Tracheobronchial regions

The respiratory system has several mechanisms for removing deposited particles (8). The walls of the nasal and tracheobronchial regions are coated with a mucous fluid. Nose blowing, sneezing, coughing, and swallowing... 

Fig. 7-2. Particle deposition as a function of particle diameter in various regions of the lung. The nasopharyngeal region consists of the nose and throat the tracheobronchial region consists of the windpipe and large airways and the pulmonary region consists of the small bronchi and the alveolar sacs. Source Task Group on Lung Dynamics, Health Phys. 12, 173 (1966).

Tracheobronchial region The middle region of the respiratory system, comprised of the trachea and the bronchi. 

Clearance of particles from the tracheobronchial region to the gastrointestinal tract is predicted to occur with a half-time of 5 h. Absorption of material from the tracheobronchial region into blood is predicted to occur with a half-time of 15 min and amount to 0.95, 0.5, and 0.01 of the deposited Class D, W, and Y substances, respectively. There are no experimental studies that provide measurements of these clearance pathways that can be compared to these model predictions for inhaled radiocerium. 

Patra, A.L., "Comparative Anatomy of Mammalian Respiratory Tracts The Nasopharyngeal Region and the Tracheobronchial Region," J. Tox. Env. Health... 

Because the mucus layer or the underlying cells may serve as either final accumulation sites of toxic gases or layers through which the gases diffuse en route to the blood, we need simplified models of these layers. Altshuler et al. have developed for these layers the only available model that can be used in a comprehensive system for calculating tissue doses of inhaled irritants. It assumes that the basement membrane of the tracheobronchial region is covered with three discrete layers an inner layer of variable thickness that contains the basal, goblet, and ciliated cells a 7-Mm middle layer composed of waterlike or serous fluid and a 7-Mm outer layer of viscous mucus. Recent work by E. S. Boatman and D. Luchtel (personal communication) in rabbits supports the concept of a continuous fluid layer however, airways smaller than 1 mm in diameter do not show separate mucus and serous-fluid layers. 

Particles with an aerodynamic diameter of 1-5 p are deposited in the airways (tracheobronchial regions) hy sedimentation under gravitational forces. As the alveolar regions are approached, the velocity of the airflow decreases significantly, allowing more time for sedimentation. The very small particles, generally less than 1 p, that penetrate to the alveoli are deposited there mainly hy diffusion. 

The rationale for basing air quality standards on smaller particles is evident from an examination of Fig. 2.12, a diagram of the human respiratory tract. Larger particles that are inhaled are removed in the head or upper respiratory tract. The respiratory system from the nose through the tracheobronchial region is covered with a layer of mucus that is continuously moved upward by the motion of small hairlike projections called cilia. Large particles deposit on the mucus, are moved up, and are ultimately swallowed. 

Figure 2.13 shows the deposition of particles in various regions of the respiratory tract as a function of particle diameter (Phalen, 1984 Phalen et al., 1991 Yeh et al., 1996). The deposition fraction of PM1() in the pulmonary and tracheobronchial regions can be quite large, so it is not surprising that health effects could be associated with these particles. Deposition in the upper portions of the respiratory system is dominated primarily by the large particles, which are readily taken out in the nose and upper airways. 

The tracheobronchial region propels foreign particles from the deep parts of the lungs to the oral cavity, where they can be either expelled with spntnm or swallowed. 

From Eq. (8.2) it is obvious that the Stokes number Stk and thus the deposition efficiency by impaction increase with increasing particle size and airflow velocity. Impaction occurs most frequently in the upper respiratory tract (pharynx, larynx, and main trachea), where particles larger than 5 p,m are trapped because of their size and the fast and turbulent airflow exerted. Also in the upper tracheobronchial region, impaction is the most prominent mechanism (Hinds 1998). 

In the tracheobronchial region, a high proportion of the epithelial cells are ciliated such that there is a near complete covering of the central airways by cilia (Figure 10.2). Towards the periphery of the... 

This is the dominant deposition mechanism for particles >1 pm in the upper tracheobronchial regions. A particle with a large momentum may be unable to follow the changing direction of the inspired air as it passes the bifurcations and as a result will collide with the airway walls as it continues on its original course. 

The deposition of inhalable uranium dust particles in the various regions of the lungs (extrathoracic, tracheobronchial, and deep pulmonary or alveolar) depends on the size of the particles. Particles larger than 10 pm are likely to be transported out of the tracheobronchial region by mucocilliary action and... 

Particles with diameters between 1 and 5 pm are deposited in the tracheobronchial region as a result of either inertial impaction at airway bifurcations or gravitational sedimentation onto other airway surfaces. Undissolved particles may then be removed by the action of the mucociliary defense system working as an escalator particles trapped in the mucus are propelled toward the pharynx by the action of thin cilia located on the surface membrane of specialized cells. Once in the pharynx, the particles may be swallowed. The efficiency of the escalator defense system may be greatly impaired by various environmental contaminants, like sulfur dioxide, ozone, and cigarette smoke that are known to paralyze the activity of the ciliated cells and consequently the upward movement of the mucus. 

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