The reason for the large interspecies variation in vertebrate red blood cell (RBC) size is poorly understood. To test the effect of RBC size on blood viscosity, blood samples from two vertebrates, the dog (Canis familiaris L., 1758) and the greater siren (Siren lacertina L., 1766), with extremely different RBC sizes were examined. In this study, we examined whether RBC size altered the relationship of viscosity and viscometer tube diameter, the well-known Fåhraeus–Lindqvist effect (FLE). We used a glass capillary viscometer that incorporates tubes with a diameter narrow enough to evoke the FLE. At similar RBC concentrations, viscosity of suspensions with the larger siren RBCs were greater than the viscosity of suspensions with the smaller dog RBCs. However, the relationship between viscosity and tube diameter is independent of RBC size. The results of this study allow us to conclude that the FLE is not related to RBC size. While packaging hemoglobin in RBCs appears adaptive by reducing blood viscosity through the FLE, RBC size does not contribute to the reductions in tube-relative viscosity.
Packed red blood cell transfusion is not associated with increased risk of necrotizing enterocolitis in premature infants