Airway anatomy deposition

The pattern of deposition of inhaled particles in a normal lung is dependent on particle size, flow rate, and airway anatomy (branching). There are also patient specific variables that influence aerosol deposition. These include respiratory rate, tidal volume, and other anatomical features. The presence of airflow obstruction characteristic of some lung diseases will affect deposition. 

The size of the fibrous particles that appear to induce disease in the animal models is compatible with the measured respiratory range in humans (Lipp-man, 1977). Most particulate deposition takes place not in the upper or conducting portion of the airways but in the alveolar region of the pulmonary tree (the respiratory unit). Some surface deposition may occur at bifurcations in the bronchial tree, but the actual amount at each location is influenced by anatomy, specific to the species—probably to an individual—as well as the variety of fiber. A large proportion of airborne particulates are rejected as part of the normal clearance mechanisms in animals, but in humans clearance mechanisms may be compromised by smoking, for example. We are unaware of any experiments on fiber toxicity using smoking rats ... 

While chemical composition is important in determining the toxicity of particles and fibers, it is equally or more important to determine where a particle or fiber will deposit in the respiratory tract and how long it will stay there. The quantity and location of particle deposition in the respiratory tract depends on factors related to both the exposed individual and the inhaled particles. The mechanism of deposition is determined by the physical (size, shape, and density) and chemical (hygroscopicity and charge) characteristics of the inhaled particles. Particle deposition is also affected by biological factors inherent to the exposed individual such as breathing pattern (volume and rate), route of breathing (mouth versus nose), and the anatomy of the airways. 

Gaseous pesticides are evenly dispersed in the air. In the case of inhalation, the anatomy and physiology of the respiratory system diminishes the pesticide concentration in inspired air. As pesticides are mostly lipid-soluble, they are usually not removed in the upper airways but tend to deposit in the distal portion of the lung, the alveoli [83] and may then be absorbed into the blood stream. 

FIGURE 11.2 Anatomy of the human respiratory tract. Deposition of nerve agent vapor or aerosols in the different regions of the lung can lead to different symptomology. Upper airway deposition can lead to immediate respiratory distress. Alveolar deposition leads to systemic distribution of the nerve agent. 

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