ABSTRACT
Although they are anatomically connected, the conducting airways and the gasexchange region of the respiratory tract clearly have a number of different functions to fulfill. Primarily, the conducting airways must provide a conduit for the transport of air, whereas the gas-exchange region (lungs) must provide a surface for gas exchange. In providing these functions they must also provide a barrier between the tissues of the body and an external environment that is often hazardous. In an average human this surface barrier may comprise up to 140 m2 of epithelial surface (1) that may be exposed to as much as 15,000 L of air per day. To highlight the potential for damage to this surface, it has been estimated (2) that, in humans, the airways may be exposed to more than 7 kg of pollutant per year, compared with 1.4 kg in the gastrointestinal tract. During this exposure, it is critically important for survival that airway function is maintained and that the constant barrage of damaging pollutants is dealt with in a rapid and efficient manner. The consequences of inadequate response to environmental insults may vary considerably between the gas-exchange region and the conducting airways. Thus, inflammation of the airways may give rise to bronchoconstriction, as in asthma, whereas fibrosis may be induced in the lung parenchyma.
