Method for Estimating Respiratory Surface Area of Mammalian Lungs from Their Physical Characteristics. ∗

Abstract

The surface area available for gas exchange in the lungs s an impotant factor in gas transfer between the alveoli and the blood 1 ., 2 Present estimates of this area are based on measurements of alveolar dimensions taken from fixed histologic specimens 3 , 4 , 5 . These estimates of total surgace area are made by measuring or calculating the area/volume ratio for a small portion of lung tissue and applying this ratio to an assumed total lung volume. The volume which must be assumed is uncertain because the degree of inflation represented in the particular specimen is difficult to determine, especially if the lungs are allowed to collapse before fixation is complete. Moreover, distortion of lung architecture inevitably occurs with standard histologic methods involving dessicatin and embedding, and such lobular subdivisions as the air-sacs and alveolar ducts, which contribute only slightly to the respiratory surface area, have in the inflated state a much larger volume than is generally realized 6 . for these reasons calculation by anatomic methods tend to result in overestimating the number of alveoli and the total lung surface.

The method of estimating lung surface described in this report is based on physical measurements of the lungs and avoids any assumption of size or configuration of respiratory structures. The calculation depends on consideration of the changes in free energy of the lungs as they are deflated. During inflation, energy is stored by deforming elasitc elements. and by the creation of a large air-liquid interface If the lungs are diflatd by allowing volume equilibrium at successive pressure decrements, the stored energy released during deflation can be measured.