A 58-year-old man is ventilated for ARDS. The team places an oesophageal balloon to measure pleural pressure. End-expiratory oesophageal pressure is 22 cmH2O and set PEEP is 14 cmH2O. Transpulmonary pressure at end-expiration is therefore -8 cmH2O. What does this negative transpulmonary pressure signify?