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Muco-ciliary transport is only effective because of the coordination of the ciliary beats (metachronous) and the harmony between mucus and cilia. The tip of the cilia is in contact with a jellyform layer of mucus propelled to the oropharynx. This jellyform layer has a complex rheological behaviour: it flows like a liquid and shapes like solid elastic. When the rheological properties of bronchial secretion are abnormal, mucociliary transport becomes inefficient. However, the most fluid secretions are not necessarily best transported, because the elasticity and viscosity to guarantee efficient muco-ciliary transport can only vary within defined limits. The mechanism regulating the ciliary beats is poorly understood; the bronchial secretions conduct impulses through the autonomic nervous system as well as mediators such as histamine and the metabolites of arachidonic acid. Mucociliary function may be studied either, directly through mucociliary transport or through mucociliary clearance. A fall in mucociliary activity can be produced by a primary ciliary disorder, by bronchial disease or the consequences of respiratory infection. General anaesthetics and Atropine slow mucociliary transport but Ipratropium bromide does not; Theophylline and sympathomimetics speed it up. The expectorants are mucolytics (proteolytic enzymes, N-acetyl-cysteine), there are agents to correct hydration anomalies of the bronchial secretion (water, hypertonic sodium chloride) iodides, antifibrins by substitution, anti-inflammatory agents and mucoregulatory agents (S-carboxymethylcysteine, bromhexine). The efficacy of the greater part of these expectorants has not been established in vivo by controlled therapeutic trials.

The air we breathe contains certain gases and particles, some of which may be a threat to man. The respiratory tract is their first target because it comes into direct contact with them, and to protect the body itself it mobilizes defense mechanisms of high complexity. To understand these it is necessary to analyze the toxic modalities of these air contaminants: composition, deposition and transit through the respiratory tract. As far as the particles are concerned, the aerodynamic diameter is an important factor in determining the manner of deposition. Also of importance are shape, density, degree of humidity and radioactivity. Some particles are infectious, some are antigenic, and some are toxic due to their physical structure or chemical composition. Emphasis is placed on the role of aerosols, particularly tobacco smoke, as pollution factors or co-factors with other pollutants. The characteristics of the exposed subject capable of defining a risk profile varying from person to person are dwelt on. This approach presupposes a separation, within the cleaning and defense mechanisms, of the mechanical means which bring physical phenomena into play on the basis of fixed structures in the respiratory tract (cilium, mucous cells, surfactant). This is followed by study of the immunologic means of defense essentially involving cellular elements residing temporarily in the lung (T or B lymphocytes, plurinuclears, alveolar histiocytes in particular). Many enzyme systems are also involved in this pulmonary defense and in keeping homeostasis working. Particular attention is focused on some of them, such as the antiproteases and cytochrome P450.(ABSTRACT TRUNCATED AT 250 WORDS)