Porth's Essentials of Pathophysiology, 4e

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Control of Respiratory Function

C h a p t e r 2 1

lined with coarse hairs, which filter and trap dust and other large particles from the air. The upper portion of the nasal cavity, which is lined with a mucous membrane that contains a rich network of small blood vessels, sup- plies both warmth and moisture to the air we breathe. The mouth serves as an alternative airway when the nasal passages are plugged or when there is a need for the exchange of large amounts of air, as occurs during exercise. The oropharynx, which extends posteriorly from the soft palate to the epiglottis, is the only opening between the nose, mouth, and lungs. Both swallowed food on its way to the esophagus and air on its way to the larynx pass through it. Obstruction of the orophar- ynx leads to immediate cessation of ventilation. Neural control of the tongue and pharyngeal muscles may be impaired in coma and certain types of neuro- logic disease. In these conditions, the tongue falls back into the pharynx and obstructs the airway, particularly if the person is lying on his or her back. Swelling of the pharyngeal structures caused by injury, infection, or severe allergic reaction also predisposes a person to air- way obstruction, as does the presence of a foreign body. Laryngotracheal Airways The larynx, or voice box, connects the oropharynx with the trachea. The walls of the larynx are supported by rigid cartilaginous structures that prevent collapse during inspiration. The functions of the larynx can be divided into two categories: those associated with speech and those associated with protecting the lungs from substances other than air. The larynx is located in a strategic position between the upper airways and the lungs and sometimes is referred to as the “watchdog of the lungs.” The cavity of the larynx is divided into two pairs of two-by-two folds of mucous membrane stretching from front to back with an opening in the middle (Fig. 21-2).

The upper pair of folds, called the vestibular folds or false vocal cords , have a protective function. The lower pair of folds, which have cordlike margins, are termed the vocal folds or vocal cords because their vibrations are required for making vocal sounds. The true vocal cords and the elongated opening between them make up the glottis . A complex set of muscles and ligaments control the opening and closing of the glottis. Speech involves the intermittent release of expired air and opening and closing of the glottis. The epiglottis, which is located above the vocal folds, is a large, leaf-shaped piece of cartilage that is covered with epithelium. During swal- lowing, the free edges of the epiglottis move downward to cover the larynx, thus routing liquids and foods into the esophagus. In addition to opening and closing the glottis for speech, the vocal folds of the larynx can perform a sphincter function in closing off the airways. When confronted with substances other than air, the laryn- geal muscles contract and close off the airway. At the same time, the cough reflex helps in removing the for- eign substance from the airway. If the muscles that con- trol the swallowing mechanism are partially or totally paralyzed, food and fluids can enter the airways instead of the esophagus when a person attempts to swallow. These substances are not easily removed, and when they are pulled into the lungs, they can cause a serious inflammatory condition called aspiration pneumonia . Tracheobronchial Airways The tracheobronchial airways, which consist of the trachea, bronchi, and bronchioles, can be viewed as a system of branching tubes (Fig. 21-3A). They are simi- lar to a tree whose branches become smaller and more numerous as they divide. There are approximately 23 levels of branching, beginning with the conducting air- ways and ending with the respiratory airways, where gas exchange takes place (Fig. 21-3B). The trachea, or windpipe, can be viewed as a con- tinuous tube that connects the larynx and the major bronchi of the lungs. The wall of the trachea consists of four distinct layers: a mucosa layer of ciliated pseu- dostratified epithelium, a submucosal layer of dense connective tissue, a cartilaginous layer, and an outer layer of connective tissue that binds the trachea to the adjacent structures. A unique feature of the trachea is the presence of a series of horseshoe- or C-shaped rings of hyaline cartilage that prevent it from collapsing when the pressure in the thorax becomes negative (Fig. 21-4). The open part of the C-shaped ring, which abuts the esophagus, is connected by smooth muscle. Since this portion of the trachea is not rigid, the esophagus can expand anteriorly as swallowed food passes through it. The trachea divides into two branches, forming the right and left main or primary bronchi, as it moves into the thorax (Fig. 21-5A). Between the main bronchi is a keel-like ridge called the carina (Fig. 21-5B). The mucosa of the carina is highly sensitive, producing vio- lent coughing, when a foreign object (e.g., suction cath- eter) makes contact with it. Initially, the bronchi have

A

Epiglottis

Glottis

False vocal cord

True vocal cord

B

Inner lining of trachea

FIGURE 21-2. Epiglottis and vocal cords viewed from above with (A) glottis closed and (B) glottis open.