You take something like 20,000 breaths a day and notice almost none of them. You can choose to hold your breath, but not for long, and the reason you give in is not the one most people would guess.
The airway
Air enters through the nose, which warms, moistens and filters it, or through the mouth. It passes the throat and the larynx, or voice box, then flows down the trachea, which splits into two bronchi, one for each lung. These branch again and again, more than twenty times, into ever narrower bronchioles. The airways are lined with cells that make sticky mucus, which traps dust and microbes, and with tiny beating hairs called cilia. In what is often called the mucus escalator, cilia sweep trapped dust and microbes up and out of the airways to the throat, where they are swallowed. Tobacco smoke paralyzes and destroys cilia, which is one reason smokers cough: coughing is the backup system.
The alveoli
The airways end in tiny air sacs called alveoli, clustered like bunches of grapes. A careful count published in 2004 put the average adult at about 480 million of them, though individuals vary widely. Spread flat, their combined surface would cover something like 70 square meters, the floor of a small apartment. Each alveolus is wrapped in capillaries, and the barrier between air and blood is thinner than a thousandth of a millimeter.
Nothing pumps gas across that barrier. Oxygen and carbon dioxide cross by diffusion, moving from where they are more concentrated to where they are less. Air in the alveoli is rich in oxygen and the arriving blood is poor in it, so oxygen moves into the blood. The blood is rich in carbon dioxide, so that gas moves out into the air to be exhaled. The huge surface and thin wall make this passive process fast enough to supply a sprinting body.
How air moves
The lungs have no muscle of their own to inflate them. Beneath them lies a dome-shaped sheet of muscle, the diaphragm. When you breathe in, the diaphragm contracts and flattens, enlarging the chest, while muscles between the ribs lift the rib cage. The larger space lowers the pressure inside the lungs below the pressure of the outside air, and air flows in. Quiet breathing out is passive: the muscles relax and the elastic lungs recoil like a stretched balloon.
The urge to breathe
The rhythm of breathing is set by the brainstem, which watches the chemistry of the blood. Surprisingly, its main signal is not oxygen. The urge to breathe comes mainly from rising carbon dioxide, which makes the blood more acidic. That explains a real danger. Swimmers who breathe very fast and deep before diving, hoping to stay under longer, blow off carbon dioxide. Hyperventilating delays the urge to breathe while oxygen keeps falling, and a swimmer can black out underwater without warning.
Carrying the gases
Almost all the oxygen in blood rides on hemoglobin inside red cells; each hemoglobin molecule can carry four oxygen molecules. Carbon dioxide travels differently. Most carbon dioxide is carried as bicarbonate dissolved in the plasma, with smaller shares bound to hemoglobin or dissolved as gas. Carbon monoxide, from faulty heaters and car exhaust, is deadly because it binds hemoglobin far more tightly than oxygen does, by a factor of more than two hundred, so the blood stops carrying oxygen even though the person keeps breathing. It has no color or smell, which is why detectors save lives.
Altitude
At 5,000 meters the air pressure is roughly half that at sea level, so each breath delivers about half as much oxygen. The body first responds by breathing faster and raising the heart rate. Over days and weeks a slower change follows: the kidneys release a hormone called EPO, which prompts the bone marrow to make more red blood cells. This is why some endurance athletes train at altitude, and why injecting synthetic EPO is banned in sport.
When lungs are damaged
In asthma, the airways become inflamed and their muscle tightens, so air struggles to move; inhalers relax the muscle or calm the inflammation. Chronic obstructive pulmonary disease, mostly caused by smoking, narrows the airways and destroys alveolar walls, shrinking the surface for gas exchange. Smoking is also the leading cause of lung cancer.
Worked example: a climb toward Everest
A climber flies to Nepal and walks up to Everest Base Camp at about 5,300 meters. On the first day she is breathless on gentle slopes and her pulse races, because each breath carries much less oxygen. Over the following weeks her kidneys release EPO and her blood gains red cells, so the same climb feels easier. Climbers call this acclimatization, and they ascend slowly to give it time. At the summit, 8,849 meters up, air pressure is about a third of that at sea level, and most climbers carry bottled oxygen. In 1978 Reinhold Messner and Peter Habeler first reached the top without it, showing how far a well-adapted body can stretch. A climber who rushes upward risks dangerous fluid buildup in the lungs or brain, and the cure is to descend.