Elasticity of the pulmonary alveolar sheet.

An analysis of experimental data on the pulmonary alveolar sheet of the cat shows: (1) As far as elasticity in the plane of the sheet is concerned, the alveolar sheet may be regarded as uniform; the compliance of the posts is the same as that of the membranes. (2) Within a physiological range of positive transmural pressure, the mean thickness of the sheet varies linearly with the pressure. Also, the stress distribution in the alveolar-capillary membrane is nonuniform and nonisotropic. A theoretical sheet thickness-pressure relationship is derived in which the effect of stress resultants (sum of elastic stress and surface tension) is explicitly linked to the compliance of the sheet thickness. The sheet-flow theory then shows that average flow is very sensitive to the arteriole pressure. The flow per alveolar sheet is (1/C) (ha* —hv ). Here ha and hv are equal to alveolar sheet thickness at the arteriole and venule, respectively. When par t > palv, ha = h0 + a(pBrt -paiv)> where p a r t is the blood pressure in the sheet at the arteriole, palv is the gas pressure in the alveolus, a. is the compliance constant for the sheet thickness, and h0 is the thickness at Ap = 0. The constant C = AfxjLal (SA), where fi is the coefficient of viscosity of blood, / is a friction parameter (on the order of 4, depending on the post geometry), L is the average length of the streamlines in the sheet, S is the vascular space-tissue ratio (on the order of 0.9), and A is the sheet area. Comparison of this formula with the experimental results of Roos et al., using h0 and a values from Glazier et al., shows reasonable agreement.