In With the Good Air

When the muscular diaphragm at the base of the chest cavity contracts, the cavity expands and air rushes into the lungs to equalize pressure. When the diaphragm relaxes, the cavity shrinks again, forcing air back out as an exhalation. An adult at rest breathes on average between 10 and 15 times a minute, although the rate can rise to more than once a second during heavy exercise. Children breathe faster: newborns inhale and exhale between 40 and 50 times a minute.

Breathing Room

Air flows into the lungs through a branching system of tubes and finally ends up in more than 600 million tiny sacs called alveoli, whose walls are filled with capillaries. Lungs are thus spongy and so light that they would float like corks in water. That sponginess is essential to their function, because the rate at which oxygen in the air can diffuse into the bloodstream (and carbon dioxide can go the other way) is limited by the amount of surface area available. The total surface area on the walls of the alveoli is between 80 and 90 square meters—roughly half of a tennis court.

Holding your Breath

The lungs have a total air capacity of about six liters—but the amount of air in each breath is far less. A normal resting breath is usually only about half a liter. You can never exhale more than about 4.8 liters because the muscles powering the lungs cannot squeeze out more. Exhaled air is still about 17 percent oxygen (fresh air is 21 percent).

Protecting the Heart

Respiration is the main purpose of the lungs, but in effect they also serve some other functions that are particularly beneficial to the heart. For example, they help to cushion the heart against physical shocks. Also, the network of fine capillaries in the lungs acts like a sieve to capture blood clots originating in the body's veins before they can block the delicate coronary arteries and damage the heart muscle.

Don't Light up

The American Cancer Society estimates that lung cancers killed more than 162,000 people in the U.S. in 2006, making them the country's leading cause of cancer deaths for both men and women. In fact, lung cancers killed more people than the next three most common cancers (colon, breast and prostate) combined. More than 90 percent of all lung cancers are probably caused by smoking. (Asbestos, air pollution and naturally occurring radon account for most of the other cases.)