Increased PVR at extremes of lung volumes. This figure represents measurements made in an animal-lobe preparation in which the transmural pressure of the capillaries is held constant. It illustrates that at low lung volumes (as may occur with atelectasis), extraalveolar vessels become narrow, and smooth muscle and elastic fibers in these collapsed vessels increase PVR. At high lung volumes, as alveolar volumes are increased and walls are thinned, capillaries are stretched, reducing their caliber and also increasing PVR. (Adapted from John West's Essential Physiology, 10th edition, Philadelphia: Lippincott & Williams, with permission).