Alveolar surface film

The situation is, however, different in the alveolar region of the lung where the respiratory gas exchange takes place. Its thin squamous epithelium is covered by the so-called alveolar surface liquid (ASL). Its outermost surface is covered by a mixture of phospholipids and proteins with a low surface tension, also often referred to as lung surfactant. For this surfactant layer only, Scarpelli et al. [74] reported a thickness between 7 and 70 nm in the human lung. For the thickness of an additional water layer in between the apical surface of alveolar epithelial cells and the surfactant film no conclusive data are available. Hence, the total thickness of the complete ASL layer is actually unknown, but is certainly thinner than 1 gm. 

The alveolar surface represents a thin liquid film formed at the interface between the alveolar gas phase and a liquid hypophase covering the epithelium. This film is stabilised by the alveolar surfactant (AS), consisting mainly of phospholipids and proteins. AS plays an important role in alveolar stabilisation in the process of breathing. It is known that AS components exist as individual molecules and as various lipid and protein/lipid micellar structures present in the so-called hypophase and, according to some researchers, form a continuous lipid monolayer at the water/air interface [e.g. 1-4]. 

The microscopic foam bilayer proved to be an appropriate model for investigation of alveolar surface and alveolar stability as well [21]. This approach is in agreement with the findings of Scarpelli that at birth the lung surfactant takes the form of intraalveolar bubbles with formation of foam films [e.g. 2,22,23],... 

These studies of AS multilayer films as well as of lipid bilayers give reason to propose a new hypothesis of the structure of the alveolar surface. According to it the continuous lipid monolayer of the alveolar surface is in contact with the multilayer or with the membrane of epithelial cells situated under it, i.e. at the contact sites as well as between the individual... 

Pulmonary surfactant is a complex lipoprotein material (King, 1974 Harwood et al.y 1975) which is absorbed at the alveolar air/liquid interface and assists in maintaining alveolar stability. It lowers the surface tension to about 10nNm and reduces the contractile force of the surface and the work of lung expansion. It also prevents atelectasis because during expiration it forms a solid film on the alveolar surface. 

The surface tension forces of the alveolar lining film are reduced when rats breath air containing 70 mg/m of respirable crystalline silica 7 h... 

Ultrastructural studies of airway mucus and alveolar surfactant have been compromised by difficulties of preservation. By conventional aqueous fixation methods, the extracellular fluid lining is dissolved in the fixative more quickly than it can be stabilized. These techniques consist of either instilling the aqueous fixatives through the airways or by immersing small sections of tissue directly into the solution. This process causes the surface film to break up and to be washed away. Better preservation of the surface film was achieved by Weibel and Gil... 

The film appears continuous over the alveolar surface and covers the pores of Kohn (74 Figs. 3 and 4). By transmission electron microscopy (TEM) the surface film is seen to be multilayered (Fig. 5), consistent with the concept of a surfactant reservoir. The mechanism of formation of this multilayered film and the ways in which respiratory forces interact with it are not understood. One model proposed by Ries and Swift (79) is shown in Figure 6. 

Figure 11.23 (a) Structure of dipalinitoyl phosphatidylcholine. (b) Aggregates c)f alveoli. The advantage of dipalmitoyl phos-phab dylcholine is that it spreads at physiological temperatures to form a thin fat film over the whole inner surface of the alveolar sacs, which reduces surface tension. There are 300-400 million alveoli in each human lung. 

The surface of the cells lining the tracheobronchial tree and the surface of most of the cells lining the nasopharyngeal region are covered with a layer of relatively thick mucous material in the alveolar region, cells are lined with a thin film of fluid. The aqueous environment provided by these surface liquids favors at least partial dissolution and eventually absorption of water-soluble particles, especially those present as liquid droplets. Various defense mechanisms may help to remove less soluble particles from their site of deposition. 

In order to determine the infants lung maturity and the necessity of surfactant therapy it is of great importance to substantiate the functionality of the alveolar surfactant, derived via invasive techniques [13], Several techniques and models have been largely used to investigate inteifacial physicochemical properties in vitro and to assess clinical efficiency of ES in vivo the Langmuir monolayer technique in combination with Wilhelmy plate method for surface tension measurements and black foam film method for determination of the ability of ES for stable film formation [14]. The pendant drop method combined with the Axisym-metric Drop Shape Analysis (ADSA) has been also used for similar purposes [4,15-18]. 

This problem is circumvented by the presence of a surface active material composed of a complex mixture of lipids and proteins, the pulmonary surfactant, at the liquid-and-air interface within the alveolar lumen. The composition of the surfactant is such that it allows dense packing of the lipid film during expiration, thereby reducing the surface tension generated by the aqueous film. In the absence of surfactant increased surface tension along the alveolar lining results in the collapse... 

In eontrast with alveolar surfactant, the source of airway surfactant is less clear (49). Since surfactant material is presumed to leave the tracheal surface by mucociliary transport, the surfactant film has to be replenished by local secretion or from the alveolar region. The two options for the origin of airway surfactant are not mutually exclusive, and it is likely that airway surfactant is derived from both alveolar and local sources. 

When the mucous layer is appropriately preserved, it shows a smooth or slightly undulating surface, as shown in scanning electron micrographs (Fig. 8). By transmission electron microscopy, the top-most layer can be seen to be coated with an osmiophilic film (Fig. 9), which may appear multilayered (Fig. 10). Lamellar bodies and other lipid structures may be present in the sol layer (6,86 Fig. 11). The periodicity of these osmiophilic lamellae has been measured at approximately 40 A (6), suggesting structural homology with the alveolar lining layer. 

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