The Respiratory System

Your respiratory system is what lets you breathe. Its main job is to bring fresh oxygen from the air into your bloodstream and to remove carbon dioxide (the waste gas your cells produce) back out into the air. The whole system is built around your two lungs, helped by the airways that connect them to your mouth and nose, and powered by a sheet of muscle below them called the diaphragm. You breathe roughly 20,000 times every day, taking in about 11,000 litres of air, without ever having to think about it.

  • Breaths per dayapprox. 20,000approx. 12 breaths per minute at rest
  • Air per dayapprox. 11,000 litresThrough your lungs
  • Lung surface areaapprox. 70 m²Half the size of a tennis court
  • Alveoli per lungapprox. 300 millionTiny air sacs where oxygen crosses to blood
  • Main muscleDiaphragmDome of muscle below the lungs
  • Time you can hold breathapprox. 30 sec to 2 minTrained free divers: 10+ minutes

The path of a breath

When you take a breath, air follows a specific path through your body.

  1. Air enters through your nose (or mouth), where tiny hairs filter out dust and your nose warms and moistens the air.
  2. It travels down your throat (pharynx) past the voice box (larynx).
  3. Then down the windpipe (trachea), a tube reinforced with rings of cartilage.
  4. The windpipe splits into two bronchi, one going to each lung.
  5. Inside each lung, the bronchi keep branching like a tree, into smaller bronchioles.
  6. The tiniest bronchioles end in clusters of alveoli: microscopic air sacs where oxygen passes into the blood.

The lungs: 70 square metres of surface

You have two lungs, sitting inside your chest and protected by your rib cage. They look spongy and feel soft, packed with the branching airways and the tiny alveoli at the ends. The right lung has three lobes (sections); the left lung has only two, to leave room for the heart between them.

Each lung contains about 300 million alveoli. If you spread out the inside surface of all the alveoli flat, they would cover an area of around 70 square metres: about half the size of a tennis court. That huge surface area is why your lungs can absorb so much oxygen so quickly.

How alveoli work

Each tiny alveolus is wrapped in a network of even tinier capillaries, the smallest blood vessels in your body. The wall between an alveolus and a capillary is just one cell thick (about 0.0005 mm). When you breathe in fresh air, oxygen molecules slip across this thin wall into the blood, where they bind to haemoglobin molecules in your red blood cells. At the same time, carbon dioxide (waste from your cells) slips the other way: out of the blood into the alveolus, ready to be breathed out.

How breathing actually works

Breathing is not done by the lungs themselves; lungs cannot move on their own. They are inflated and deflated by the muscles around them.

  • Diaphragm: a dome-shaped sheet of muscle below your lungs. When it contracts (flattens), it pulls the lungs downwards, making more room and sucking air in.
  • Intercostal muscles: between your ribs. When they contract, they pull the rib cage up and out, also expanding the chest.

To breathe out, the muscles relax. The lungs spring back to their smaller size and push the air out. You usually only notice when you breathe heavily (during exercise) or hold your breath, but the diaphragm is moving constantly, all day and all night, doing one of the most important jobs in your body.

Fact Your lungs are not the same shape on both sides. The left lung is slightly smaller and has only 2 lobes (sections), while the right lung has 3 lobes. The difference is because the left lung has to share space with your heart, which sits slightly to the left in your chest. The "heart-shaped" indentation in your left lung is called the cardiac notch.

Why we need oxygen

Your cells need oxygen to release energy from food. Inside every cell, tiny power stations called mitochondria combine glucose with oxygen to produce energy (and carbon dioxide and water as waste products). This is called cellular respiration, and it is the chemistry that powers everything your body does, every second of your life.

Without oxygen, brain cells start dying within 4 to 6 minutes, which is why holding your breath is dangerous and why CPR (cardiopulmonary resuscitation) needs to start as soon as possible if someone stops breathing.

Did you know? Yawning has nothing to do with low oxygen. Modern research suggests that yawning probably helps cool down the brain: pulling cool air into the body and increasing blood flow through the head. Yawns are also contagious, possibly because being able to mirror someone else's state was useful for early human social groups. Even some animals (dogs, chimps) catch yawns from humans.
Deeper dive: how athletes train at high altitude

The amount of oxygen in the air depends on altitude. At sea level, oxygen makes up about 21% of the atmosphere and the air pressure is high enough that each breath fills your lungs with plenty of oxygen. At the top of Mount Everest (8,849 m), the percentage of oxygen is still 21%, but the air pressure is so low that each breath delivers only about 30% as much oxygen as at sea level. This is why climbers have to use bottled oxygen on the highest mountains.

People who live at high altitudes (the Andes, Himalayas, Ethiopian highlands) have evolved special adaptations over many generations. Tibetans have a genetic mutation in a gene called EPAS1 that helps them use oxygen more efficiently, possibly inherited from an ancient relative of modern humans called the Denisovans. Andean people produce extra red blood cells, increasing the oxygen-carrying capacity of their blood. Ethiopian highlanders have higher levels of haemoglobin in each red blood cell.

Endurance athletes (marathon runners, cyclists, cross-country skiers) often train at altitude to take advantage of the same physiological effects. After several weeks at 2,000 to 3,000 m above sea level, the body produces extra red blood cells in response to the lower oxygen. When the athlete returns to sea level for a race, their blood can now carry more oxygen than before, giving them a competitive edge that lasts a few weeks before fading. Many top endurance athletes spend months a year at high-altitude training camps for exactly this reason.

For the blood that carries oxygen, see the circulatory system. For what your cells do with the oxygen, see parts of a cell (specifically the mitochondria).