EFFECT OF INCREASING FLOW ON DISTRIBUTION OF PULMONARY CAPILLARY TRANSIT TIMES

The complex morphology of the pulmonary capillary network causes capil lary transit times to be dispersed about a mean. It is known that flow -induced decreases in mean capillary transit time are partially offset by capillary recruitment and distension, but the effect of these fact ors on the rest of the distribution of transit times is unknown. We ha ve studied the relationship between blood flow, capillary recruitment, and the distribution of transit times in isolated canine lungs with v ideomicroscopy. Doubling baseline lobar blood flow recruited capillari es. All transit times in the distribution decreased, as did relative d ispersion. Doubling flow again caused a further decrease in transit ti mes, but neither capillary recruitment nor relative dispersion changed significantly. We conclude that capillary transit times become more h omogeneous as lobar flow increases from low to intermediate levels. Fu rther increases in flow across a fully recruited network are associate d with decreases in transit times but not with more homogeneous capill ary perfusion.