We have studied the comparative rheology of individual red blood cells from humans and rainbow trout (Oncorhynchus mykiss) at their natural body temperatures. Trout red blood cells were large ellipsoids (about 16 microns x 11.5 microns x 2.5 microns) with a mean volume of 250 fl, a surface area of approximately 350 microns 2 and an elongated nucleus of about 9 microns x 5 microns. Although much larger than human red cells (diameter 8 microns, V = 92 fl, A = 136 microns 2), both theoretical calculation and experimental aspiration into micropipettes indicated that the limiting size of a cylindrical vessel that both types of cell could enter was approximately 3 microns. Nevertheless, individual trout red cells had much longer transit times through 5 microns filter pores and were much slower to enter 3-4 microns diameter micropipettes. Interestingly, the relative deformability of the trout cells depended on the pore size and applied pressure, with entry times for trout and human cells converging as pipette diameter increased. The relatively poor overall cellular deformability of the trout cells reflected their membrane rigidity (shear elastic modulus 4-5 times higher than that of human membrane), as well as their large size and the presence of a prominent nucleus. Capillary diameters in trout muscle are similar to those in the human microcirculation (about 3 microns), while systemic driving pressures are much lower. Therefore, either red cell deformability is a less critical circulatory parameter than has previously been thought, or the apparently disadvantageous blood rheology of trout is adequate because of the lower demand for tissue perfusion.

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