Relations among recoil pressure, surface area, and surface tension in the lung.

The difference between energy stored in air- and saline-filled lungs is the sum of surface energy and the energy of tissue distortion caused by surface tension. The surface energy is zeta gamma dS, where gamma is surface tension and S is surface area. There is no corresponding relation between tissue energy and measurable variables. However, two equations can be obtained from the expression for the total energy difference. One is the statement that the total energy of the lung is minimum at equilibrium, and the other is the statement of conservation of mechanical energy as lung volume changes. The expression for tissue energy is eliminated between the two equations to obtain a single relation among the variables of interest: recoil pressure, surface area, and surface tension. Published data on recoil pressure and surface area of saline-filled, air-filled, and detergent-washed rabbit lungs are used in these equations to determine surface tension as a function of lung volume. The values of surface tension deduced from this analysis are lower than the values that would be obtained if the additional tissue forces in the air-filled lung were neglected. The contribution of tissue forces to the added recoil of the air-filled lung increases with increasing lung volume and accounts for approximately half the additional recoil at high lung volume.