Blood-tissue exchange in liver and heart: the influence of heterogeneity of capillary transit times.

The single-injection multiple indicator-dilution technique has been used to explore blood-tissue exchange in the heart and the liver. Analysis of the classical description of exchange at the level of the single capillary following an impulse injection indicates that the loss of the material from the impulse in each capillary depends on the transit time of that capillary. Thus, in a multiple indicator-dilution experiment, early in time, for barrier-limited substances, the relation between the reference and diffusible substances as a function of time will reflect the manner in which the underlying capillary transit times have varied. In the heart, labeled sucrose exchange was used to quantitate the underlying pattern of capillary heterogeneity. With maximal coronary vasodilation, the capillary transit times appeared uniform (and there was a corresponding maximal heterogeneity of large vessel transit times). With more normal coronary tone, both the underlying capillary and large vessel transit times were found to increase along the outflow curve with the outflow arrival time. In the liver, in contrast, the transit times through the exchange area were found to show a maximal heterogeneity (and there was, in the large vessels, a corresponding uniform transit time).