Background and objectives: In capillaries with diameters less than those of resting RBCs, the cells have to deform to pass through such narrow vessels. Since RBCs of fetuses, neonates and adults differ in their geometrical properties the flow behavior of RBCs with different sizes in uniform tubes with diameters of 3 to 6 μm were studied by means of a micropipette system and a mathematical model. Assumptions in this model include an RBC flow velocity of 1 mm/s, axisymmetric cell shape and a gap between the RBC and the vessel wall that allows sufficient lubrication. The flow resistance depends on the surface area and volume of RBCs, the plasma viscosity and the vessel diameter.
Methods: Surface area and volume of different RBC populations (20 fetuses, 20 preterm neonates, 10 term neonates and 10 adults) were determined by means of a micropipette system and plasma viscosity was measured using a capillary tube viscometer. The flow behavior of RBCs with different volumes (61, 83 and 127 fl) was studied by direct microscopic observation using a micropipette system. The micropipettes had diameters of 3.5, 4.1, 5.1, and 6.0 μm. The flow velocity of the RBC in the pipettes was 1 mm/s and the calculated and measured cell lengths were compared.
Results: Volume and surface area of RBCs were 140 ± 29 fl and 172 ± 20 μm2, respectively, in the fetuses, decreased with increasing maturity (term neonates: 110 ± 20 fl and 149 ± 18 μm2) and reached the lowest values in adults (93 ± 14 fl and 136 ± 12 μm2). Plasma viscosities increased with increasing maturity due to rising plasma protein concentrations. The flow model leads to the following conclusions: During the passage of 3- to 6-μm vessels, the large fetal and neonatal RBCs are more elongated than the smaller adult RBCs. The critical vessel diameter, i.e., when the rear of the RBC becomes convex during passage of a narrow capillary, was 4.1 μm for fetal RBCs, 3.6 μm for neonatal RBCs and 3.3 μm adult cells. Suspended in the same medium, fetal and neonatal RBCs require 27% (term neonates) to 100% (fetuses) higher driving pressures than adult RBCs to achieve the necessary elongation for passing through a 4.5-μm capillary. However, the different RBCs require similar driving pressures if the cells are suspended in the corresponding autologous plasma. Cell lengths of the RBCs with different geometry determined during the flow experiments agreed with the predicted values.
Conclusion: We conclude that the large size of fetal and neonatal RBCs may cause impaired flow in narrow vessels with diameters below the critical values of 3.6 to 4.1 μm. In vessels with diameters above the critical diameter (Dcr), the disadvantage of the large size of neonatal and fetal RBCs appears to be completely compensated for by the lower plasma viscosity.
Keywords: Capillaries; RBC; axisymmetric flow; fetus; neonate.