What forces keep the air spaces of the lung dry?

An alternative view of the role(s) of surfactant and the alveolar model

Currently, the study of surfactant proteins is much in vogue, but, in the early days, the physics underlying surfactant function was treated somewhat superficially, leaving assumptions that have become culturally embedded, such as the "bubble" model of the alveolus. This review selectively reexamines these assumptions, comparing each combination of alveolar model and role of surfactant for compatibility with the major features of pulmonary mechanics and alveolar stability, morphology, and fluid balance.

Particulate air pollution and acute health effects

Epidemiological studies have consistently shown an association between particulate air pollution and not only exacerbations of illness in people with respiratory disease but also rises in the numbers of deaths from cardiovascular and respiratory disease among older people. Meta-analyses of these studies indicate that the associations are unlikely to be explained by any confounder, and suggest that they represent cause and effect. We propose that the explanation lies in the nature of the urban particulate cloud, which may contain up to 100000 nanometer-sized particles per mL, in what may be a gravimetric concentration of only 100-200 micrograms/m3 of pollutant. We suggest that such ultra-fine particles are able to provoke alveolar inflammation, with release of mediators capable, in susceptible individuals, of causing exacerbations of lung disease and of increasing blood coagulability, thus also explaining the observed increases in cardiovascular deaths associated with urban pollution episodes. This hypothesis is testable both experimentally and epidemiologically.

Stability and permeability of amphiphile bilayers

In this review the rupture and permeability of bilayers are considered on the basis of a mechanism of the formation of microscopic holes as fluctuations in the bilayers. The hole formation is treated as a nucleation process of a new phase in a two-dimensional system with short-range intermolecular forces. Free rupture and deliberate rupture (by alpha-particles) of foam bilayers (Newtonian black films) are discussed. A comparison is made between the rupture of foam and emulsion bilayers. Experimental methods for obtaining foam and emulsion bilayers from thin liquid films are considered. Methods for investigating the stability and permeability of foam bilayers, which are based on a microscopic model allowing the use of amphiphile solutions with very low concentrations, are described. Experimental dependences of the lifetime of bilayers, the probability of observing the foam bilayer in a foam film, the gas permeability of bilayers, etc. on the concentration of amphiphile molecules in the solution are reported. The influence of temperature and external impact (e.g. alpha-particle irradiation) have also been experimentally studied. A good agreement between theory and experiment is established, allowing determination of several characteristics of foam and emulsion bilayers obtained from ionics or non-ionics: the specific edge energy of bilayer holes, equilibrium surfactant concentration below which the bilayer is thermodynamically metastable, work for the formation of a nucleus hole, number of vacancies in the nucleus hole, coefficient of gas diffusion through the bilayer, etc. On the basis of the effect of temperature on the rupture of foam bilayers the binding energy of a surfactant molecule in the bilayer is determined. The adsorption isotherm of surfactant vacancies in the foam bilayer is obtained which shows a first-order phase transition. Some applications to scientific, technological and medical problems are considered. The foam bilayer is used as a model for investigating short-range forces in biological structures, the interaction between membranes and cell fusion. It is also shown that the foam bilayer is a suitable model for studying the alveolar surface and stability. On that basis a clinical diagnostic method is developed for assessment of the human foetal lung maturity.

Pulmonary lymphatics in SIDS--a comparative morphometric study

Pulmonary lymphatics in a group of SIDS cases were compared with a control group of a similar age distribution. The causes of death in the controls were known to be associated with varying degrees of pulmonary edema. The surface areas of pleural, interstitial, peribronchial, periarterial and perivenous lymphatics were planimetrically measured. The data were statistically tested. The tests showed that the pulmonary lymphatics of the SIDS group were significantly more distended than those of the control group as expressed by highly significant differences in their surface areas. On the whole 2142 lymphatics were measured; 1255 in the SIDS group, which consisted of slides from 10 cases and 887 in the control group which consisted of slides from 11 controls. Although the sample size imposes a limit on the general applicability of the results, the clear distinction between the two groups as shown by the tests do indicate a possible feature of SIDS--lymphatic stasis--which might be secondary to some impairment of lymphatic drainage. This in turn could induce subtle fibrotic changes in the connective tissue framework of these lungs. The results are presented in the light of the forensic significance of the signs of asphyxia frequently found in SIDS and the diagnostic difficulties encountered in differentiating between SIDS and mechanical asphyxia e.g. due to smothering.

Ventilation and ventilators--an update

In the five years which have passed since the previous review, the literature has been concerned more with the ways in which ventilators may be applied to patients and the effects of differing patterns of ventilation than with the design philosophy of the ventilators themselves. This account should be read in conjunction with that of 1982 [1].

Solute permeability of the alveolar capillary barrier

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Physiological aspects of intermittent positive pressure ventilation

The mechanical properties of the lungs and chest wall dictate the relationship between tidal volume, flow rate and airway pressure developed during intermittent positive pressure ventilation (IPPV). The increase in intrathoracic pressures associated with IPPV has consequences for the intrapulmonary distribution of ventilation and perfusion (hence gas exchange), cardiac output and regional blood flows. Barotrauma is a potential hazard. IPPV also affects the homeostatic mechanisms that keep the air spaces dry. Strategies to maximise the benefits and minimise the side effects of IPPV include positive end-expiratory pressure, intermittent mandatory ventilation, differential lung ventilation and high frequency ventilation. Understanding the physiological effects of IPPV and associated therapies allows a rational approach to the adjustment of ventilation against pulmonary, cardiovascular and systemic responses so as to optimise gas exchange and peripheral oxygen delivery.

Alveolar liquid lining: Langmuir method used to measure surface tension in bovine and canine lung extracts

The Langmuir trough has been used to study monolayers of surfactant from beef lung extracts, dog lavage and the principal component (dipalmitoyl lecithin; DPL) in which surface tension has been simultaneously monitored on each film by the Wilhelmy method and the original Langmuir method whose readings are independent of contact angle. Readings on the Wilhelmy balance at 80% film compression reached the near-zero values recorded in previous studies, averaging 2.0 dyn cm-1, but Langmuir readings on the same films averaged 13.5 dyn cm-1 for DPL with no reading below 7.0 dyn cm-1. Similar differences were found for bovine extracts and dog lung lavage and have been attributed to a serious contact-angle artifact in the Wilhelmy method widely used in studying pulmonary surfactant. Higher minimum values of surface tension determined by the Langmuir method are shown to be incompatible with normal physiological function in the smaller mammals on the basis of the traditional bubble model of the alveolus. When Langmuir or Wilhelmy values are determined under more physiological conditions of area change, pH, temperature, humidity etc., values for surface tension are even higher and less compatible with the concept of a continuous liquid lining for the alveolar diameter of any mammal.