Unit Six. Animal Life
The oxygen-gathering mechanism of mammals, although less efficient than that of birds, adapts them well to their terrestrial habitat. Mammals, like all terrestrial vertebrates, obtain the oxygen they need for metabolism from air, which is about 21% oxygen gas. A pair of lungs is located in the chest, or thoracic, cavity. As you can see in figure 24.5, the two lungs hang free within the cavity, connected to the rest of the body only at one position, where the lung’s blood vessels and air tube enter. This air tube is called a bronchus (plural, bronchi). It connects each lung to a long tube called the trachea, which passes upward and opens into the rear of the mouth. The trachea and both the right and left bronchi are supported by C-shaped rings of cartilage.
Figure 24.5. The human respiratory system.
The respiratory system consists of the lungs and the passages that lead to them.
Air normally enters through the nostrils into the nasal cavity where it is moistened and warmed. In addition, the nostrils are lined with hairs that filter out dust and other particles. As the air passes through the nasal cavity, an extensive array of cilia further filters it. The air then passes through the back of the mouth, through the pharynx (the common passage of food and air), and then through the larynx (voice box) and the trachea. Because the air crosses the path of food at the back of the throat, a special flap called the epiglottis covers the trachea whenever food is swallowed, to keep it from “going down the wrong pipe.” From there, air passes down through several branchings of bronchi in the lungs and eventually to bronchioles that lead to alveoli. Mucous secretory ciliated cells in the trachea and bronchi also trap foreign particles and carry them upward to the pharynx, where they can be swallowed. The lungs contain millions of alveoli, tiny sacs clustered like grapes. The alveoli are surrounded by an extremely extensive capillary network. All gas exchange between the air and blood takes place across the walls of the alveoli.
The mammal respiratory apparatus is simple in structure and functions as a one-cycle pump. The thoracic cavity is bounded on its sides by the ribs and on the bottom by a thick layer of muscle, the diaphragm, which separates the thoracic cavity from the abdominal cavity. Each lung is covered by a very thin, smooth membrane called the pleural membrane. This membrane also folds back on itself to line the interior of the thoracic cavity, into which the lungs hang. The space between these two layers of membrane is very small and filled with fluid. This fluid causes the two membranes to adhere to each other in the same way a thin film of water can hold two plates of glass together, effectively coupling the lungs to the walls of the thoracic cavity.
Similar to the way a layer of fluid causes the pleural membrane of the lungs to adhere to the inside of the thoracic cavity, fluid inside the alveoli can cause these air sacs to collapse. This doesn’t occur because epithelial cells that line the alveoli secrete a mixture of lipoprotein molecules called surfactant. Surfactant molecules form a thin layer on the inner lining of the alveoli and reduce the surface tension that would otherwise cause the alveoli to collapse. Premature babies sometimes suffer from respiratory distress syndrome because surfactant isn’t produced in adequate amounts until the seventh month of gestation.
Air is drawn into the lungs by the creation of negative pressure—that is, pressure in the lungs is less than atmospheric pressure. The pressure in the lungs is reduced when the volume of the lungs is increased. This is similar to how a bellow pump or accordion works. In both cases, when the bellow is extended the volume inside increases, causing air to rush in. How does this occur in the lungs? The volume in the lungs increases when muscles that surround the thoracic cavity contract, causing the thoracic cavity to increase in size. Because the lungs adhere to the thoracic cavity, they also expand in size. This creates negative pressure in the lungs, and air rushes in.
The Mechanics of Breathing
The active pumping of air in and out of the lungs is called breathing. During inhalation, muscular contraction causes the walls of the chest cavity to expand so that the rib cage moves outward and upward. The diaphragm, the red-colored lower border of the lung in the Key Biological Process illustration above, is dome-shaped when relaxed, as in panel 1, but moves downward and flattens during this contraction, as in panel 2. In effect, we have enlarged the bellow in the accordion.
During exhalation (in panel 3) the ribs and diaphragm return to their original resting position. In doing so, they exert pressure on the lungs. This pressure is transmitted uniformly over the entire surface of the lung, forcing air from the inner cavity back out to the atmosphere. In a human, a typical breath at rest moves about 0.5 liters of air, called the tidal volume. The extra amount that can be forced into and out of the lung is called the vital capacity and is about 4.5 liters in men and 3.1 liters in women. The air remaining in the lung after such a maximal expiration is the residual volume, or dead volume, typically about 1.2 liters.
Because the diffusion surfaces of the lungs are not exposed to fully oxygenated air, but rather to a mixture of fresh and partly oxygenated air, the respiratory efficiency of mammalian lungs is far from maximal. As explained earlier, a bird, whose lungs do not retain a residual volume, is able to achieve far greater respiratory efficiency.
Key Learning Outcome 24.4. In mammals, the lungs are located within a thoracic cavity that is surrounded by muscles. By contracting and relaxing, these muscles expand or reduce the volume of the cavity, drawing air into the lungs or forcing it out.