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1 Department of Program in Biomedical Engineering, University of Arizona, Tucson, AZ, USA
2 Department of Program in Biomedical Engineering, University of Arizona, Tucson, AZ, USA; Department of Physiology, University of Arizona, Tucson, AZ, USA
* To whom correspondence should be addressed. E-mail: secomb{at}u.arizona.edu.
A previously developed Krogh-type theoretical model is used to estimate capillary density in human skeletal muscle, based on published measurements of oxygen consumption, arterial partial pressure of oxygen and blood flow during maximal exercise. The model assumes that oxygen consumption in maximal exercise is limited by the ability of capillaries to deliver oxygen to tissue, and is therefore strongly dependent on capillary density, defined as number of capillaries per unit cross-section area of muscle. Based on an analysis of oxygen transport processes occurring at the microvascular level, the model allows estimation of the minimum number of straight, evenly spaced capillaries required to achieve a given oxygen consumption rate. Estimated capillary density values are determined from measurements of maximal oxygen consumption during knee extensor exercise and during whole body cycling, and range from 459 to 1468 capillaries/mm2. Measured capillary densities, obtained using either histochemical staining techniques or electron microscopy on quadriceps muscle biopsies from healthy subjects, are generally lower, ranging from 123 to 515 capillaries/mm2. This discrepancy is partly accounted for by the fact that capillary density decreases with muscle contraction, and muscle biopsy samples typically are strongly contracted. The results imply that estimates of maximal oxygen transport rates based on capillary density values obtained from biopsy samples do not fully reflect the oxygen transport capacity of the capillaries in skeletal muscle.
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