Comparative hemorheology.

Comparative data on blood composition and blood flow properties indicate different levels of interspecies variation for several parameters. Hematocrit and hemoglobin levels have relatively low variability among mammals, while mean cell volume and red blood cell (RBC) count are more variable. There is also a difference of variability between high and low shear rate blood viscosity in mammals, with low shear rate viscosity having a higher degree of interspecies variation. This observation parallels the diversity of RBC aggregation among mammalian species. Allometric relations for blood rheology parameters indicate highly significant correlations of low shear rate blood viscosity and RBC aggregation with body weight, especially if species belonging to the order Artiodactyla are excluded. These allometric relations can be used to formulate various hypotheses about the evolutionary histories of circulatory functions, as well as hypotheses related to the role of RBC aggregation in the mammalian circulatory system. Such comparative studies and analytical reasoning based on comparative data may contribute to answering the "why" questions, and accordingly may improve our understanding of circulatory functions and hence may have possible clinical importance. During the last several decades, similar approaches to various medical concepts have yielded a new approach to medicine that is now known as Evolutionary Medicine.