Alveolar surface forces and lung architecture

The entire alveolar surface is lined by a thin fluid continuum. As a conseq uence, surface forces at the air-liquid interface are operative, which in p art are transmitted to the delicate lung tissue. Morphologic and morphometr ic analyses of lungs show that the alveolar surface forces exert a moulding effect on alveolar tissue elements. In particular, in lungs at low degrees of inflation, equivalent to the volume range of normal breathing, there is a derecruitment of alveolar surface area with increasing surface tensions which reflects equilibrium configurations of peripheral air spaces where th e sum of tissue energy and surface energy is minimum. Thus, changes in surf ace tension alter the recoil pressure of the lung directly and indirectly b y deforming lung tissue and hence changing tissue tensions. However, the in terplay between tissue and surface forces is rather complex, and there is a marked volume dependence of the shaping influence of surface forces. With increasing lung volumes the tissue forces transmitted by the fiber scaffold of the lung become the predominant factor of alveolar micromechanics: at l ung volumes of 80% total lung capacity or more, the alveolar surface area-v olume relation is largely independent of surface tension. Most important, w ithin the range of normal breathing, the surface tension, its variations an d the associated variations in surface area are small. The moulding power o f surface forces also affects the configuration of capillaries, and hence t he microcirculation, of free cellular elements such as the alveolar macroph ages beneath the surface lining layer, and of the surfaces of the periphera l airways. Still enigmatic is the coupling mechanism between the fluid cont inua of alveoli and airways which might also be of importance for alveolar clearance. As to the surface active lining layer of peripheral air spaces. which determines alveolar surface tension, its structure and structure-func tion relationship are still ill-defined owing to persisting problems of fil m preservation and fixation. Electron micrographs of alveolar tissue, of li ning layers of captive bubbles, and scanning force micrographs of surfactan t films transferred on mica plates reveal a complex structural pattern whic h precludes so far the formulation of an unequivocal hypothesis. (C) 2001 E lsevier Science Inc. All rights reserved.