Physiological and ecological implications of hemorheological variations in marine and terrestrial mammals

Abstract

The possible significance of variations in interspecific hemorheological properties related to diving behavior was studied in eight species of marine mammals with humans and pigs as terrestrial controls. Diving duration was positively correlated with elevated blood hemoglobin, oxygen capacity and viscosity among animals of the same class. No acclimatization response to activity was evident from studies of blood drawn from newly-captured northern elephant seals and sea otters and those in captivity for extended periods which justified the use of captive animals for rheological studies. Adaptations of marine mammals to diving were evident from comparisons of phocid seal and pig hemorheology. Seals had increased oxygen storage (six times) with less viscosity-dependent reductions in oxygen transport ($-$22%) when compared to pigs at equal packed cell volume. Phocid seal blood samples were compared with those of pigs and humans for erythrocyte aggregation and blood viscoelasticity to study the mechanics of viscometric variations. Viscous and elastic components of seal blood viscosity were 20 to 73% lower than those of pigs due to decreased aggregation extent and rate (P $<$ 0.05). Lower plasma fibrinogen and increased erythrocyte electrophoretic mobility are believed to contribute to lowered seal blood aggregation. Comparisons of the in vivo effects of blood viscosity on whole body and myocardial oxygen consumption by manipulation of whole body hematocrit in seals and pigs revealed that optimal hematocrit ranges for seals were shifted to the right of those from pigs (SEALS: 25%-55%; PIGS: 25%-45%; P $<$ 0.05). Seals showed significantly less viscosity-dependence in total body oxygen transport and oxygen consumption than did pigs. Myocardial oxygen consumption data were variable and showed no statistically significant differences among seals and pigs. The seals' lower erythrocyte aggregation, decreased low-shear viscosity and a greater ability to compensate for viscosity changes may represent adaptations to reduce the stress necessary to reinitiate flow in stagnant venous sinuses thereby reducing blood-flow resistance during dive-recovery. These adaptations may help maintain circulatory perfusion to vital organs, while flow is restricted to less oxygen-dependent tissues during underwater submergence without sacrificing the advantage of increased blood oxygen storage.