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In any case, within the respiratory acinus, the distance from the convective tidal front to alveolar surfaces is short enough so that efficient CO exchange takes place by molecular diffusion.By contrast, airborne particles, with diffusion coefficients smaller by orders of magnitude than those for gases, tend to remain suspended in the tidal air, and can be exhaled without deposition.It reflects the minimal lung deposition between 0.1 and 1 µm, where deposition is determined largely by the exchange, in the deep lung, between tidal and residual lung air.Deposition increases below 0.1 µm as diffusion becomes more efficient with decreasing particle size.Soluble materials deposited as, on, or within particles can diffuse into and through surface fluids and cells and be rapidly transported by the bloodstream throughout the body.Aqueous solubility of bulk materials is a poor guide to particle solubility in the respiratory tract.The complex structure and numerous functions of the human respiratory tract have been summarized concisely by a Task Group of the International Commission on Radiological Protection (ICRP 1994), as shown in figure 10.1 .The conductive airways, also known as the respiratory dead space, occupy about 0.2 litres.
The essentially particle-free residual lung air that accounts for about 15% of the expiratory tidal flow tends to act like a clean-air sheath around the axial core of distally moving tidal air, such that particle deposition in the respiratory acinus is concentrated on interior surfaces such as airway bifurcations, while interbranch airway walls have little deposition.
In addition, the system must also: (1) remove an equal volume of carbon dioxide entering the lungs from the alveolar capillaries; (2) maintain body temperature and water vapour saturation within the lung airways (in order to maintain the viability and functional capacities of the surface fluids and cells); (3) maintain sterility (to prevent infections and their adverse consequences); and (4) eliminate excess surface fluids and debris, such as inhaled particles and senescent phagocytic and epithelial cells.
It must accomplish all of these demanding tasks continuously over a lifetime, and do so with high efficiency in terms of performance and energy utilization.
They condition the inhaled air and distribute it, by convective (bulk) flow, to the approximately 65,000 respiratory acini leading off the terminal bronchioles.
As tidal volumes increase, convective flow dominates gas exchange deeper into the respiratory bronchioles.