538 
HEMODYNAMICS 
shear rates (<10 sec"^), blood viscosity de- 
pends strongly on shear rate and human blood 
viscosity may exceed model viscosity by more 
than a factor of 10. Plasma fibrinogen concen- 
tration is important at low shear rates. 
Cell deformability is related to the sphericity 
index because spherical cells, if of fixed surface 
area, are capable of no deformation. Evidence 
v^'as presented that the index is a function of 
cell volume, and the RBC of man is more de- 
formable than model cells. The capability to de- 
form enhances rouleaux formation, increases 
cell vulnerability, and permits cells to penetrate 
holes much smaller than their disc diameter. 
Augmented diffusion due to rigid red cell ro- 
tation and collision is expected to increase lin- 
early with cell size. However, qualitative con- 
siderations suggests cell induced diffusion 
diminishes with increased deformability, the 
latter increasing with cell size. Model experi- 
ments are expected to slightly underestimate 
augmented diffusion in human blood and the 
diffusion rate should be proportional to the ratio 
of cell diameter to a power between one and 
zero. 
Size and deformability conspire in producing 
the cell depleted layer. Model experiments are 
expected to underestimate the extent of the 
layer in human blood. 
Recalling the fact that red blood cell proper- 
ties are interrelated, from the standpoint of the 
mechanical properties of blood the animal that 
best models man is one whose red blood cell 
volume is the same as that of man. 
SUMMARY 
The red cells in mammalian and non-mam- 
malian species display a range of size, shape, 
membrane flexibility, tendency to aggregate and 
intracellular viscosity characteristic of each spe- 
cies. These parameters influence the ability of 
the cells to resist mechanical trauma, and along 
with the properties of the plasma, influence the 
rheological behavior of the blood. In this paper : 
(1) The relevant mechanical properties of 
the erythrocytes of several species ob- 
tained in the laboratory of Shu Chien 
are summarized along with the result- 
ing rheological influences. 
(2) The mechanical hemolysis results of 
Forstrom's jet test for several species 
are summarized in the light of the 
mechanical properties of the erythro- 
cytes. 
(8) The role of cell size and deformability 
in influencing the diffusion of extra- 
cellular solutes in the presence of a 
velocity gradient is calculated (but 
not demonstrated experimentally). 
(4) The role of cell size and deformability 
in influencing the cell concentration 
distribution near walls during flow is 
qualitatively predicted (but not dem- 
onstrated experimentally) . 
(5) The way these factors determine the 
degree to which model experiments 
can be related to human blood is sum- 
marized. 
REFERENCES 
1. Chien, S., Usami, S., Dellenback, R. J., and Bry- 
ant, C. A.: Comparative Hemorheology — Hema- 
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Blood Viscosity, Biorheology, 8:35, 1971. 
2. Goldsmith, H. L., and Mason, S. G.: The Micro- 
rheology of Dispersions, in Rheology, Vol. 4 (Edited 
by Eirich, F. R.) p. 850, Academic Press, New 
York, 1967. 
3. COKELET, G. R. and Meiselman, H. J.: Rheological 
Comparison of Hemoglobin Solutions and Erythro- 
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4. Rand, R. P.: Mechanical Properties of the Red 
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6. LaCelle, P. L. and Weed, R. I.: Alteration of 
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7. FORSTROM, R. J.: Ph.D. Thesis, Minneapolis, Uni- 
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8. CoLLiNGHAM, R. E. : Ph.D. Thesis, Minneapolis, 
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