The mucociliary transport system of the lungs and the role of surfactant

Many pollution particles enter the organism via the lung. In the lung, on a surface of 140 m(2), the blood is separated from the air by a tissue barri er of only 1/1000 mm. The conducting airways (trachea, bronchi, bronchioli) are a very effective aerodynamic filter for inhaled particles. The mucocil iary transport system functions like a self-cleaning mechanism within the f ilter. Inhaled particles and particles deposited in the lungs play a crucia l aetiological and therapeutic role. The discussion in health policy on the relationship between the increase in air pollution and lung damage is of g reat importance at the present time. Epidemiological studies of recent year s have shown very clearly that there is a correlation between morbidity and mortality as a consequence of respiratory and cardiogenic problems and the concentration of PM10 particles in ambient air. So far, however, this corr elation has not been explained. The intrathoracic airways are coated by a respiratory epithelium. This has an irregular coating of viscous liquid, consisting of a low viscous sol pha se and a high viscous gel phase. It seems, however, that those phases are n ot clearly distinguishable. The gel phase is moved towards the pharynx by t he metachronal ciliary beat transporting the particles out of the lungs. Fu rthermore, at the air-liquid interface, there exists a continuous surfactan t film which reduces the surface tension as is the case in the alveoli. When particles are deposited on the airway wall, that is, on the surfactant film, they are wetted by surface forces and displaced into the liquid phas es. Thus, the surfaces of the particles are probably changed by the surfact ant or by surfactant components. Many of these particles are transported in the liquid (gel phase) towards the pharynx (mucociliary transport), wherea s some of them remain in close association with the epithelium (sol phase). Such particles remain in the airways for days or even weeks. They are eith er phago-cytised by macrophages and carried off via the airways or taken up by dendritic cells and transported into the tissue from where they reach t he lymph nodes via lymph drainage and are presented to the T-lymphocytes. The displacement of particles into the liquid phases, caused by the surfact ant, can be considered as the initial step in a complex cascade of defence processes in the lungs. The surface of the particles is probably modified b y surfactant or surfactant components. These modified particles may be dire cted to that clearance pathway which is most beneficial for our health, tha t is, out of the lungs or into the lymphatic glands, where an immune reacti on can be triggered. We therefore consider surfactant to be a primary immun e barrier.