FIGURE 27–8 Lungs Terrestrial vertebrates breathe with lungs. Lungs with a larger surface area can take in more oxygen and release more carbon dioxide. Mammals have the greatest lung surface area among animals. Infer Why do mammals require a large surface area with which to process oxygen?

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▸ Amphibian, Reptilian, and Mammalian Lungs The internal surface area of lungs increases from amphibians to reptiles to mammals, as shown in Figure 27–8. A typical amphibian lung is little more than a sac with ridges. Reptilian lungs are often divided into chambers that increase the surface area for gas exchange. Mammalian lungs branch extensively, and their entire volume is filled with bubblelike structures called alveoli (al VEE uh ly; singular: alveolus). Alveoli provide an enormous surface area for gas exchange. The structure of mammalian lungs enables mammals to take in the large amounts of oxygen required by their high metabolic rates. However, in the lungs of mammals and most other vertebrates, air moves in and out through the same tracheal passageway. For this reason, some stale, oxygen-poor air is trapped in the lungs. In humans, this stale air is typically equivalent to about one third of the air inhaled in a normal breath.

▸ Bird Lungs In birds, the lungs are structured so that air flows mostly in only one direction. No stale air gets trapped in the system. A unique system of tubes and air sacs in birds' respiratory systems enables this one-way airflow. Thus, gas exchange surfaces are continuously in contact with fresh air. This highly efficient gas exchange helps birds obtain the oxygen they need to power their flight muscles at high altitudes for long periods of time.

27.2 Assessment

Review Key Concepts

1. 1. Review In what ways are the respiratory structures of all animals similar?

   2. Apply Concepts Explain why it is important that respiratory surfaces are moist and selectively permeable.

2. 1. Review Which groups of aquatic animals breathe with gills? With lungs?

   2. Relate Cause and Effect Why do some animals actively pump water over their gills?

3. 1. Review How do terrestrial invertebrates and terrestrial vertebrates breathe?

   2. Interpret Visuals Contrast the structures of amphibian, reptilian, and mammalian lungs, as shown in Figure 27–8.

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4. Describe the events that occur when a mammal respires, including the path of air through its lungs.

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