account for, preferential leukocyte adhesion in these vessels . Recently, the presence of a significant endothelial glycocalyx surface layer has been discovered , which is up to 0.5 mm thick and effectively reduces shear stress on the endothelial surface to zero . The shear forces experienced by adherent or rolling leukocytes can be calculated from the cell geometry and an interfacial shear stress present at the interface between glycocalyx and lumen. This interfacial shear stress is, on average, five times higher than the Newtonian wall shear stress estimated for a homogenous fluid and no glycocalyx [3 a].
In human blood, most leukocytes are neutrophils, with a smaller fraction of lymphocytes, monocytes, eosinophils, and basophils. In mouse blood, lymphocytes are more abundant than neutrophils. The lymphocyte compartment contains B cells, various types of T cells, natural killer cells, and others. Blood lymphocytes comprise hundreds of pheno-types distinguishable by flow cytometry.
Under most conditions, leukocyte interactions with post-capillary venules are dominated by neutrophils. Typically, more than 90 percent of all rolling and adherent cells are neutrophils, with the balance monocytes and eosinophils. Lymphocytes rarely roll or adhere in postcapillary venules of nonlymphatic tissues, but roll vigorously in high endothe-lial venules of lymph nodes and other secondary lymphatic organs. Therefore, reports of leukocyte rolling and adhesion observed by intravital microscopy in nonlymphatic organs almost always refer to neutrophils, although this is not always specifically mentioned. Recently, mice have become available in which certain leukocyte subsets express green fluorescent protein (GFP), so that specific intravital microscopic studies become possible for neutrophils, monocytes, and certain T cells.
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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.