In an artificially ventilated dog, the varying tracheal pressure causes a density fluctuation in the blood sampled from the aorta. We cross-correlated the tracheal pressure with the density to determine the time shift or delay of the latter from the former waveform for a ventilation frequency in the range of 6-30 CPM. The delay time was found to be 29% of the mean transit time (MTT) of the pulmonary vasculature and independent of the ventilation frequency. A comparison of this percentage with the reported arterial-to-capillary-to-venous fractional volumes of the lung suggested that the delay time may be the MTT time for blood flowing through the venous network of the lung and the cross-correlation may serve as an in vivo means to partition the MTT of the pulmonary vasculature at its capillaries. These results and an analysis on the deformation of the viscoelastic, pulmonary capillaries indicated that the tracheal pressure, acting primarily through the viscous part of the viscoelasticity, deforms the capillaries to produce the density fluctuation in blood outflowing from the lung.