While the pulmonary circulation does not have as many demands on it or as many control mechanisms, resistance is altered by a number of factors passive and active. As explained on the last page, increases both pulmonary arterial pressure and left atrial pressure result in decreases in PVR.
Increased interstitial pressure and blood viscosity increase PVR. The former occurs because fluid fills the interstitial space, squeezing the blood vessels to smaller calibers, thus increasing flow resistance. Increasing viscosity means the blood is thicker and it takes more energy to move it through blood vessels.
Increased lung blood volume decreases PVR by expanding pulmonary blood vessels, decreasing flow resistance.