The Respiratory System
The primary function of the respiratory system is to allow oxygen from the air to enter the blood and carbon dioxide from the blood to exit into the air. During inspiration, or inhalation (breathing in), and expiration, or exhalation (breathing out), air is conducted toward or away from the lungs by a series of cavities, tubes, and openings, illustrated in Figure 14.1.
The respiratory system also works with the cardiovascular system to accomplish these four respiratory events:
1. breathing, the entrance and exit of air into and out of lungs;
2. external respiration, the exchange of gases (oxygen and carbon dioxide) between air and blood;
3. internal respiration, the exchange of gases between blood and tissue fluid;
4. transport of gases to and from the lungs and the tissues.
Cellular respiration, which produces ATP, uses the oxygen and produces the carbon dioxide that makes gas exchange with the environment necessary. Without a continuous supply of ATP, the cells cease to function. The four events listed here allow cellular respiration to continue.
Figure 14.1 The respiratory tract extends from the nasal cavities to the lungs, which are composed of air sacs called alveoli. Gas exchange occurs between the air in the alveoli and the blood within a capillary network that surrounds the alveoli. Notice in the blow-up that the pulmonary arteriole is colored blue it carries O2 poor blood away from the heart to the alveoli. Then carbon dioxide leaves the blood, and oxygen enters the blood. The pulmonary venule is colored red-it carries O2 rich blood from the alveoli toward the heart.
The Respiratory Tract
Table 14.1 traces the path of air from the nose to the lungs. As air moves in along the airways, it is cleansed, warmed, and moistened. Cleansing is accomplished by coarse hairs just inside the nostrils and by cilia and mucus in the nasal cavities and the other airways of the respiratory tract. In the nose, the hairs and the cilia act as screening devices. In the trachea and other airways, the cilia beat upward, carrying mucus, dust, and occasional bits of food that “went down the wrong way” into the pharynx, where the accumulation can be swallowed or expectorated. The air is warmed by heat given off by the blood vessels lying close to the surface of the lining of the airways, and it is moistened by the wet surface of these passages. Conversely, as air moves out during expiration, it cools and loses its moisture. As the air cools, it deposits its moisture on the lining of the trachea and the nose, and the nose may even drip as a result of this condensation. The air still retains so much moisture, however, that upon expiration on a cold day, it condenses and forms a small cloud.
The nose, a prominent feature of the face, is the only external portion of the respiratory system. Air enters the nose through external openings called nostrils. The nose contains two nasal cavities, which are narrow canals separated from one another by a septum composed of bone and cartilage (Fig. 14.2). Mucous membrane lines the nasal cavities. The nasal conchae are bony ridges that project laterally into the nasal cavity. They increase the surface area for moistening and warming air during inhalation and for trapping water droplets during exhalation. Odor receptors are on the cilia of cells located high in the recesses of the nasal cavities. The tear (lacrimal) glands drain into the nasal cavities by way of tear ducts. For this reason, crying produces a runny nose. The nasal cavities also communicate with the paranasal sinuses, air-filled spaces that reduce the weight of the skull and act as resonating chambers for the voice. If the ducts leading from the sinuses become inflamed, fluid may accumulate, causing a sinus headache. The nasal cavities are separated from the oral cavity by a partition called the palate, which has two portions. Anteriorly, the hard palate is supported by bone, and posteriorly the soft palate is not so supported.
The pharynx is a funnel-shaped passageway that connects the nasal and oral cavities to the larynx. Consequently, the pharynx, commonly referred to as the “throat,” has three parts: the nasopharynx, where the nasal cavities open posterior to the soft palate; the oropharynx, where the oral cavity joins the pharynx; and the laryngopharynx, which opens into the larynx. The soft palate has a soft extension called the uvula that can be seen projecting into the oropharynx. The tonsils form a protective ring at the junction of the oral cavity and the pharynx. Being lymphatic tissue, the tonsils contain lymphocytes that protect against invasion of inhaled pathogens. Here, both B cells and T cells are prepared to respond to antigens that may subsequently invade internal tissues and fluids. In this way, the respiratory tract assists the immune system in maintaining homeostasis. In the pharynx, the air passage and the food passage cross because the larynx, which receives air, is anterior to the esophagus, which receives food. The larynx lies at the top of the trachea. The larynx and trachea are normally open, allowing air to pass, but the esophagus is normally closed and opens only when a person swallows.
Figure 14.2 The path of air. This drawing shows the path of air from the nasal cavities to the trachea, which is a part of the lower respiratory tract. The other organs are in the upper respiratory tract.