The airways of the lungs consist of a series of branching tubes; each level of branching results in another generation of airways. As they branch, the airways become narrower, shorter, and more numerous. There are a total of 23 generations of airways with the alveoli comprising 23rd generation.

Air is carried to and from the lungs by the trachea extending toward the lungs from the larynx. The trachea divides into the right and left main bronchi; these primary bronchi each supply a lung. The primary bronchi branch and form the secondary, or lobar, bronchi, one for each lobe of lung. The left lung consists of two lobes and the right lung has three lobes. The lobar bronchi branch and form the tertiary, or segmental, bronchi, one for each of the functional segments within the lobes. These bronchi continue to branch and move outward toward the periphery of the lungs. The smallest airways without alveoli are the terminal bronchioles. Taken together, the airways from the trachea through and including the terminal bronchioles are referred to as conducting airways. This region, which consists of the first 16 generations of airways, contains no alveoli, so no gas exchange takes place in this area. Consequently, it is also referred to as anatomical dead space. The volume of the anatomical dead space is approximately 150 ml (or about 1 ml per pound of ideal body weight).

The conducting airways carry out two major functions. The first is to lead inspired air to the more distal gas-exchanging regions of the lungs. The second is to warm and humidify the inspired air as it flows through them. The alveoli are delicate structures and may be damaged by excessive exposure to cold, dry air.

Branching from the terminal bronchioles are the respiratory bronchioles. This is the first generation of airways to have alveoli in their walls. Finally, there are the alveolar ducts which are completely lined with alveolar sacs. This region, from the respiratory bronchioles through the alveoli, is referred to as the respiratory zone, which comprises most of the lungs and has a volume of about 3000 ml at the end of a normal expiration.

Epithelium. All of the conducting airways (trachea through terminal bronchioles) are lined with pseudostratified ciliated columnar epithelium. Interspersed among these epithelial cells are mucus-secreting goblet cells. Furthermore, mucus glands are found in the larger airways. Consequently, the surface of the conducting airways consists of a mucus-covered ciliated epithelium. The cilia beat upward at frequencies between 600 and 900 beats per minute. As a result, the cilia continuously move the mucus away from the respiratory zone and up toward the pharynx. This mucociliary escalator provides an important protective mechanism that removes inhaled particles from the lungs. Mucus that reaches the pharynx is usually swallowed or expectorated. Interestingly, the nicotine found in cigarette smoke paralyzes the cilia, impairing their ability to remove any toxic substances. The respiratory bronchioles are lined with cuboidal epithelial cells that gradually flatten and become squamous type cells. As mentioned previously, the alveoli are composed of large, flat simple squamous epithelium.

Cartilage. The trachea and primary bronchi contain C-shaped cartilage rings in their walls; the lobar bronchi contain plates of cartilage that completely encircle the airways. The cartilage in these large airways provides structural support and prevents collapse of the airways. As the bronchi continue to branch and move out toward the lung periphery, the cartilage diminishes progressively until it disappears in airways about 1 mm in diameter. Airways with no cartilage are referred to as bronchioles. As the cartilage becomes more sparse, it is replaced by smooth muscle. Therefore, the bronchioles, which have no cartilage to support them and smooth muscle capable of vigorous constriction, are susceptible to collapse under certain conditions, such as an asthmatic attack.

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