To examine the effects of aging on the elasticity of the lungs a single exponential function (V = A - B exp (-KP)), where V is lung volume, P is recoil pressure, and A, B, and K are constants) and a fourth-order polynomial were fitted to the static pressure-volume data from 124 healthy nonsmokers (83 males). K, the index of compliance, was independent of sex and increased with age. B/A and recoil pressures at various lung volumes were higher in males than females, but decreases with age were similar in both sexes. Static compliance (expressed as percent of total lung capacity/cmH2O), derived from the polynomial expression, was the same in males and females and increased with age. The results show that the lungs of males and females have the same intrinsic elasticity, and differences in recoil pressure depend on differences in lung size and in maximum distending forces. Loss of elasticity with age is consistent with an increase in the unstressed dimensions of alveoli and a decrease in their elastic fibers.

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To improve the quantitative representation of elastic recoil, an exponential function (V = A - B exp (-KP), where V is the lung volume, P is the recoil pressure, and A, B, and K are constants), has been fitted to static pressure-volume (PV) data by a least-squares technique using a digital computer. The PV points below about 50% total lung capacity (TLC) usually deviate from an exponential expression. Increasing the lower volume limit to which the exponential was fitted from 40 to 75% of TLC in 20 subjects, increased K and B/A significantly. Residual variance was lowest (approx 1.0 +/- 0.5%) when the lower volume limit was between 50 and 60% TLC. The PV points in the lower volume range deviated significantly less from exponential in a group of older subjects than for young subjects. The coefficient of variation of K and B/A for duplicate studies in 10 subjects was 9 and 14%, respectively. For inflation PV data, K was significantly lower and A significantly higher than for deflation. For PV data obtained after a submaximal inspiration, K did not change significantly. From this exponential function, the PV curve above 50% TLC can be satisfactorily described by K, B/A, and recoil pressure at TLC; other conventional parameters may be derived.

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