Geometrical Relationship Between Capillaries and Muscle Fibers in Chronic Hypoxia

Abstract

Capillary-to-fiber geometrical relationships are important for several aspects of blood—tissue transfer in skeletal muscles. Fiber size determines maximal diffusion distances within the muscle fiber. Transmembrane flux depends on the amount of capillary and fiber surfaces. From the Fick law of diffusion, the increased capillary surface area found after endurance training indicates a reduction of the pressure drop necessary for a given O₂ flux (23). Ellis et al. (7) stressed the importance of capillary geometry for the efficiency of blood—tissue transfer. The dramatic change in capillary configuration with muscle shortening (22, 25) tends to create a more uniform circumferential O₂ supply around the muscle fibers. At the extreme (circumferentially homogeneous O₂ supply), the Hill model (8) predicts that the muscle fiber can achieve a five-fold increase in O₂ flux for the same drop in Po_e from capillary to the center of a 50-µm fiber compared with the Krogh geometry (7, 11). This chapter reviews our morpho-metric data on capillary-to-fiber geometry in skeletal muscles in response to hypoxia. We addressed the question of whether or not systematic difference(s) are found in the arrangement of capillaries relative to the muscle fibers after chronic exposure or adaptation, or both, to hypoxia. Specifically, we examined fiber size, capillary amount, and configuration in muscles of deer mice native to 3800 m compared with sea-level deer mice, in rats and pigeons kept at 3800 m for 5 months with sea-level controls, and in animals tolerant to prolonged periods of anoxia (harbor seal) compared with dogs. These animal models were selected to provide a wide range of functional demands or constraints.