Gas exchange, but bitesize.

Part 1What is the respiratory system?

You breathe about 20 000 times a day, usually without thinking about it. Each breath, air travels from your nose all the way down to tiny sacs deep in your lungs. The job of this whole system is ventilation — the movement of air (gases) INTO and OUT OF the lungs.

Why do you do it? You breathe IN to take in oxygen, which your body needs. You breathe OUT to get rid of carbon dioxide, a waste gas. Air does not jump straight to your lungs — it follows a clear pathway, getting into smaller and smaller tubes.

nose → trachea → bronchi → bronchioles → alveoli

The trachea is your windpipe — the main tube. It branches into two bronchi (one per lung), which split into tiny bronchioles, which end in the alveoli (the air sacs where gases are swapped). Around all this, your ribs protect the lungs, and the diaphragm (a sheet of muscle) sits below them.

Part 2The trachea is built for the job

Press gently on the front of your throat and you can feel hard rings under the skin. Those are rings of cartilage. They hold the trachea OPEN so it cannot collapse under pressure when you breathe — like the rings in a vacuum-cleaner hose.

The inside of the trachea also keeps itself clean. It is lined with two kinds of cell. Goblet cells release sticky mucus that traps dust and germs. Ciliated epithelial cells are covered in cilia — tiny hairs that sweep the dirty mucus UP and OUT towards your throat, where it is swallowed or coughed away. ('Epithelial' just means a surface cell.)

So the two work as a team: mucus traps the dirt, and cilia sweep it away — keeping the airway clean.

Part 1The lungs can't move themselves

Here is the surprise: your lungs have no muscles of their own. They cannot pull air in by themselves. So what makes air rush IN when you breathe in? The answer is a change in volume and pressure, driven by two sets of muscles around the lungs.

Those two muscles are the intercostal muscles (between the ribs) and the diaphragm (the sheet of muscle under the lungs). When they move, they change the size of the chest — and that changes everything.

The key rule to lock in: a bigger space holds gas at a LOWER pressure; a smaller space holds gas at a HIGHER pressure. And air always flows from HIGH pressure to LOW pressure.

Part 2Breathing IN and breathing OUT

When you breathe IN: the intercostal muscles and diaphragm CONTRACT. The ribcage moves UP and OUT, and the diaphragm flattens DOWN. This makes the chest volume INCREASE. A bigger volume means the pressure inside DROPS — below the pressure outside. So air rushes IN, from high pressure outside to low pressure inside.

When you breathe OUT: the same muscles RELAX. The ribcage drops down and in, and the diaphragm domes back up. The chest volume DECREASES, so the pressure inside RISES — above the pressure outside. So air is forced OUT.

It is the change in chest VOLUME that changes the PRESSURE — and the difference in pressure is what moves the air.

Part 1What's actually in a breath

People often say "we breathe in oxygen and breathe out carbon dioxide." That's a neat story, but it isn't quite right. Most of every breath — IN and OUT — is a gas your body never uses at all: nitrogen, about 78%. It goes in and comes straight back out, unchanged.

Only two gases change much between inhaled and exhaled air:

Oxygen FALLS from about 21% to about 16%. You only use about 5% of each breath's oxygen — but that's enough to live on. Carbon dioxide RISES from about 0.04% to about 4% — that's roughly 1000 times more than you breathed in. You also breathe out more water vapour than you breathe in (which is why your breath mists a cold mirror).

Part 2Why the air changes

Why is there LESS oxygen in exhaled air? Because about 5% of the oxygen has diffused into your blood to be used by your cells. Why is there MORE carbon dioxide? Because your body makes CO2 as a waste product, and it is breathed out. The nitrogen doesn't change because your body never touches it.

This is why exhaled air is still good enough for the "kiss of life" — rescue breaths. It still contains about 16% oxygen, which is enough to keep another person's cells supplied.

Part 1The swap in the alveoli

The air you breathe IN is rich in oxygen but low in carbon dioxide. The blood arriving at your lungs is the opposite: low in oxygen, high in carbon dioxide. In the tiny air sacs — the alveoli — the two swap over.

Each alveolus is wrapped in tiny blood vessels called capillaries. Here is the rule:

Oxygen is HIGH in the alveolus and LOW in the blood, so oxygen diffuses INTO the blood. Carbon dioxide is HIGH in the blood and LOW in the alveolus, so carbon dioxide diffuses OUT OF the blood, into the alveolus, to be breathed out.

Both gases move by diffusion — the net movement of particles from a HIGH to a LOW concentration. And diffusion is passive: it needs NO energy.

Part 2Writing the 6-mark answer

A common exam question asks you to explain the whole swap using concentration and diffusion. Here is a full-mark answer, with each (1) showing where a mark is earned.

Part 1Four features, one goal: FASTER diffusion

An adult has around 300 million tiny alveoli. Spread flat, they would cover about half a tennis court. Their walls are just one cell thick, and they are always kept moist. Why does that matter? Because every one of these features makes diffusion FASTER. Here are the four to learn:

1. Large surface area. Millions of folded alveoli give a huge area, so more gas can diffuse across at once.
2. Thin walls (one cell thick). The alveolus and capillary walls are one cell thick, giving a SHORT diffusion pathway — gases cross quickly.
3. Good blood supply. A network of capillaries carries gases away, keeping the concentration difference BIG so diffusion keeps going.
4. Moist surface. Gases dissolve in the moisture and diffuse faster.

Part 2Writing the 6-mark answer

This is a classic exam question. Name at least three adaptations and link each one to faster diffusion. Here's the full-mark version with each (1) marked.

Part 1Why exercise speeds up breathing

After a sprint you pant hard for a minute or two, then your breathing settles. Why? During exercise your muscles work harder, so they need much more oxygen. They also produce much more carbon dioxide as waste. To cope, your breathing rate RISES — you breathe faster and deeper to take in oxygen and remove the extra CO2.

When you stop, your breathing stays fast for a while, until the extra carbon dioxide has been cleared out. A fitter person's breathing rate returns to normal more quickly afterwards — so a recovery graph can tell you who is fitter. (And when you're asleep, with your muscles resting, your breathing rate is LOWER than at rest.)

Part 2What asthma does to the airways

Someone having an asthma attack can struggle to breathe even sitting still. Asthma is a condition where the lining of the airways becomes irritated and inflamed. Three things then happen at once: the muscles around the airways contract (tightening them), the swollen lining narrows them further, and extra mucus is made. The result: the airways become NARROWER, so LESS air can move in and out — making it hard to breathe.

Asthma has no cure. But it can be managed: an inhaler reduces the swelling and relaxes the muscles, widening the airways again. Common triggers include pollen, dust, smoke, cold air and exercise.

Part 1Damage to the airways and alveoli

Cigarette smoke is hot and contains tar, nicotine and carbon monoxide. It damages the gas exchange system at every point you've learned about.

Smoke damages the cilia — the tiny sweeping hairs in the airways. With the cilia damaged, mucus is no longer swept away, so it builds up and the body coughs to clear it: the "smoker's cough." Hot smoke and tar also damage and inflame the bronchi, causing bronchitis.

Deeper down, the folded alveoli membranes break down, leaving larger air spaces. This means a SMALLER surface area — and a smaller surface area means slower, less efficient gas exchange. On top of all this, the tar is a carcinogen — a cancer-causing chemical that can lead to mouth, throat or lung cancer.

Part 2The chemicals in smoke

Three named chemicals do the harm. Tar is the sticky carcinogen that coats the lungs and causes cancer. Nicotine is the addictive chemical; it raises heart rate and blood pressure, leading to heart disease. Carbon monoxide is a gas that takes oxygen's place on red blood cells — so the blood carries LESS oxygen around the body.

So smoking harms both the lungs (less gas exchange) and the heart and blood (less oxygen carried). That double hit is why a smoker can become breathless even climbing stairs.

Part 3Writing the 6-mark answer

Putting it all together — how smoking damages gas exchange AND cuts the oxygen reaching your cells. Each (1) marks a scoring point.