Distributions of vascular pressure and resistance in the lung.

The low-viscosity bolus method was used to determine the longitudinal distributions of vascular resistance and intravascular pressure with respect to cumulative vascular volume from the lobar artery to the lobar vein in isolated dog lung lobes near functional residual capacity under zone 3 conditions. We found that the resistance distribution had two modes, a larger one upstream and a smaller one downstream from a local minimum. Over the range of vascular pressures studied the total vascular resistance decreased and the vascular volume increased with increasing vascular pressure. However, the shape of the normalized resistance distribution was independent of vascular pressure. Comparisons of the resistance distributions with the distributions of arterial, capillary, and venous volumes suggest that the modes represent regions of relatively high resistance proximal and distal to the capillary bed. These results are consistent with the concept that within the lobar vascular bed the highest resistance per unit blood volume is in the smallest arteries and veins, as suggested by morphometric data from other sources.