CAPILLARY RECRUITMENT AND TRANSIT-TIME IN THE RAT LUNG

Increasing pulmonary blood flow and the associated rise in capillary p erfusion pressure cause capillary recruitment. The resulting increase in capillary volume limits the decrease in capillary transit time. We hypothesize that small species with relatively high resting metabolic rates are more likely to utilize a larger fraction of gas-exchange res erve at rest. Without reserve, we anticipate that capillary transit ti me will decrease rapidly as pulmonary blood flow rises. To test this h ypothesis, we measured capillary recruitment and transit time in isola ted rat lungs. As flow increased, transit time decreased, and capillar ies were recruited. The decrease in transit time was Limited by an inc rease in the homogeneity of the transit time distribution and an incre ased capillary volume due, in part, to recruitment. The recruitable ca pillaries, however, were nearly completely perfused at flow rates and pressures that were less than basal for the intact animal. This sugges ts that a limited reserve of recruitable capillaries in the lungs of s pecies with high resting metabolic rates may contribute to their inabi lity to raise O-2 consumption manyfold above basal values.