Friction and lubrication of pleural tissues

The frictional behaviour of rabbit’s visceral pleura sliding against parietal pleura was assessed in vitro while oscillating at physiological velocities and amplitudes under physiological normal forces. For sliding velocities up to 3 cm s−1 and normal compressive loads up to 12 cm H2O, the average value of the coefficient of kinetic friction (μ) was constant at 0.019±0.002 (S.E.) with pleural liquid as lubricant. With Ringer-bicarbonate solution, μ was still constant, but significantly increased (Δμ=0.008±0.001; P < 0.001). Under these conditions, no damage of the sliding pleural surfaces was found on light and electron microscopy. Additional measurements, performed also on peritoneum, showed that changes in nominal contact area or strain of the mesothelia, temperature in the range 19–39 °C, and prolonged sliding did not affect μ. Gentle application of filter paper increased μ 10-fold and irreversibly, suggesting alteration of the mesothelia. With packed the red blood cells (RBC) between the sliding mesothelia, μ increased appreciably but reversibly on removal of RBC suspension, whilst no ruptures of RBC occurred. In conclusion, the results indicate a low value of sliding friction in pleural tissues, partly related to the characteristics of the pleural liquid, and show that friction is independent of velocity, normal load, and nominal contact area, consistent with boundary lubrication.