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Fig. 2 Neutrophil recruitment during acute inflammation. Red blood cells induce leukocytes to
marginate to venular endothelium and make their first contacts with the vascular wall. Selectins
and their ligands (PSGL-1) mediate leukocyte capture and rolling on the endothelium to maintain
these two cells in close proximity. Upon activation by cytokines, rolling cells arrest on the
endothelium via CD18 integrin-ICAM-1 interactions and migrate along the vessel wall until they
eventually spread and transmigrate across the endothelium (i.e., diapedesis). The leukocytes then
migrate in the tissues to the site of trauma/infection
3.2 The Impact of Neutrophil Activation on Microvascular
Blood Flow
Under physiological conditions, neutrophils are second only to the red blood cells
(RBCs) in terms of their impact on microvascular flow. Typically, neutrophils
patrol the circulation as spherical, inactivated cells that exhibit a non-adhesive
state. Notably, this rounded, non-adhesive, and passively deformable state enables
the neutrophils to efficiently pass through the microcirculation with minimal
impact on microvascular blood flow (Fig.
3
, Normal flow).
The influence of neutrophil activity on microvessel resistance, X, is exemplified
by the following proportionality relationship based on a Hagen-Poiseuille approx-
imation of blood flow:
X
/
l
=
A
ð
1
Þ
where l is cell suspension viscosity and A is vessel area. Thus, under constant
flow, changes in either the cell suspension (e.g., blood) viscosity or the micro-
vessel network geometry impacts flow resistance.
In the smallest capillaries, a relatively high degree of cell deformability is
critical for passage of the leukocytes. In this regard, neutrophils with dimensions
on the order of 12-15 lm must undergo deformations in order to pass through the
capillaries with diameters of 5-10 lm. On the other hand, it has been shown that
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