A buoyancy-driven squeeze-film model of intrapleural fluid dynamics: Basic concepts

We propose a new model of pleural liquid mechanics in the apneic animal based on the observation that the lung, in its position within the serous fluid of the chest, is a buoyant object with few physical connections to the chest. The buoyancy force due to the lung's low density causes the lung to rise within the chest, which in turn causes fluid to be squeezed out from the regions above the lung. The result is the transient component of the downward flow of intrapleural liquid and the less-than-hydrostatic vertical intrapleural pressure gradient observed by other investigators. For the purposes of mathematical simplicity, we have modeled the lung and chest wall of a horizontal animal as a pair of concentric cylinders separated by a narrow gap representing the pleural space. In this first version of the model, we treat the pleurae of the lung and chest wall as impermeable rigid boundaries, but despite these limitations, our mathematical analysis agrees with observations from a number of groups and explains the flow direction from top to bottom as well as the reported changes in vertical pressure gradient in response to a change in body orientation.