Geometry of the capillary network in skeletal muscle

The capillary branching pattern and the dimensions of the capillary network were studied in M. sartorius in frogs ( Rana pipiens) after filling the microvasculature with the silicone elastomer “Microfil.” In contrast to a simple, binary-branching system, the arteriolar-capillary-venular system diverges immediately at the terminal arteriole and passes, via parallel interconnecting pathways to the venule, leading to a compact network without areas of insufficient supply. The mean value of total length for the capillary paths (3.58 ± 1.64 SD; n = 238) is governed by the distance between distributing artery and collecting vein, and the frequency distribution is Gaussian. Most paths branch repeatedly, forming one to nine segments; the number of branch points per path follows a Poisson distribution and, as would therefore be expected, the segment length distribution is exponential (mean = 0.85 mm ± 0.64 SD; n = 620). This suggests that, during growth of the network, branching occurs on a random basis along the vessel length. Possible reasons for capillary interconnections are considered. Measurements were made, for each successive order of branch point, of the number of capillaries arising, frequency of convergent and divergent branchings, and the percentage of rejoined vessels. By means of such data the geometry of the capillary network in different striated muscles could be compared, in a search for the general concepts underlying microvascular architecture.