Figure 1. Regional pulmonary blood flow as a function of vertical height in the lungs in one sheep under four conditions: prone and awake (A), prone and breathing on its own with pentobarbital anesthesia (B), prone with pentobarbital anesthesia and mechanical ventilation (C), and supine with pentobarbital anesthesia and mechanical ventilation (D). The direction of gravity along the x axis is from left to right. Note the large heterogeneity of flow within isogravitational planes and the counter gravitational slopes with spontaneous breathing.