Different constraints shaped red blood cell characteristics in vertebrates

The correlation between body mass and hematological parameters has been investigated in mammals many times in the past, and to a much lesser degree in other vertebrates. Since the pivotal role of red blood cells (RBC) in delivering oxygen to tissues, also a correlation between hematological parameters and metabolic rates could be expected. In the present study, I investigated these correlations using data from 513 vertebrate species. Most parameters showed poor (but significant) correlation with either body mass or metabolic rates. Nonetheless, hematocrit was significantly higher in endotherms compared to ectotherms. One of the most interesting parameters is RBC volume (MCV), also because this, together with hemoglobin concentration, determines all other hematological parameters. MCV is correlated with metabolic rates (and thus, indirectly, with body mass) and hemoglobin-oxygen affinity. Moreover, in species with nucleated RBC (i.e., non-mammals), it is strongly correlated with genome size. This study shows how hematological parameters coevolved with metabolic demands and the constraint posed by a factor extrinsic to the gas exchange system (i.e., genome size).