Lungs and Breathing

You have done it about 20,000 times today without thinking about it once. Breathing is so automatic that it continues through sleep and unconsciousness, controlled by the oldest part of the brain, and so essential that the body cannot survive without it for more than a few minutes. Every breath brings in the oxygen that your cells burn for energy and carries away the carbon dioxide that burning produces.

The journey of a breath

Air enters through the nose, where it is warmed, moistened and filtered by hairs and a lining of sticky mucus that traps dust and microbes. It passes down the throat and into the windpipe, a tube held open by rings of cartilage you can feel in your neck. At the top sits the larynx, the voice box, where air passing over two folds of tissue makes them vibrate to produce speech. A flap closes the windpipe when you swallow, which is why you cannot breathe and swallow at the same time, and why food that goes down the wrong way makes you cough.

In the chest the windpipe splits into two bronchi, one to each lung, and these divide again and again, about 23 times, into tubes finer than a hair. The airways are lined with cells that beat tiny hairs upward, sweeping mucus and the debris it has caught back toward the throat, where it is swallowed. Smoking paralyses these hairs, which is why smokers cough.

The surface where it happens

At the ends of the smallest airways hang clusters of tiny sacs called alveoli, about 480 million of them, each a fraction of a millimetre across, like bunches of microscopic grapes. Their walls are one cell thick and wrapped in capillaries. Here, across a barrier less than a thousandth of a millimetre, oxygen dissolves from the air into the blood and carbon dioxide passes the other way, in less than a second. Flattened out, the alveoli of both lungs would cover about 70 square metres, roughly a tennis court, all folded into two organs that together weigh about a kilogram.

The lungs themselves have no muscle. They are soft and spongy, and they fill and empty because the chest around them changes size. The diaphragm, a dome of muscle under the lungs, flattens downward; the muscles between the ribs lift the ribcage up and out; the chest expands and air is drawn in. Relax, and the chest springs back and the air flows out. At rest, each breath moves about half a litre of air. During hard exercise, with extra muscles joining in, it can move more than three litres and the breathing rate triples.

Why you breathe faster

Breathing is controlled from the brainstem, which monitors not oxygen, as you might expect, but carbon dioxide. When carbon dioxide in the blood rises, as it does during exercise, the brainstem makes you breathe harder to blow it off. This is why holding your breath becomes unbearable after a minute or so, long before you are short of oxygen, and why hyperventilating before diving is dangerous: it lowers carbon dioxide, delays the urge to breathe, and can let a swimmer black out from lack of oxygen without warning.

Oxygen in, carbon dioxide out

The air you breathe in is 21 percent oxygen and almost no carbon dioxide; the air you breathe out is about 16 percent oxygen and 4 percent carbon dioxide. The oxygen is used by every cell to burn sugar and fat for energy, and carbon dioxide is the exhaust. Most of the mass you lose when you lose weight leaves your body this way: the fat is broken down, combined with oxygen, and breathed out as carbon dioxide and water vapour, about a kilogram of it a day.

When breathing goes wrong

Asthma narrows the airways with inflammation and muscle spasm, making breathing out especially hard; it affects more than 250 million people and is treated with drugs that relax the airway muscles. Pneumonia fills the alveoli with fluid so that oxygen cannot cross. Smoking, over decades, destroys alveoli and stiffens the lungs in a condition called COPD, and causes most lung cancers. At high altitude, the air is thinner and each breath carries less oxygen; the body adapts over weeks by making more red blood cells, but above about 8,000 metres it cannot keep up, which is why most Everest climbers carry oxygen.

Why it matters

Breathing is the fastest link between you and the world outside. Food can wait days, water a day or two; air cannot wait minutes. The lungs are also where the atmosphere meets the blood directly, which makes them the organ most exposed to whatever is in the air, from smoke and dust to viruses. Clean air is not a luxury; it is the raw material of the 20,000 breaths you will take tomorrow.