Tissue pressures and fluid dynamics of the lungs.

A pressure more negative than pleural surface pressure has to be applied to water on the pleural surface to keep it from being absorbed; yet, there is a thin layer of liquid in the pleural space under normal physiological conditions. How is the pressure of the liquid related to the pressure on the surface of the pleural membranes? There is evidence that the liquid pressure might be less than the surface pressure, and the balance is achieved by points of contact between the pleural membranes with tissue deformation forces. If it is possible for liquid and surface pressure to be different in the intrapleural space, the same mechanisms might be active within the lung tissue. Information about the tissue pressures can be estimated by determining the pressures acting on the outer surfaces of intrapulmonary vessels. There is evidence that some vessels of the lungs have a surface pressure close to alveolar pressure (alveolar vessels) and others have a surface pressure that falls relative to pleural pressure as alveolar pressure is increased relative to pleural pressure (extra-alveolar vessels). There appears to be a complete dissociation between the surface pressures and the liquid pressure surrounding the pulmonary blood vessels. These observations provide strong support for the concept that the pressure of liquid within tissue need not be the same as the tissue pressure.