Nature and distribution of vascular resistance in hypoxic pig lungs

We used the vascular occlusion technique in pig lungs isolated in situ to describe the efffects of hypoxia on the distribution of vascular resistance and to determine whether the resistive elements defined by this technique behaved as ohmic or Starling resistors during changes in flow at constant outflow pressure, changes in outflow pressure at constant flow, and reversal of flow. During normoxia, the largest pressure gradient occurred across the middle compliant region of the vasculature (ΔPm). The major effect of hypoxia was to increase ΔPm and the gradient across the relatively noncompliant arterial region (ΔPa). The gradient across the noncompliant venous region (ΔPv) changed only slightly, if at all. Both ΔPa and ΔPv increased with flow but ΔPm decreased. The pressure at the arterial end of the middle region was independent of flow and, when outflow pressure of the middle region exceeded 8.9 Torr during normoxia and 18.8 Torr during hypoxia. Backward perfusion increased the total pressure across the lung, mainly because of an increase in ΔPm. These results can be explained by a model in which the arterial and venous regions are represented by ohmic resistors and the middle region is represented by a Starling resistor in series and proximal to an ohmic resistor. In terms of this model, hypoxia exerted its major effects by increasing 1) the critical pressure provided by the Starling resistor of the middle region and 2) the ohmic resistance of the arterial region.