INTRODUCTION TO LUNG STRUCTURE
The student will be able to describe the architectural layers, components, and relationships to adjacent tissue of all major elements of the normal respiratory system, including nasal cavities, sinuses, oropharynx, larynx, bronchi, and lungs.
The student will be able to recognize the histologic appearance of all major elements of the normal respiratory system.
The student will be able to describe the major stages of lung development and the gestational ages when these milestones normally occur.
The respiratory system is classically divided into two parts, based on their structures and functions. The first is the conducting zone, being the organized array of progressively smaller airways through which air moves into and out of the lungs during tidal ventilation. The conducting zone starts at the nares (and/or mouth) and includes the nasal cavities, pharynx, larynx, trachea, bronchi, and bronchioles. The second part is the respiratory parenchyma, being the densely packed collection of membrane walls, thin enough to support molecular diffusion between the inspired air and blood coursing through the microvasculature of the lung (Fig. 2.1). Constituents of the respiratory parenchyma begin where the conducting zone ends and include the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. The gross and microscopic appearances of each component are the primary focus of this chapter, with the principal functions for each presented with more detail in subsequent chapters.
The main divisions of the respiratory tract. The structures illustrated are not drawn to scale. From Junqueira et al. Basic Histology, 6th ed. Norkwalk, CT: Appleton & Lange; 1989.
The major histologic features of each constituent of the respiratory tract are broadly summarized in Table 2.1. Within the lungs are blood and lymphatic vessels and nerves. The pulmonary arteries and their branches travel beside similarly sized airways, while pulmonary veins and lymphatics travel in the connective tissue septa that form the boundaries of each lung lobule. Bronchial arteries have narrower diameters and thicker walls than the pulmonary arteries, reflecting the smaller quantity of blood they carry (but under higher pressure). They also travel with airways but are inconspicuous due to their small size. Within the connective tissue surrounding larger airways are afferent and efferent nerve fibers. The external surface of the lung is covered by mesothelium, a simple squamous epithelium which normally is in contact with the mesothelial lining of the hemithoraces. The mesothelial layer that covers the entire outer surface of the lungs is called the visceral pleura, while the mesothelium that lines the hemithoraces is called the parietal pleura.
Table 2.1Structural changes in the respiratory tract