Gas Exchange in Humans
How oxygen gets into the blood and carbon dioxide gets out, and how breathing keeps this going.
Gas Exchange Surfaces
The human gas exchange surface is the wall of the alveoli, tiny air sacs in the lungs. Oxygen passes into the blood and carbon dioxide passes out.
| Feature | In the alveoli | Effect |
|---|---|---|
| Large surface area | Millions of alveoli | More gas can diffuse at once |
| Thin surface | Walls one cell thick | Short distance for diffusion |
| Good blood supply | Network of capillaries | Blood carries gases away and brings more |
| Good ventilation with air | Breathing replaces the air | Fresh air keeps oxygen high |
Worked example (naming the feature): A student says that alveoli are “one cell thick”. Which feature of a gas exchange surface is this? Step 1. One cell thick means the surface is very thin. Step 2. This is the thin surface feature. Answer: Thin surface.
The Breathing System
Air passes along a series of tubes to reach the alveoli in the lungs.
| Part | Description |
|---|---|
| Larynx | At the top of the trachea |
| Trachea | The windpipe, which divides into two bronchi |
| Bronchi | One tube to each lung |
| Bronchioles | Smaller tubes, ending in alveoli |
| Alveoli | Air sacs, surrounded by capillaries |
| Ribs, intercostal muscles, diaphragm | Ribs and intercostal muscles surround the lungs; the diaphragm lies below. Together they move air in and out |
ExtendedIntercostal muscles and cartilage
There are two sets of intercostal muscles between the ribs: the external intercostal muscles on the outside and the internal intercostal muscles on the inside. The trachea has rings of cartilage in its wall; they support the trachea and keep it open.
Ventilation
Breathing moves fresh air into the lungs and stale air out. This ventilation keeps the air in the alveoli fresh, which is one feature of a good gas exchange surface.
ExtendedRibs, intercostal muscles and diaphragm
The muscles change the volume of the thorax, and so the pressure in it. Air always moves from a region of higher pressure to one of lower pressure.
ExtendedBreathing in and breathing out
| Breathing in | Breathing out | |
|---|---|---|
| External intercostal muscles | Contract | Relax |
| Internal intercostal muscles | Relax | Contract |
| Ribs | Move up and out | Move down and in |
| Diaphragm | Contracts and flattens | Relaxes and moves up |
| Volume of thorax | Increases | Decreases |
| Pressure in thorax | Decreases | Increases |
| Air moves | In | Out |
Worked example (explaining inhalation): Explain how the diaphragm causes air to enter the lungs. Step 1. The diaphragm contracts and flattens. Step 2. The volume of the thorax increases, so the pressure falls. Step 3. Air moves in from the higher pressure outside. Answer: Volume up, pressure down, air in.
Inspired and Expired Air
The air that is breathed in (inspired) differs in composition from the air that is breathed out (expired).
| Gas | Inspired air | Expired air |
|---|---|---|
| Oxygen | About 21% | About 16% (less) |
| Carbon dioxide | About 0.04% | About 4% (more) |
| Water vapour | Variable, usually low | High (more) |
Investigation: limewater test
Limewater is a test for carbon dioxide. Breathe in through one tube and out through another, each leading into limewater, or pass the two kinds of air through separate samples. The limewater through which expired air passes turns milky quickly; with inspired air it stays clear for much longer.
ExtendedExplaining the differences
Expired air has less oxygen because oxygen diffuses into the blood and is used in aerobic respiration. It has more carbon dioxide because carbon dioxide is produced by respiration and diffuses out of the blood. It has more water vapour because water evaporates from the moist surface of the alveoli.
Breathing and Exercise
During physical activity both the rate and the depth of breathing increase.
Investigating the effect of exercise
Count the number of breaths in one minute at rest, and estimate the depth of each breath, for example by the chest movement. Take exercise for a set time, then count again straight afterwards and at regular intervals until the rate returns to its resting value. Keep the person, the time and the type of exercise the same in each trial, and repeat to find a mean.
ExtendedWhy exercise changes breathing
The muscles respire faster, so they produce more carbon dioxide. The carbon dioxide concentration in the blood increases and is detected by the brain. The brain sends signals that increase the rate of breathing and make each breath deeper, so more carbon dioxide is removed and more oxygen taken in.
Protecting the Breathing System
ExtendedThe lining of the airways
The air that is breathed in contains pathogens and particles such as dust. The airways are lined by goblet cells and ciliated cells, which protect the breathing system.
ExtendedHow they work
| Part | Role |
|---|---|
| Goblet cells | Make mucus |
| Mucus | Traps pathogens and particles |
| Ciliated cells | Cilia move the mucus, with the trapped pathogens and particles, up and away from the lungs towards the throat |
Worked example (protecting the lungs): Explain how dust breathed in is prevented from reaching the alveoli. Step 1. Goblet cells make mucus, which traps the dust. Step 2. Ciliated cells move the mucus away from the lungs. Answer: Mucus traps it, cilia move it away.
Exam advice
Common mistakes
Model answer
Recall checklist
- Describe the four features of a gas exchange surface.
- Label the parts of the breathing system.
- Compare inspired and expired air.
- Describe the limewater test and its result.
- Describe the effect of exercise on rate and depth.
- Explain how the ribs, intercostals and diaphragm ventilate the lungs (Extended)
- State the function of cartilage in the trachea (Extended)
- Explain how breathing responds to carbon dioxide (Extended)
- Explain the roles of goblet cells, mucus and cilia (Extended)
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