Gas exchange in the lungs occurs across an estimated 300 million tiny (0.25 to 0.50 mm in diameter) air sacs, known as alveoli. Their enormous number provides a large surface area (60 to 80 square meters, or about 760 square feet) for diffusion of gases. The diffusion rate is further increased by the fact that each alveolus is only one cell-layer thick, so that the total "air-blood barrier" is only two cells across (an alveolar cell and a capillary endothelial cell), or about 2 |im. This is an average distance because there are two types of cells in the alveolar wall, type I and type II, and the type II alveolar cells are thicker than the type I cells (fig. 16.1). Where the basement membranes of capillary en-dothelial cells fuse with those of type I alveolar cells, the diffusion distance can be as small as 0.3 |im (fig. 16.2), which is about 1/100th the width of a human hair.
Alveoli are polyhedral in shape and are usually clustered, like the units of a honeycomb. Air within one member of a cluster can enter other members through tiny pores (fig. 16.3). These clusters of alveoli usually occur at the ends of respiratory bronchioles, the very thin air tubes that end blindly in alveolar sacs. Individual alveoli also occur as separate outpouchings along the length of respiratory bronchioles. Although the distance between each respiratory bronchiole and its terminal alveoli is only about 0.5 mm, these units together constitute most of the mass of the lungs.
The air passages of the respiratory system are divided into two functional zones. The respiratory zone is the region where gas exchange occurs, and it therefore includes the respiratory bronchioles (because they contain separate outpouchings of alveoli)
■ Figure 16.2 An electron micrograph of a capillary within the alveolar wall. Notice the short distance separating the alveolar space on one side (left, in this figure) from the capillary. (EP = epithelial cell of alveolus; RBC = red blood cell; BM = basement membrane; IS = interstitial connective tissue.)
and the terminal alveolar sacs. The conducting zone includes all of the anatomical structures through which air passes before reaching the respiratory zone (fig. 16.4; see also fig. 16.21).
Air enters the respiratory bronchioles from terminal bronchioles, which are narrow airways formed from many successive divisions of the right and left primary bronchi. These two large air passages, in turn, are continuous with the trachea, or windpipe, which is located in the neck in front of the esophagus (a muscular tube that carries food to the stomach). The trachea is a sturdy tube supported by rings of cartilage (fig. 16.5).
Air enters the trachea from the pharynx, which is the cavity behind the palate that receives the contents of both the oral and nasal passages. In order for air to enter or leave the trachea and lungs, however, it must pass through a valvelike opening called the glottis between the vocal folds. The ventricular and vocal folds are part of the larynx, or voice box, which guards the entrance to the trachea (fig. 16.6). The projection at the front of the throat, commonly called the "Adam's apple," is formed by the largest cartilage of the larynx.
■ Figure 16.3 A scanning electron micrograph of lung tissue. (a) A
small bronchiole passes between many alveoli. (b) The alveoli are seen under higher power, with an arrow indicating an alveolar pore through which air can pass from one alveolus to another.
If the trachea becomes occluded through inflammation, excessive secretion, trauma, or aspiration of a foreign object, it may be necessary to create an emergency opening into this tube so that ventilation can still occur. A tracheotomy is the procedure of surgically opening the trachea, and a tracheostomy involves the insertion of a tube into the trachea to permit breathing and to keep the passageway open. A tracheotomy should be performed only by a competent physician because of the great risk of cutting a recurrent laryngeal nerve or the common carotid artery.
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■ Figure 16.4 The conducting and respiratory zones of the respiratory system. The conducting zone consists of airways that conduct the air to the respiratory zone, which is the region where gas exchange occurs. The numbers of each member of the airways and the total number of alveolar sacs are shown in parentheses.
Right primary bronchus
■ Figure 16.5 The conducting zone of the respiratory system. (a) An anterior view extending from the larynx to the terminal bronchi and (b) the airway from the trachea to the terminal bronchioles, as represented by a plastic cast.
The conducting zone of the respiratory system, in summary, consists of the mouth, nose, pharynx, larynx, trachea, primary bronchi, and all successive branchings of the bronchioles up to and including the terminal bronchioles. In addition to conducting air into the respiratory zone, these structures serve additional functions: warming and humidification of the inspired air and filtration and cleaning.
Regardless of the temperature and humidity of the ambient air, when the inspired air reaches the respiratory zone it is at a temperature of 37° C (body temperature), and it is saturated with water vapor as it flows over the warm, wet mucous membranes that line the respiratory airways. This ensures that a constant internal body temperature will be maintained and that delicate lung tissue will be protected from desiccation.
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Ventricular fold (false vocal cord)
Vocal fold (true vocal cord)
■ Figure 16.6 A photograph of the larynx showing the true and false vocal cords and the glottis. The vocal folds (true vocal cords) function in sound production, whereas the ventricular folds (false vocal cords) do not.
Mucus secreted by cells of the conducting zone structures serves to trap small particles in the inspired air and thereby performs a filtration function. This mucus is moved along at a rate of 1 to 2 centimeters per minute by cilia projecting from the tops of epithelial cells that line the conducting zone (fig. 16.7). There are about 300 cilia per cell that beat in a coordinated fashion to move mucus toward the pharynx, where it can either be swallowed or expectorated.
As a result of this filtration function, particles larger than about 6 |im do not normally enter the respiratory zone of the lungs. The importance of this function is evidenced by black lung, a disease that occurs in miners who inhale large amounts of carbon dust from coal, which causes them to develop pulmonary fibrosis. The alveoli themselves are normally kept clean by the action of resident macrophages (see fig. 16.1). The cleansing action of cilia and macrophages in the lungs is diminished by cigarette smoke.
■ Figure 16.7 A scanning electron micrograph of cilia in a bronchial wall. The cilia that project from the tops of the epithelial cells help to cleanse the lung by moving trapped particles.
Test Yourself Before You Continue
1. Describe the structures involved in gas exchange in the lungs and explain how gas exchange occurs.
2. Describe the structures and functions of the conducting zone of the respiratory system.
3. Describe how each lung is compartmentalized by the pleural membranes. What is the relationship between the visceral and parietal pleurae?
Ventricular fold (false vocal cord)
Vocal fold (true vocal cord)
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