This has the effect of increasing the effective cross-sectional area of the pulmonary vasculature and thus reducing its resistance. As the cross-sectional area increases, it's as if you are using a bigger pipe to pass the flow through. Although above we discussed the resistance to flow in the lung vasculature overall, it turns out that flow varies from region to region. While some regions are well perfused, others receive very little flow. The terms upstream and downstream refer to the direction of flow.
Flow is from upstream to downstream during cardiac systole in the pulmonary vasculature.
Pressure is greatest in the dependent regions of the lung . Pressure must be greater than local alveolar pressure in order for flow to occur. Essentially, the downstream pressure can be either the pulmonary venous pressure OR alveolar pressure , depending on local conditions. I alveolar pressure is greater than BOTH local pulmonary arterial and venous pressures . Other Influences on PVR Effects of lung volume Because the pulmonary vasculature is found largely within the lung, changes in lung volume have the potential to profoundly alter pulmonary vascular resistance.