Selected contribution: Measuring the response time of pulmonary capillary recruitment to sudden flow changes

To determine how rapidly pulmonary capillaries recruit after sudden changes in blood flow, we used an isolated canine lung lobe perfused by two pumps running in parallel. When one pump was turned off, flow was rapidly halved; when it was turned on again, flow immediately doubled. We recorded pulmona ry capillary recruitment in subpleural alveoli using videomicroscopy to mea sure how rapidly the capillaries reached a new steady state after these ste p changes in blood flow. When flow was doubled, capillary recruitment reach ed steady state in <4 s. When flow was halved, steady state was reached in similar to 8 s. We conclude that the pulmonary microcirculation responds ra pidly to step changes in flow, even in the capillaries that are most distan t from the hilum.