Understanding Gas exchange in humans: IGCSE Biology Explained

Every breath is a gas swap

You take roughly 20,000 breaths a day without thinking about it. Each one brings oxygen into your blood and removes carbon dioxide. That exchange sounds simple, but the IGCSE Biology exam expects you to explain exactly how it happens, where it happens, and what makes the system so efficient. This is one of those topics where understanding the structure of the breathing system unlocks every question the examiners can throw at you.

The breathing system: knowing the parts

Before you can explain gas exchange, you need to be able to label a diagram of the breathing system. Cambridge examiners regularly present a diagram and ask you to identify structures, so this is non-negotiable knowledge for your IGCSE exam.

Air enters through the nose or mouth, passes through the larynx (the voice box), and travels down the trachea (windpipe). The trachea is reinforced with C-shaped rings of cartilage that keep it open. At its base, the trachea splits into two bronchi (singular: bronchus), one leading to each lung. Inside the lungs, the bronchi divide repeatedly into smaller tubes called bronchioles. At the tips of the finest bronchioles sit clusters of tiny air sacs called alveoli (singular: alveolus). This is where gas exchange actually takes place.

Surrounding each alveolus is a dense network of capillaries carrying blood. Below the lungs sits the diaphragm, a dome-shaped sheet of muscle. The lungs themselves are enclosed by the ribcage, with intercostal muscles running between the ribs.

How ventilation works

Ventilation is the mechanical process of moving air into and out of the lungs. It is not the same as gas exchange or respiration. Mixing up these three terms is one of the fastest ways to lose marks on a Cambridge IGCSE paper.

Breathing in (inhalation)

1. The external intercostal muscles contract, pulling the ribs upward and outward.
2. The diaphragm contracts and flattens, moving downward.
3. These two movements increase the volume of the thoracic (chest) cavity.
4. The increased volume causes the pressure inside the lungs to drop below atmospheric pressure.
5. Air rushes in through the nose or mouth to equalise the pressure.

Breathing out (exhalation)

1. The internal intercostal muscles contract (during forced exhalation) or the external intercostal muscles simply relax.
2. The ribs move downward and inward.
3. The diaphragm relaxes and returns to its dome shape, pushing upward.
4. The volume of the thoracic cavity decreases.
5. Pressure inside the lungs rises above atmospheric pressure.
6. Air is pushed out.

Gas exchange at the alveoli

The alveoli are where the real business of the breathing system takes place. Oxygen passes from the air inside the alveolus into the blood in the surrounding capillary. Carbon dioxide moves in the opposite direction, from the blood into the alveolus, to be breathed out. Both gases move by diffusion, travelling down their respective concentration gradients.

Oxygen concentration is high in the alveolar air and low in the blood arriving from the body (because cells have been using it for respiration). So oxygen diffuses into the blood. Carbon dioxide concentration is high in the blood (a waste product of cell respiration) and low in the alveolar air. So carbon dioxide diffuses into the alveolus.

Why alveoli are so good at gas exchange

The alveoli have four features that make them extremely efficient gas exchange surfaces. Examiners ask about these constantly, so it is worth knowing both the feature and the reason it helps.

[Extended/Supplement] The capillary walls are also only one cell thick, so the total barrier between alveolar air and blood is just two cells across. The constant flow of blood through the capillaries and the constant ventilation of the alveoli together maintain a steep concentration gradient for both oxygen and carbon dioxide. This gradient is the driving force for diffusion, and without it, gas exchange would slow and eventually stop.

Inspired versus expired air

The Cambridge IGCSE syllabus expects you to know the approximate composition of the air you breathe in compared with the air you breathe out. The differences are small in absolute terms but significant in biological terms.

Notice that you still breathe out a lot of oxygen (16%). Your lungs do not extract all of it. This is why mouth-to-mouth resuscitation works: the air you blow into another person's lungs still contains enough oxygen to keep them alive.

The limewater experiment

A classic Cambridge practical asks you to compare the carbon dioxide content of inspired and expired air using limewater (calcium hydroxide solution). The setup works like this:

1. Two test tubes or flasks are set up, each containing limewater.
2. A delivery tube arrangement allows you to breathe in through one tube (so atmospheric air bubbles through the first flask of limewater) and breathe out through the other (so expired air bubbles through the second flask).
3. After several breaths, compare the two flasks.

Result: The limewater in the expired-air flask turns milky (cloudy) much faster than the limewater in the inspired-air flask. This is because expired air contains roughly 100 times more carbon dioxide than inspired air. Carbon dioxide reacts with limewater to form insoluble calcium carbonate, which causes the cloudiness.

The inspired-air flask will eventually turn slightly milky too, because atmospheric air does contain a small amount of carbon dioxide (0.04%), but the change is far slower.

Effects of physical activity on breathing

When you exercise, your muscles respire faster. They use more oxygen and produce more carbon dioxide. Your body responds by increasing both the rate (breaths per minute) and the depth (volume of air per breath) of breathing. This ensures more oxygen reaches the alveoli per minute and more carbon dioxide is removed.

[Extended/Supplement] The increased carbon dioxide concentration in the blood is detected by chemoreceptors. These send signals to the breathing centre in the brain, which sends nerve impulses to the intercostal muscles and diaphragm, increasing the rate and depth of ventilation. This is an example of a homeostatic response.

You can investigate this in a practical by counting breaths per minute at rest, then immediately after two minutes of step-ups or jogging, then at one-minute intervals during recovery. A typical result shows breathing rate peaking straight after exercise and returning to the resting rate within three to five minutes.

Effects of smoking on the gas exchange system

Smoking damages nearly every part of the breathing system. Cambridge IGCSE Biology expects you to link specific substances in cigarette smoke to specific health effects.

• Tar settles on the lining of the airways and alveoli. It contains carcinogens (cancer-causing chemicals) and is linked to lung cancer. Tar also damages the cilia (tiny hair-like structures) that line the trachea and bronchi. Healthy cilia beat upward to sweep mucus and trapped particles out of the airways. When they are destroyed, mucus accumulates, leading to the persistent cough known as "smoker's cough" and increasing the risk of bronchitis.
• Nicotine is the addictive substance in tobacco. It raises heart rate and blood pressure, increasing the risk of cardiovascular disease over time.
• Carbon monoxide binds to haemoglobin in red blood cells more tightly than oxygen does. This forms carboxyhaemoglobin, which cannot carry oxygen. The result is that less oxygen reaches body tissues. In pregnant women, this can reduce oxygen supply to the developing foetus.
• Smoke particles and irritants cause inflammation of the bronchial lining, narrowing the airways (chronic bronchitis). Long-term exposure can destroy the walls of alveoli, reducing the surface area for gas exchange. This condition is called emphysema. Patients with emphysema become breathless during even mild activity because their lungs can no longer absorb enough oxygen.

Worked example

Question: Explain how the structure of the alveoli makes gas exchange efficient. [4 marks]

Model answer:

1. Alveoli have a very large total surface area, providing more area over which diffusion of oxygen and carbon dioxide can occur. [1 mark]
2. The alveolar walls are only one cell thick, giving a short diffusion distance so gases can cross rapidly. [1 mark]
3. Each alveolus is surrounded by a dense network of capillaries, maintaining a good blood supply that carries oxygen away and brings carbon dioxide, keeping steep concentration gradients. [1 mark]
4. Ventilation continually replaces the air in the alveoli, ensuring the oxygen concentration stays high and the carbon dioxide concentration stays low, which maintains the concentration gradient for diffusion. [1 mark]

Notice how each point names the feature and then explains the benefit. That two-part structure is what earns full marks.

Common exam mistakes

1. Saying "we breathe in oxygen and breathe out carbon dioxide": This implies that inspired air is pure oxygen and expired air is pure carbon dioxide. In fact, both gases are present in both directions. The percentages change, but neither gas is completely absent from either type of air.
2. Confusing ventilation, gas exchange, and respiration: Ventilation is the physical movement of air in and out of the lungs. Gas exchange is the diffusion of oxygen and carbon dioxide at the alveolar surface. Respiration is the chemical reaction inside cells that releases energy from glucose. They are three distinct processes.
3. Writing that the diaphragm "moves down" during inhalation without saying it contracts and flattens: Cambridge expects precise language. The diaphragm contracts, becoming flatter, which increases thoracic volume.
4. Forgetting to explain why concentration gradients are maintained: Students often state that oxygen diffuses from the alveolus into the blood but forget to explain that blood flow and ventilation keep the gradient steep. Without that explanation, you miss a mark on extended-response questions.
5. Listing smoking effects without linking substance to damage: "Smoking causes cancer" is too vague. You need to say that tar contains carcinogens that cause lung cancer. Always name the specific substance and its specific effect.

Self-check questions

1. Name the four features that make alveoli efficient gas exchange surfaces and explain how each one contributes to efficient diffusion.
2. Describe the changes in the diaphragm and intercostal muscles during inhalation and exhalation.
3. Draw a table comparing the composition of inspired and expired air for oxygen, carbon dioxide, nitrogen, and water vapour.
4. Explain why a person with emphysema becomes breathless during mild physical activity.
5. Describe how you would use limewater to demonstrate that expired air contains more carbon dioxide than inspired air. Include the expected result and the role of the control.
6. Explain why breathing rate and depth both increase during vigorous exercise.