Stress at the pleural surface.

The transmission of forces between the visceral and parietal pleural is modelled as being mediated by two distinct pathways: (a) a contact pressure (Pcon) acting on an area of contact (Acon) and (2) a pleural liquid pressure (Pliq) acting on the area of liquid anfractuosities (Aliq). Summation of the forces yields a new equation relating pleural surface pressure (Ppl) to Pcon, Acon, Pliq, and Aliq. A complete solution of the equation is restricted by present limitations on measurement technology. However, assuming that the volume of pleural liquid is constant during tidal breathing, analysis of the model explains the findings that tidal swings in Pliq (deltaPliq) are sometimes greater than tidal swings in Ppl (deltaPpl). It also predicts that the influence of a change of lung elastic recoil on Pliq may be different in magnitude than its effect on Ppl. Therefore deltaPliq is not necessarily equivalent to deltaPpl.