Figure 17-8 Total pulmonary vascular resistance relates to lung volume as an asymmetric U-shaped curve. The trough of the curve occurs when lung volume equals functional residual capacity (FRC). Total pulmonary resistance is the sum of the resistance in small vessels (increased by increasing lung volume) and the resistance in large vessels (increased by decreasing lung volume). The end point for increasing lung volume (toward total lung capacity [TLC]) is the creation of zone 1 conditions, and the end point for decreasing lung volume (toward residual volume [RV]) is the creation of low ventilation-perfusion (V̇A/) and atelectatic (atel) areas that demonstrate hypoxic pulmonary vasoconstriction (HPV). The curve represents a composite of data from references ^{[20] [21]} and ^[23] .