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Learning Objectives

  • The student will be able to define the general contributions of airway design, receptors, reflexes, and secretions in lung defense against microbes and other respiratory particulates.

  • The student will be able to explain how impaction, sedimentation, Brownian motion, and mucociliary transport act to clear inhaled particles of various sizes.

  • The student will be able to identify key leukocytes of airways and parenchyma, their roles in recognizing pathogens in proximal and distal airspaces, and their secretions of cytokines, chemokines, and other host defense molecules.

Ventilation of the lung requires the daily inhalation of many thousands of liters of ambient air (Chap. 4), none of which can be presumed to be free of suspended microbes, inorganic particulates, organic fumes, or other noxious gases. Nevertheless, the delicate alveolar parenchyma remains remarkably intact and sterile in most individuals, unless airway defense mechanisms are disturbed. Those mechanisms include physical processes such as sedimentation, as well as biological barriers composed of resident leukocytes that begin in the nares and extend in various forms to, and even through the septal barrier.


As described in Chap. 2, the nasal cavities are lined with vibrissae that are interspersed over the distal nasal turbinates, starting at the keratinized epithelial layers that exist inside the nares even before transition to the ciliated respiratory epithelium (Fig. 10.1). With their large size and scattered spacing, these hairs are sufficiently strong to ensnare larger objects that might be inhaled accidentally, such as small insects and large particles of ash. Tactile stimulation of these hairs is often sufficient to induce sneeze, cough, or glandular secretions that collectively act to consolidate and expel such foreign materials. However, they filter few objects that are smaller than the size that can be resolved with the unassisted human eye (~100 μm).


As presented in Chap. 2, nasal vestibules contain vibrissal hair follicles interspersed among eccrine and sebaceous glands that originate in the dermis and beneath the stratified squamous epithelium. The vestibule is keratinized distally but not proximally.

Less obvious are the delicate sensory nerve endings also present throughout the upper nasal passages. Collectively, these afferent fibers serve as irritant receptors capable of responding to both physical deformations, such as inhalation of a gnat, and physico-chemical stimuli like smoke, mineral dust, cold air, capsaicin, and ammonia vapors. Several classes of receptors present in this region send myelinated or unmyelinated fibers to the CNS via the vagus, trigeminal, and perhaps other cranial nerves. Additional details are presented in Chap. 11 about their sensitivity thresholds and their relative rapidity of adaptations to continued stimulation. Activating these receptors can induce bronchoconstriction, coughs, or sneezes that physically propel foreign debris out of the nasal passages. Receptor activation also stimulates serous or ...

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