Vertical gradient in regional vascular resistance and pre to post capillary resistance ratios in the dog lung

The ratios of pre-capillary (Ra) and post-capillary (Rv) resistances to total vascular resistance (RT) were measured at different vertical distances in the lungs of supine dogs. Capillary pressures were estimated as the algebraic sum of the alveolar absorption pressure for Tyrode's solution and the plasma colloid osmotic pressure. Segmental resistance fractions (Ra/RT, Rv/RT) were calculated using the pressure drops between arterial, capillary, and venous pressures at each vertical distance within the lung. In a second group of animals, total vascular resistance was calculated at each lung level using regional blood flow as measured by radiolabelled microspheres and the corresponding regional vascular pressures. The total vascular resistance was lowest in Zone III portions of the lung, but Ra/RT and Rv/RT were constant throughout this zone (n = 22) averaging .63 +/- .02 (SEM) and .37 +/- .02 (SEM, respectively, resulting in Ra/Rv of 1.70). Total vascular resistance increased markedly in Zone II portions of the lung, but the relative contribution of Ra/RT decreased from .62 +/- .07 (SEM) to .45 +/- .05 (SEM). Thus Ra/Rv decreased from 1.70 to .82 in Zone II. The absolute values of pre- and post-capillary resistances were lowest in Zone III but post-capillary resistance increased to a greater degree than pre-capillary resistance up Zone II.