Physiology of the alveolar surface network

The alveolar surface network (ASN) is the totally fluid intraacinar conformation of the alveolar surface liquid (ASL) continuum circulating, both in series and in parallel, through ultrathin (to <7 nm) molecular conduits formed by appositions of unit bubbles of alveolar gas. The ASN is the analogue of foam in vitro. Appositions of unit bubble films, namely foam films, include (a) bubble-to-bubble at the alveolar entrance, across alveolar ducts, and at pores of Kohn ('classical foam films'); (b) bubble-to-epithelial cell surface ('cell-surface foam film'); and (c) bubble-to-open surface liquid layer of the terminal conducting airways ('surface foam film'). These appositions of monolayer bubble films create (a) 'macrochannels' ('pressure points', 'reservoirs') that modulate ASL transfers, volume and flow throughout the acinus and between acinar surface and both the interstitium and the terminal conducting airways surfaces, and (b) 'microchannels' along the broadest surfaces of the appositions. 'Microchannels', which are expectedly bilayer, serve several functions, including (a) virtually frictionless orientation of unit bubbles and ASL to fill the acinar air space; (b) virtually unrestricted diffusion of respiratory gases; (c) architectural support ('infrastructure') against the 'mass' and 'recoil' force of the interstitium; and (d) provision of 'gate' and 'bridge' dynamics that further modulate and direct ASL circulation. The physiological and anatomical boundary between acinar ASN and the bubble-free open liquid surfaces of the conducting airways is marked by the surface foam film. The ASN operates as outlined above in all regions of the lung, at all lung volumes, beginning at the onset of air-breathing at birth and continuing throughout life. Reports of its discovery (Pulmonary Physiology of the Fetus, Newborn and Child (1975) 116; Pediatr. Res. 12 (1978) 1070) and subsequent confirmatory research including the adult lung are summarized in this review by progressive development of each function. These functions, which are normal for a relatively dry foam such as the ASN (where gas:liquid volume ratio is >99:1) cannot be duplicated by the conventional theories and models of an open 'alveolar lining layer'. The unfortunate research technologies upon which these theories and models have been formulated have, indeed, obfuscated recognition of the ASN in vivo. They are also presented and critiqued in this review.