Respiratory defense mechanisms

Panicles entrained in the airstream deposit along the airway as a function of size, density, airstream velocity, and breathing frequency. Sizes of rougjily spherical or irregularly shaped particles arc commonly characterized by relating the settling velociiy of the particle to that of an idealized spherical particle. For example, an irregular particle which settles at the same rate as a 5 pm spherical particle has a mean mass aerodynamic diameter (MMAD) of. 5 pm. Since spherical particle mass, is a function of particle diameter, J 

Finer particles ( 3 pm), termed respirable particles, pass beyond the ex-trathoracic airways and enter the tracheobronchial tree. Impaction plays a significant role near the tracheal jet, but sedimentation predominates as the effects of rapid conduit expansion dampen in the distal trachea and beyond. Sedimentation occurs when gravitational forces exerted on a particle equal drag forces, i.e., when particle velocity falls to u . As mean inspiratory air-stream velocity gradually declines along the tracheobronchial tree, particle momentum diminishes and 0.5-3 pm MMAD particles settle out of the airflow and onto mucosal surfaces. 

Mean airflow velocities approach zero as the inspired airstream enters the lung parenchyma, so particle momentum also approaches zero. Most of the particles reaching the parenchyma, however, are extremely fine ( 0.5 pm MMAD), and particle buoyancy counteracts gravitational forces. Temperature gradients do not exist between the airstream and airway wall because the inspired airstream has been warmed to body temperature and fully saturated before reaching the parenchyma. Consequently, diffusion driven by Brownian motion is the only deposition mechanism remaining for airborne particles. Diffusivity, can be described under these conditions by 

FIGURE 5.28 Estimated overall airway deposition as a function of initial particle size and particle hygroscopicity for particles with mass median aerodynamic diameters (MMAD) between 0.1 and 10 p.m. ° Geometric dispersion, a measure of particle size distribution, principally affects only smaller MMAD, 

Consequently, any breathing pattern which increases pulmonary residence times, such as breath-holding, increases fine particle deposition throughout the airway. 

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Proctor, D. E, Andersen, E, and Lundqvist, G. R. (1977b), Nasal mucociliary funerion in humans, Part 1. In Respiratory Defense Mechanisms pp. 427- 52. Marcel Dekkcr, New York. 

Leak, L. V. (1977). Respiratory defense mechanisms. In Brain, J. D., D. F. Proctor, and L. W. Reid, eds. Respiratory Defense Mechanisms. Marcel Dekker, New York. 

ACUTE HEALTH RISKS irritation to respiratory tract can overwhelm normal respiratory defense mechanisms and result in temporary difficulty in breathing asbestos splinters may penetrate skin and cause asbestos "corns". 

Particles (such as those present in mists, and in fumes, smokes, and dusts) present a more complex distribution pattern because the particle size affects its deposition at various levels of the airway. Such factors as sedimentation and impact rates also control particle deposition. Therefore, heavier particles may settle in the nasopharynx or upper airways, whereas lighter or smaller particles may reach more-peripheral airways. Once they have impacted, particles are susceptible to a variety of respiratory defense mechanisms. These mechanisms determine the efficiency with which particle removal progresses, thereby determining the particle s ultimate degree of adverse effects. 

Sleigh MA. The nature and action of respiratory tract ciUa. In Brain JD, Proctor DF, Reid L, eds. Respiratory Defense Mechanisms. New York Marcel Dekker,... 

Respiratory Defense Mechanisms (in two parts), edited by J D Brain, D F Proctor, and L M Reid... 

Can cause respiratory problems, especially in asthmatics. Reduces respiratory defense mechanisms and increases infection rates. Lxmg-term exposure may cause structural damage to the lungs. Produces a smelly, dark haze. Contributes to brown haze... 

The mineral asbestos possesses some unique properties that cause lung injury and necessitate microscopic methods for evaluating its hazard. The characteristic fibrous shape of asbestos particles [see Fig. 1.4(b)] enables them to get past respiratory defense mechanisms and cause asbestosis (scarring of lung tissue), mesothelioma (cancer of the lining of the lung), and lung cancer. Microscopy is used to identify those asbestos fibers whose size and shape enable them to cause these diseases. 

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