In normal people, the pressure in pleural space is negative with respect to the alveolar pressure during the entire respiratory cycle. The pressure gradient between the alveoli and pleural space, the transpulmonary pressure is the result of the inherent elastic recoil of the lung. During spontaneous breathing the pleural pressure is also negative with respect to atmospheric pressure.
When communication develops between an alveolus or other intrapulmonary air space and the pleural space, air flows from the alveolus into the pleural space until there is no longer a pressure difference or until the communication is sealed