How Your Lungs Exchange Oxygen and Carbon Dioxide

When you breathe in, air usually enters through the nose or mouth and passes down the trachea. The trachea divides into two bronchi, one leading to each lung. These branch repeatedly into smaller bronchioles, creating a wide network that carries air deep into the lungs.

Rings of cartilage help keep the larger airways open. Mucus traps dust and microorganisms, while tiny moving hairs called cilia help carry trapped material towards the throat. This cleaning system protects the delicate gas exchange surfaces farther inside. Sneezing and coughing provide extra ways to remove irritants when the quieter defences are not enough.

Breathing movements are produced mainly by the diaphragm and muscles between the ribs. When the chest cavity expands, pressure inside the lungs falls and air moves in. When the space becomes smaller, pressure rises and air moves out. Your lungs are not empty bags being filled by enthusiasm. They follow pressure differences.

Each bronchiole ends in clusters of alveoli. Their combined surface area is very large, giving many oxygen and carbon dioxide molecules space to cross at the same time. The alveolar walls and nearby capillary walls are each only one cell thick, so the diffusion distance is short.

Oxygen concentration is higher in fresh alveolar air than in blood arriving from the body. Oxygen therefore diffuses across the thin walls and enters red blood cells, where it binds to haemoglobin. Carbon dioxide concentration is higher in the arriving blood, so that gas diffuses in the opposite direction and is breathed out.

The alveoli have a moist lining because gases must dissolve before crossing cell membranes. A substance called surfactant helps stop the tiny sacs from collapsing. Dense capillary networks continually bring deoxygenated blood and carry oxygenated blood away, maintaining the concentration differences needed for diffusion.

Exercise increases the amount of oxygen your cells use and the carbon dioxide they produce. Your breathing becomes faster and deeper, while your heart pumps more blood through the lungs. These changes maintain useful gradients and deliver gases at a higher rate.

To remember an effective exchange surface, look for:

• •A large surface area.
• •A very thin barrier.
• •A moist surface.
• •A strong blood supply.
• •Ventilation that refreshes the air.

A common simplification says you breathe in oxygen and breathe out carbon dioxide. In fact, the air you inhale is roughly one fifth oxygen, and the air you exhale still contains most of that oxygen. Only part of it crosses into the blood during each breath, while the carbon dioxide content rises from a tiny trace to a few per cent. That leftover oxygen is the reason rescue breaths during first aid can still help someone who has stopped breathing.

The urge to breathe is also driven less by oxygen than many people assume. Sensors in the brain and major blood vessels respond mainly to rising carbon dioxide, which makes the blood slightly more acidic. This explains a real danger for swimmers. Breathing rapidly before a long underwater swim lowers carbon dioxide and delays the signal to surface, but it does not add much stored oxygen. Oxygen can then fall low enough to cause sudden blackout before the swimmer feels any need to breathe.

A common belief is that mountain air contains less oxygen. In fact, the air high up is still about one fifth oxygen, the same proportion as at sea level. What changes is the pressure. With less air pressing down from above, each breath contains fewer molecules of every gas, including oxygen. The pressure of oxygen in the alveoli is therefore lower, the difference between alveolar air and arriving blood is smaller, and oxygen diffuses into the blood more slowly. Climbers feel breathless after effort that would be easy at home.

The body responds in stages. Breathing becomes faster and deeper within minutes, and the heart rate rises. Over days and weeks, the kidneys release more of a hormone called erythropoietin, often shortened to EPO, which stimulates the bone marrow to make more red blood cells, so the blood can carry more oxygen. This process is called acclimatisation, and it is why mountaineers ascend gradually. Some endurance athletes train at altitude hoping to raise their red blood cell count before competing nearer sea level.

Carbon dioxide does not simply travel in the blood as a dissolved gas. Only a small fraction, usually under a tenth, moves that way. A larger share, roughly a fifth, binds loosely to proteins, including haemoglobin. Most of it, around seven tenths, is converted into hydrogencarbonate ions. Inside red blood cells, an enzyme called carbonic anhydrase speeds up the reaction between carbon dioxide and water, forming carbonic acid, which splits into hydrogencarbonate and hydrogen ions. The hydrogencarbonate moves out into the plasma and is carried back towards the lungs.

In the lung capillaries the process runs in reverse. As carbon dioxide diffuses into the alveoli, hydrogencarbonate is turned back into carbon dioxide and water, keeping the outward flow going. The two gases also influence each other. In busy tissues, extra carbon dioxide and acidity make haemoglobin release oxygen more readily, a pattern called the Bohr effect after the Danish scientist Christian Bohr. In the lungs, where carbon dioxide is being removed, haemoglobin holds on to oxygen more tightly. Each gas therefore helps the other to load or unload in exactly the place where it is needed.

The surfactant that lines the alveoli matters most at the very start of life. Water lining a tiny sac pulls inwards through surface tension, and the smaller the sac, the stronger that pull, so without help the alveoli would tend to collapse each time a baby breathed out. Surfactant lowers the surface tension. The lungs begin making it only in the later part of pregnancy, so babies born very early often lack enough. In 1959 the American researcher Mary Ellen Avery showed that this shortage caused the breathing failure, now called respiratory distress syndrome, that was then a leading cause of death in premature babies.

The condition gained public attention in 1963, when Patrick Kennedy, the newborn son of President John F. Kennedy, died of it two days after being born several weeks early. Two treatments have since transformed survival. From the early 1970s, research in New Zealand showed that giving steroid injections to mothers expected to deliver early speeds up the development of their babies' lungs, including surfactant production. From around 1990, doctors also began delivering artificial or animal-derived surfactant directly into the lungs of premature babies through a breathing tube. Together these steps have saved a great many lives in neonatal units.

Your lungs exchange gases by bringing air and blood close together across thin, moist alveolar walls. Oxygen diffuses into the blood, while carbon dioxide diffuses into the alveoli for removal. Follow the route from airway to alveolus, then connect each structural feature with faster diffusion, and the respiratory system becomes a clear journey.