The respiratory system

Gas exchange

Every cell is respiring all the time, using oxygen and making carbon dioxide. The respiratory system brings oxygen-rich air close to the blood and takes carbon dioxide out of it. Swapping one gas for the other is called gas exchange.

Don't confuse the words. Breathing (ventilation) is moving air in and out of the lungs. Cellular respiration is the chemical reaction in every cell's mitochondria: glucose + oxygen → carbon dioxide + water + energy. Breathing supplies the oxygen for respiration and removes the carbon dioxide it makes.

The path of air

1. Nose and mouth. Air enters through the nostrils, where hairs and sticky mucus trap dust and germs, and the air is warmed and moistened.
2. Pharynx (throat). Shared by food and air. A flap called the epiglottis closes over the airway when you swallow, so food goes down the oesophagus instead. If it doesn't close in time, you choke and cough.
3. Larynx (voice box). Contains the vocal cords, which vibrate as air passes over them.
4. Trachea (windpipe). A tube held open by C-shaped rings of cartilage, so it doesn't collapse. Its lining has tiny hairs called cilia that sweep mucus and trapped dirt up to the throat, where you swallow it.
5. Bronchi. The trachea splits into two bronchi (one is a bronchus), one for each lung.
6. Bronchioles. Each bronchus branches again and again, like an upside-down tree, into tens of thousands of narrow bronchioles.
7. Alveoli. Every bronchiole ends in a cluster of tiny air sacs. This is where gas exchange happens.

Alveoli: built for gas exchange

Your two lungs contain around 480 million alveoli (one is an alveolus), each about a fifth of a millimetre across. Each one is wrapped in a mesh of capillaries.

Gases cross by diffusion, moving from where they are more concentrated to where they are less concentrated. Air in the alveolus has lots of oxygen and the blood arriving from the body has little, so oxygen diffuses into the blood. Blood arriving from the body has lots of carbon dioxide, so it diffuses out into the alveolus to be breathed out.

Alveoli are good at this for the same reasons villi are good at absorbing food:

• Huge surface area: all those sacs add up to about 70 m², roughly a third of a tennis court, folded into your chest.
• Very thin walls: the wall of the alveolus and the wall of the capillary are each one cell thick, so gas only has to travel about a thousandth of a millimetre.
• Moist lining: gases dissolve in the moisture before diffusing across.
• Rich blood supply: blood keeps carrying oxygen away and bringing carbon dioxide in, so the difference in concentration stays large.

Notice that breathed-out air still has plenty of oxygen. That's why mouth-to-mouth resuscitation works.

How you breathe

Lungs have no muscle of their own. They are moved by the diaphragm, a dome-shaped sheet of muscle under the lungs, and the intercostal muscles between the ribs.

The key is pressure. When the chest gets bigger, the air inside is spread out and its pressure drops, so air from outside is pushed in. You can model this with a bell jar: the jar is the rib cage, two balloons are the lungs and a rubber sheet at the bottom is the diaphragm.

At rest you breathe about 12 to 20 times a minute, moving roughly half a litre of air each breath. During hard exercise you can breathe faster and much deeper, moving over 100 litres a minute, to supply your muscles with oxygen and remove all the carbon dioxide they make.

When things go wrong

Asthma affects about 1 in 9 Australians, one of the highest rates in the world. During an asthma attack, the muscles around the bronchioles tighten, the lining swells and extra mucus is made, so the airways narrow. Less air can get through, causing wheezing, coughing, a tight chest and shortness of breath. Triggers include pollen, dust mites, smoke, cold air, exercise and colds. Reliever puffers relax the airway muscles within minutes; preventers taken every day reduce the swelling. A severe attack is a medical emergency.

Smoking damages almost every part of the respiratory system. Tobacco smoke contains thousands of chemicals, including at least 70 that cause cancer.

• Tar coats the lungs and paralyses the cilia, so mucus builds up. Smokers cough to clear it.
• Emphysema: chemicals in smoke break down the walls between alveoli, merging them into bigger, baggier sacs with far less surface area. People with severe emphysema can be breathless just walking across a room.
• Carbon monoxide in smoke binds to haemoglobin far more tightly than oxygen does, so the blood carries less oxygen.
• Lung cancer: about 9 in 10 cases are linked to smoking.

Vaping is newer, so its long-term effects are still being studied, but vapes deliver nicotine (which is highly addictive) and other chemicals deep into the lungs, and have been linked to lung injury. Since 2024 vapes in Australia can only be sold in pharmacies.