ABSTRACT
Blood is a two-phase suspension of formed elements (i.e., red blood cells [RBCs],
white blood cells [WBCs], platelets) suspended in an aqueous solution of organic molecules,
proteins, and salts called plasma. The apparent viscosity of blood depends on the
existing shear forces (i.e., blood behaves as a non-Newtonian fluid) and is determined
by hematocrit, plasma viscosity, RBC aggregation, and the mechanical properties of
RBCs. RBCs are highly deformable, and this physical property significantly contributes
to aiding blood flow both under bulk flow conditions and in the microcirculation.
The tendency of RBCs to undergo reversible aggregation is an important determinant
of apparent viscosity because the size of RBC aggregates is inversely proportional
to the magnitude of shear forces; the aggregates are dispersed with increasing shear
forces, then reform under low-flow or static conditions. RBC aggregation also affects
the in vivo fluidity of blood, especially in the low-shear regions of the circulatory
system. Blood rheology has been reported to be altered in various physiopathological
processes: (1) Alterations of hematocrit significantly contribute to hemorheological
variations in diseases and in certain extreme physiological conditions; (2) RBC deformability
is sensitive to local and general homeostasis, with RBC deformability affected by
alterations of the properties and associations of membrane skeletal proteins, the
ratio of RBC membrane surface area to cell volume, cell morphology, and cytoplasmic
viscosity. Such alterations may result from genetic disorders or may be induced by
such factors as abnormal local tissue metabolism, oxidant stress, and activated leukocytes;
and (3) RBC aggregation is mainly determined by plasma protein composition and surface
properties of RBCs, with increased plasma concentrations of acute phase reactants
in inflammatory disorders a common cause of increased RBC aggregation. In addition,
RBC aggregation tendency can be modified by alterations of RBC surface properties
because of RBC in vivo aging, oxygen-free radicals, or proteolytic enzymes. Impairment
of blood fluidity may significantly affect tissue perfusion and result in functional
deteriorations, especially if disease processes also disturb vascular properties.
KEYWORDS
Hemorheology - hemodynamics - viscosity - erythrocyte deformability - erythrocyte
aggregation - tissue perfusion - blood flow
