Life on land is very different from life underwater, but the fundamental energetic needs remain. ATP is still the chemical that provides the energy for all the reactions necessary for life, and cells still need oxygen to produce ATP. Consequently, an organism must put air in contact with the cells that need it. Ultimately, oxygen must get into the cells and carbon dioxide must get out. These are universal challenges facing all terrestrial animals.

Terrestrial vertebrates have a general solution to these challenges. First, they suck in air through their mouth or nose. The air moves down a trachea into lungs. In the lungs, O₂ diffuses from air to blood, while CO₂ diffuses from blood to air. Finally, the oxygen-depleted air is exhaled and the process begins again. The specific design of the lungs and respiratory system differs a bit from one taxon to another—we review the most notable distinctions below—but the general process is the same.

Mammalian respiration begins with a deep breath. Let’s trace the air through the respiratory process in humans (FIGURE 21-33). Air enters through the nose, filling the nasal cavity, where it becomes warm and moist. Additional air can be taken in through the mouth. In either case, these two entry points for air join together at the throat (also called the pharynx), at the back of the mouth. The air passes through the throat and moves through the voice box, or vocal cords—also called the larynx. The voice box’s location can be seen as the bump on the front of your neck (more pronounced in men, but also present in women), called the “Adam’s apple.” From the larynx, the air moves into the trachea (also called the windpipe), a long tube that takes the air into the chest cavity. Once there, the trachea branches into two smaller tubes called bronchi. One bronchus goes to the left lung and the other goes to the right lung.

Figure 21-33A terrestrial mammal. Overview of the human respiratory system.

Figure 21-34Alveoli in the lungs: where air meets blood.

When the bronchi enter the lungs, which are like stretchy, elastic bags, they branch again. And again. And again. With each successive branching, the bronchi get smaller. Under a certain size they are called bronchioles. Eventually, the bronchioles reach a dead end. These dead ends are tiny elastic sacs, the alveoli, and it is here that the air meets the blood vessels (FIGURE 21-34).

There are about 300 million alveoli in each human lung, with a total surface area roughly the size of a movie screen. Alveoli are made up of the most delicate cells in our bodies and have ultra-thin walls. Completely surrounding the alveoli, the way your fingers might completely surround a small ball you are grasping, are tiny capillaries; like the alveoli, the capillaries have extremely thin walls. Oxygen in the air in the alveoli dissolves in moisture on the cells lining them. It can then pass right through the two sets of thin membranes—alveolar and capillary—and get picked up by the bloodstream. Simultaneously, carbon dioxide can diffuse from the blood into the alveoli. In the short time you hold it in your lungs, the breath you inhaled is changed. When exhaled, it is depleted of O₂ and laden with CO₂.

Amphibians and reptiles are almost identical to mammals when it comes to the respiratory system. The lungs of amphibians are a bit smaller in relation to body size, but amphibians make up for some of this reduced lung capacity by conducting a bit of gas exchange through their skin. This is why they must keep their skin moist at all times. Reptiles are generally too thick-skinned and scaly to achieve any respiration through their skin, but they have slightly larger lungs than amphibians to pick up the slack. Among the terrestrial vertebrates, birds are the champions of respiratory efficiency. We explore some of their unique adaptations in Section 21-16.

Smoking introduces thousands of different chemicals into the respiratory system, many of which—such as formaldehyde, ammonia, and benzene—have powerfully destructive effects on its cells. These dangerous chemicals can kill immune system cells that help fight off infections, further reducing our immune response to pathogens. The chemicals in smoke also trigger mucous secretions that can block airways and lead to other respiratory difficulties. After chronic exposure to smoke, the walls of the alveoli become brittle, reducing respiratory capacity. Toxic particles in tobacco smoke can also damage the tiny, hair-like cilia on the cells lining the trachea, thus reducing their ability to filter out dirt and microorganisms from the air we breathe. Finally, and perhaps most significantly, carcinogenic chemicals in tobacco smoke can trigger unrestrained cell multiplication in lung tissues, causing cancer.

Although smoking is destructive in numerous ways—it causes almost half a million deaths in the United States every year—stopping smoking at any point can begin the process of reversing some of the damage. By the end of the first year of non-smoking, the risk of death from lung cancer and heart disease begins to decrease, and after 15 years of non-smoking, the risk of death from these causes returns to the same levels as for individuals who have never smoked.