Rheology of fetal and maternal blood.

Rheological parameters were measured in 10 pairs of mothers and newborns. Whole blood viscosity was similar despite a higher fetal hematocrit (47.0 +/- 5.1 versus 35.5 +/- 12.0%, mean +/- SD, p less than 0.05). When the hematocrit of the suspension of red cells in plasma was adjusted to 45%, the viscosity was significantly lower in the fetal blood over a wide range of shear rates (0.52-208 S-1). The main reason for the lower viscosity in the fetal blood was the lower plasma viscosity as compared to the maternal blood (1.08 +/- 0.05 versus 1.37 +/- 0.08 centipoise, p less than 0.05); this in turn was attributable to a lower total plasma protein concentration (4.74 +/- 0.71 versus 6.47 +/- 0.64 g/dl, p less than 0.05). All protein fractions were lower in the fetal plasma. The assessment of red cell deformability by filtration through polycarbonate sieves revealed that the resistance of a fetal red cell was three times higher than that of a maternal red cell in a 2.6-micron pore, but there was no significant difference in resistance for these red cells in 6.9-micron pores. This higher filtration resistance of fetal red cells through the small pores was mainly due to their large volume (115.4 +/- 10.8 versus 93.5 +/- 5.9 fl, p less than 0.001). Measurements on membrane-free hemoglobin solutions indicated that the internal viscosity of these two types of red cells was not different.(ABSTRACT TRUNCATED AT 250 WORDS)