Vasomotor control of capillary transit time heterogeneity in the canine coronary circulation

A set of indicator-dilution studies of the coronary circulation of the intact functioning heart was carried out in the dog to provide a data base for defining the effect of changes in vasomotor control on the exchange of materials across the myocardial capillaries. The reference substance used was 125I-labeled albumin; the diffusible substance used was 14C-labeled sucrose. Model analyses of the data were carried out. In previous models of the capillary exchange in the coronary circulation, it had been assumed that a single capillary transit time is representative of the whole. The data acquired here indicate that there is a very large heterogeneity of capillary transit times in the intact heart, and that the single transit time model is approximately true only when the resistance vessels are maximally dilated. The present pattern of findings is explained best by a model of the coronary microcirculation based on capillary-large vessel units with a variable heterogeneity of flow or capillary lengths, hence of transit times. We conclude that a major determinant of the extraction of each diffusible substance, on a microscopic level, is the distribution of capillary transit times.