Biomedical Engineering Reference
In-Depth Information
with a geometrical asymmetry.
19
)
changes by swelling
20
and shrinking (crenated RBC)
21
in hypotonic and hypertonic
fluids, respectively. RBC Membrane has thermal agitation. These spontaneous
undulations are explained by the high membrane deformability. Furthermore, the
membrane is excited by permanent impacts from water molecules.
Red blood cells defend against osmotic changes by controlling their intracellular
ion content, especially keeping low [Na
+
]
i
andhigh[K
+
]
i
by the activities of the
AT P - d r iv e n N a
+
-K
+
pumps and ion exchangers. Hereditary
stomatocytosis
and
spherocytosis
are characterized by an uncontrolled excessive membrane permeabil-
ity to Na
+
and K
+
associated with amino acid substitutions in the intramembrane
domain of the anion exchanger SLC4a1 (or AE1) [
112
].
In narrow capillaries (conduit bore slightly
Red blood cells respond to osmotic pressure (
Π
particle size), the 2 essential control
parameters of the collective particle flow behavior are the hematocrit and flow
velocity. At high H
t
values, 3 distinct phases can be predicted [
113
]: (1) disordered
biconcave disc-shape; (2) parachute-shape, RBCs being aligned in a single file; and
(3) slipper-shape, RBCs being arranged in 2 parallel interdigitated rows.
>
3.5.3
Plasma Membrane
The membrane is constituted by a phospholipid bilayer with protein inclusions,
ion channels for chloride Cl
−
and bicarbonate HCO
3
in particular, and sialogly-
copeptides (blood groups),
22
with glucids (
10%), and with cytoskeleton meshing
and mooring proteins. A peripheral skeleton, a mesh made of spectrin, actin, and
other proteins (tubulin,
23
among others), resides at the inner surface of the plasma
membrane. It binds to RBC membrane (ankyrin-spectrin bonds) [
114
]. It gives
the RBC shape. Large, repeated RBC deformation associated with maintenance of
structural integrity is due to the cortical membrane-associated cytoskeleton.
<
19
The phospholipid outer layer of the RBC membrane has slightly more molecules and a greater
surface area than the inner layer.
20
In low-osmolarity solutions, RBCs becomes spherical after swelling (spherocyte). With further
swelling (hemoglobin solution leakage), RBCs are reduced to the membrane (erythrocyte ghost).
Hyperosmotic shock triggers the release of prostaglandin-E2 (PGe2) [
111
]. The latter activates
Ca
2
+
channels in the RBC membrane. Subsequent Ca
2
+
entry induces ankyrin-R degradation and
RBC shrinkage.
21
Two types of spiculed cells exist: echinocyte and acanthocyte, whether spicules are uniformly
distributed or not.
22
Erythrocytes have surface antigens associated with blood groups (glycosphingolipids ABO, Rh,
MN, Duffy, Lewis, and Kell).
23
Microtubules, involved in marginal bundles of RBCs, as well as in mitotic spindles and
centrioles, are composed of tubulins (Vol. 1 - Chap. 6. Cell Cytoskeleton).
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