Cross-sectional PO2 distributions in Krogh cylinder and solid cylinder models.

Radial profiles, gradients and frequency distributions of PO2 are calculated for two cylindrical tissue models with uniform O2 diffusion properties and O2 consumption: Krogh's cylinder with O2 supplied from a central capillary (model A), and a solid cylinder with O2 supplied from the outer surface (model B). Because the O2 diffusion flux is divergent in model A, and convergent in model B, the PO2 gradient flattens with increasing distance from the supplying surface more rapidly in model A than in model B. In model A, the frequency distribution of PO2 with respect to unit cross-sectional area is highly asymmetric, being skewed to low PO2, whereas it is uniform in model B. Model A is applicable to parallel capillaries uniformly distributed across tissue cross-section. For Model B a large number of capillaries surrounding a cylindrical structure is required. Model A appears to be much more adequate than model B to describe the normal morphometry in skeletal muscle (capillary number/fiber number ratio of about 2, capillary-to-muscle fiber radius ratio of about 0.1). The experimental finding of relatively low PO2 and small PO2 gradients within muscle fiber cross-sections is in agreement with both models, but agrees better with model A than with model B.