Structural determinants of maximal O₂ transport in muscles of exercising foxes

The arctic blue fox (Alopex lagopus) has a specific maximal oxygen consumption (V̇(O₂) max/Mb = 3.6 ml O₂ · s^-1 kg^-1) that is approximately 1.6-fold greater than those of dogs and horses. The fox has one of the highest body mass specific skeletal muscle mitochondrial volumes (V(mt,m)/Mb = 44 cm³ · kg^-1) among mammalian athletic species matching its higher V̇(O₂) max/Mb. The structural components related to capillaries, such as specific capillary length density (J(c)/Mb = 348 km · kg^-1) and specific capillary volume (V(c)/Mb = 4.8 ml · kg^-1), are not greater in the fox than in the larger athletes. Because a greater specific muscle diffusing capacity for oxygen (DT(O₂)/Mb) is not utilized by the fox to achieve a higher V̇(O₂) max/Mb, a higher pressure difference for diffusion in the muscle capillaries is the alternative explanation for the fox's higher V̇(O₂) max/Mb. This mechanism is suggested by the fox's higher arterial and mixed venous capillary P(O₂) (120 mm Hg and 37 mm Hg, respectively) and its shorter mean muscle capillary transit time for blood (t̄c = 0.28 sec) compared to larger species.