Page 105 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Flow Properties of Blood molecular weight proteins (" B) as well as
ions and non-charged substances with low
The viscosity (η) of blood is higher than that of molecular weights are dissolved in plasma.
plasma due to its erythrocyte (RBC) content. The sum of the concentrations of these parti-
Viscosity (η) = 1/fluidity = shearing force cles yields a plasma osmolality of 290 mOsm/
(τ)/shearing action (γ) [Pa · s]. The viscosity of kgH 2O (" pp. 164, 377). The most abundant
+
blood rises with increasing hematocrit and cation in plasma is Na , and the most abundant
–
–
decreasing flow velocity. Erythrocytes lack the anions are Cl and HCO 3 . Although plasma
major organelles and, therefore, are highly de- proteins carry a number of anionic net charges
formable. Because of the low viscosity of their (" C), their osmotic efficacy is smaller because
contents, the liquid film-like characteristics of the number of particles, not the ionic valency,
their membrane, and their high surface/ is the determining factor.
volume ratio, the blood behaves more like an The fraction of proteins able to leave the
emulsion than a cell suspension, especially blood vessels is small and varies from one
when it flows rapidly. The viscosity of flowing organ to another. Capillaries in the liver, for ex-
blood (η blood) passing through small arteries (! ample, are much more permeable to proteins
20µm) is about 4 relative units (RU). This is than those in the brain. The composition of in-
twice as high as the viscosity of plasma (η plasma terstitial fluid therefore differs significantly
= 2 RU; water: 1 RU = 0.7 mPa · s at 37 "C). from that of plasma, especially with respect to
Because they are highly deformable, normal
Blood RBCs normally have no problem passing protein content (" C). A completely different
+
composition is found in the cytosol, where K is
4 through capillaries or pores in the splenic ves- the prevailing cation, and where phosphates,
sels (see p. 89 B), although their diameter (! proteins and other organic anions comprise
# 5 µm) is smaller than that of freely mobile the major fraction of anions (" C). These frac-
RBCs (7 µm). Although the slowness of flow in tions vary depending on cell type.
small vessels causes the blood viscosity to in- Sixty percent of all plasma protein (" B) is
crease, this is partially compensated for albumin (35–46 g/L). Albumin serves as a ve-
(η blood!) by the passage of red cells in single hicle for a number of substances in the blood.
file through the center of small vessels (diame- They are the main cause of colloidal osmotic
ter # 300 µm) due to the Fåhraeus–Lindqvist pressure or, rather, oncotic pressure (" pp. 208,
effect (" A). Blood viscosity is only slightly 378), and they provide a protein reserve in
higher than plasma viscosity in arterioles times of protein deficiency. The α 1, α 2 and %
(! ! 7 µm), but rises again in capillaries globulins mainly serve to transport lipids
(! ! 4 µm). A critical increase in blood viscos- (apolipoproteins), hemoglobin (haptoglobin),
ity can occur a) if blood flow becomes too slug- iron (apotransferrin), cortisol (transcortin),
gish and/or b) if the fluidity of red cells and cobalamins (transcobalamin). Most
decreases due to hyperosmolality (resulting in plasma factors for coagulation and fibrinolysis
crenation), cell inclusion, hemoglobin malfor- are also proteins. Most plasma immuno-
mation (e.g., sickle-cell anemia), changes in globulins (Ig, " D) belong to the group of γ
the cell membrane (e.g., in old red cells), and so globulins and serve as defense proteins (anti-
forth. Under such circumstances, the RBCs un- bodies). IgG, the most abundant immuno-
dergo aggregation (rouleaux formation), in- globulin (7–15 g/L), can cross the placental
creasing the blood viscosity tremendously (up barrier (maternofetal transmission; " D). Each
to 1000 RU). This can quickly lead to the cessa- Ig consists of two group-specific, heavy protein
tion of blood flow in small vessels (" p. 218). chains (IgG: γ chain, IgA: α chain, IgM: µ chain,
IgD: δ chain, IgE: ε chain) and two light protein
chains (λ or κ chain) linked by disulfide bonds
Plasma, Ion Distribution
to yield a characteristic Y-shaped configura-
Plasma is obtained by preventing the blood tion (see p. 95 A).
from clotting and extracting the formed el-
92 ements by centrifugation (" p. 89 C). High

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