The Respiratory System: How We Breathe

Why we have to breathe

Try holding your breath. Within a minute, your body forces you to gasp for air. That powerful urge is a clue to how important breathing is. Every cell in your body needs oxygen to release energy from food, in a process called respiration. And every cell produces a waste gas, carbon dioxide, that must be removed before it builds up to dangerous levels. The respiratory system is the body's gas-exchange machine: it brings oxygen in and takes carbon dioxide out, over and over, your whole life.

Notice that breathing and "respiration" are linked but not the same. Breathing is the physical act of moving air in and out of your lungs. Cellular respiration is the chemical process inside cells that uses that oxygen. The respiratory system's job is to supply the oxygen and clear the waste so cellular respiration can happen everywhere in the body.

The journey of a breath

Follow a breath of air on its journey into your body.

1. Nose and mouth. Air enters mainly through your nose, which is the better doorway. Tiny hairs and sticky mucus trap dust and germs, and the nose warms and moistens the air before it goes deeper — gentler on your lungs than cold, dry air.
2. Trachea (windpipe). The cleaned air travels down the trachea, a tube held open by C-shaped rings of cartilage so it never collapses. Its lining is coated in mucus and microscopic hairs called cilia that sweep trapped dirt back up to your throat.
3. Bronchi. At the bottom, the trachea splits into two tubes called bronchi, one going into each lung.
4. Bronchioles. Inside the lungs, each bronchus branches again and again into smaller and smaller tubes called bronchioles, like the branches of an upside-down tree.
5. Alveoli. At the very end of the smallest bronchioles sit clusters of tiny air sacs called alveoli — and this is where the magic happens.

The alveoli: where gases are swapped

You have around 300 to 500 million alveoli in your lungs. Each one is a microscopic balloon with an incredibly thin wall, just one cell thick, wrapped in a net of tiny blood vessels called capillaries.

This is where gas exchange takes place. The blood arriving in the capillaries is loaded with carbon dioxide and low on oxygen. Because the alveolus walls are so thin and so close to the blood:

• Oxygen passes out of the air in the alveolus and into the blood.
• Carbon dioxide passes out of the blood and into the alveolus, ready to be breathed out.

Gases move by diffusion — naturally spreading from where there is a lot of them to where there is less. No energy or pump is needed for the swap itself; it happens automatically because of the difference in concentration.

Why so many tiny alveoli instead of two big lungs like balloons? Surface area. If you could unfold and flatten all your alveoli, they would cover an area roughly the size of a tennis court. A huge surface means a huge amount of gas can be exchanged very quickly — fast enough to keep up with your body even when you sprint. The freshly oxygenated blood then leaves for the heart, where the circulatory system pumps it out to the body.

The diaphragm: the engine of breathing

Your lungs cannot move on their own — they have no muscles inside them. Instead, breathing is powered mostly by a large, dome-shaped muscle beneath your lungs called the diaphragm, helped by the intercostal muscles between your ribs.

Breathing in (inhaling):

• The diaphragm contracts and flattens downwards.
• The rib muscles pull the ribs up and out.
• This makes the chest cavity bigger, lowering the air pressure inside.
• Air rushes in from outside to fill the larger space.

Breathing out (exhaling):

• The diaphragm relaxes and domes upwards.
• The ribs drop down and in.
• The chest cavity gets smaller, raising the pressure.
• Air is pushed back out.

In other words, your body changes the size of your chest, and air flows in or out to balance the pressure — just like how squeezing and releasing a sealed bag of air pushes air out and sucks it back in.

Most of the time you never think about this. The brainstem monitors the carbon dioxide in your blood and automatically adjusts how fast and deep you breathe — even while you sleep. When you exercise and produce more carbon dioxide, your brain speeds up your breathing without you deciding to.

A team effort

The respiratory system is useless on its own — it collects oxygen, but cannot deliver it. That is the job of the circulatory system, which carries the oxygen in the blood to every cell. And the oxygen is needed to release energy from the nutrients supplied by the digestive system. When you go for a run, all three systems crank up together: you breathe faster, your heart beats harder, and energy from your last meal powers your muscles. It is a perfect example of how the body's systems cooperate.

Looking after your lungs matters too. Smoke and polluted air damage the delicate cilia and alveoli, making gas exchange harder. Exercise, fresh air and avoiding smoking keep this remarkable system working well.

Try it yourself: measure your lung capacity

You can estimate how much air your lungs can move using a simple balloon.

1. Take a deep breath in — as much air as you can hold.
2. Blow it all out into a balloon in one single breath. Pinch the neck shut so no air escapes.
3. Lay the balloon next to a ruler and measure its width (diameter) at the widest point. Write it down.
4. Let the air out and repeat three times, then take the average.
5. Now try it again after doing 30 star jumps, and compare.

What it shows: the size of the balloon reflects your lung capacity — how much air your lungs can push out in one breath. After exercise you may take deeper breaths, and people who do lots of sport often have larger lung capacities because their respiratory muscles are stronger. Compare results with friends or family members of different ages, and discuss why taller or older people often have bigger lungs.