Gas Exchange in the Lungs

The deepest part of a breath

Take a slow, deep breath. Air rushes in through your nose, down your windpipe and into your lungs. But have you ever wondered what actually happens to that air once it is inside you? Why do we need air at all, and what comes back out when we breathe out?

The answer is a process called gas exchange — one of the most important swaps in your whole body. This lesson goes deeper than simply "breathing in and out", which you can review in the respiratory system. We will travel right to the tiniest corners of your lungs to see the swap happen.

Why your body needs the swap

Every cell in your body makes energy by combining food with oxygen. This process is called respiration, and it produces a waste gas: carbon dioxide. So your cells constantly need two things to happen:

1. Fresh oxygen delivered to them.
2. Waste carbon dioxide taken away.

Your lungs are where both jobs get done at the same time. Bringing oxygen in and carbon dioxide out — that double swap is gas exchange.

Inside the lungs: the alveoli

When you breathe in, air travels down your windpipe, which splits into smaller and smaller tubes, like the branches of an upside-down tree. At the very end of the smallest tubes are millions of tiny air sacs called alveoli.

The alveoli are where the magic happens. Each one is:

• Tiny and round, like a microscopic bubble or a bunch of grapes.
• Wrapped in capillaries — the smallest blood vessels — carried there by the circulatory system: heart and blood.
• Lined with a wall that is just one cell thick, so gases can cross almost instantly.

There are around 300 to 500 million alveoli in an adult's lungs. Spread out flat, their surface would cover about half a tennis court! This enormous surface area is the secret to swapping huge amounts of gas very fast.

How the gases swap: diffusion

Gases move during gas exchange by a process called diffusion. The rule of diffusion is simple: a gas spreads from a place where there is more of it to a place where there is less of it, until it is evenly spread.

Here is how it works at the alveoli:

• The air you breathe in is rich in oxygen. The blood arriving at the alveoli is low in oxygen (it has just come back from the body). So oxygen diffuses out of the air sac and into the blood.
• At the same time, the blood is full of carbon dioxide waste, while the fresh air has very little. So carbon dioxide diffuses out of the blood and into the air sac, ready to be breathed out.

Because the alveoli walls are so thin and there are so many of them, this swap happens in a fraction of a second, again and again, with every breath.

The journey of the gases

Gas exchange is part of a bigger round trip:

1. You breathe in fresh air to the alveoli.
2. Oxygen diffuses into the blood and is grabbed by red blood cells.
3. The blood carries oxygen to every cell in your body, working with the systems of the human body.
4. Cells use the oxygen and release carbon dioxide into the blood.
5. The blood carries carbon dioxide back to the lungs.
6. It diffuses into the alveoli, and you breathe it out.

When you exercise, your cells use oxygen and make carbon dioxide faster, so your body speeds up your breathing and heartbeat to keep the swap going. That is why you puff and pant after running.

Looking after your lungs

Healthy lungs swap gases efficiently. To protect them:

• Stay active to keep your lungs and breathing muscles strong.
• Breathe clean air and avoid smoke. Smoking damages and clogs the delicate alveoli, so fewer can do their job.
• Keep healthy when ill, as some infections fill the alveoli with fluid and make breathing harder.

Try it: build a working lung model

This classic model shows how breathing pulls air into the lungs so gas exchange can happen.

You will need a clear plastic bottle, two small balloons, a rubber band, scissors and tape (ask an adult to help with cutting).

1. Carefully cut the bottom off the plastic bottle.
2. Push one balloon inside the bottle and stretch its neck over the bottle's mouth. This balloon is a lung.
3. Cut the neck off the second balloon and stretch the wide part across the open bottom of the bottle, securing it with the rubber band. This is your diaphragm, the muscle under your lungs.
4. Now pull the bottom balloon down. Watch the lung balloon inside fill with air and inflate.
5. Push the bottom balloon up and watch the lung balloon empty.

Why it works: When you pull the "diaphragm" down, it makes more space inside the bottle, so air is drawn in and the lung inflates — just like when your real diaphragm pulls down and you breathe in. That breath fills your alveoli with fresh, oxygen-rich air, so gas exchange can take place. Push the diaphragm up and the lung empties, like breathing out the carbon dioxide. Your model shows the very first step that makes the whole gas swap possible.