Heterogeneity of capillary diameters in skeletal muscle of the frog

Capillary diameters in sartorius muscle of frogs were measured in vivo by means of a new computer video method, based on the passage of red blood cells (RBCs) through the capillary (C. Ellis, R. Sanfranyos, and A. Groom (1983), Microvasc. Res. 26, 139-150). The distribution of capillary diameters from 21 frogs was represented by a histogram with a mean +/- SD of 16.7 +/- 4.4 microns (N = 83). The measured dimensions (mean +/- SD) of frog RBCs, which have a flattened ellipsoidal shape, were: major axis = 24.1 +/- 2.6 microns (N = 149); minor axis = 16.5 +/- 1.5 microns (N = 158); thickness at center = 5.4 +/- 0.8 microns (N = 32). Frog RBCs travel through capillaries with their major axes predominantly parallel to the direction of flow; therefore, RBCs pass through capillaries without deformation provided that the diameter of the capillary is larger than the minor axis of the cell. By standardizing the measured values of capillary diameter in terms of mean minor dimension of the RBCs (ratio of means for frog being 1.0, approx), we were able to compare the diameter distribution in an amphibian with that in a mammal (rat). If RBC size alone mattered, both standardized distributions should superimpose; however, that for frog was shifted to the right of that for rat, indicating that frog RBCs are less deformable than RBCs of rat. This highlights the necessity, in the microcirculation, for matching capillary diameter to both size and deformability of the red cell.