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Gas exchange is the process by which oxygen enters the blood and carbon dioxide is removed. This takes place in the alveoli — tiny air sacs in the lungs. The lungs are beautifully adapted to make this process as efficient as possible.
The lungs are the organs of gas exchange. Air travels through the following structures to reach the alveoli:
Mouth/nose → trachea (windpipe) → bronchi (singular: bronchus) → bronchioles → alveoli
| Structure | Description |
|---|---|
| Trachea | Windpipe; held open by C-shaped rings of cartilage; lined with mucus and cilia |
| Bronchi | Two branches of the trachea, one going to each lung |
| Bronchioles | Smaller branches of the bronchi; smooth muscle walls can constrict or dilate |
| Alveoli | Tiny air sacs at the ends of bronchioles; site of gas exchange |
The alveoli (singular: alveolus) are tiny, balloon-shaped air sacs clustered at the end of the bronchioles. There are approximately 300 million alveoli in the lungs.
| Adaptation | How it increases efficiency |
|---|---|
| Enormous surface area | ~300 million alveoli provide a combined surface area of ~70 m² (roughly half a tennis court) — more area for diffusion |
| Walls one cell thick | The alveolar wall and the capillary wall are each only one cell thick — this gives a very short diffusion distance (about 0.2 μm total), so gases diffuse rapidly |
| Rich blood supply | A dense network of capillaries surrounds each alveolus — blood constantly flows past, carrying oxygen away and bringing CO₂, maintaining a steep concentration gradient |
| Moist lining | The inside of alveoli is coated with a thin film of moisture — gases dissolve in this moisture before diffusing across the membrane |
| Good ventilation | Breathing constantly refreshes the air in the alveoli — this maintains a high oxygen concentration in the alveoli and a low CO₂ concentration, keeping the concentration gradient steep |
Exam tip: When explaining efficient gas exchange, always mention FOUR factors: (1) large surface area, (2) thin walls (short diffusion distance), (3) good blood supply (maintains concentration gradient), and (4) ventilation (maintains concentration gradient). A 4-mark question on alveolar adaptations is very common.
Gas exchange in the alveoli occurs by diffusion — the net movement of particles from an area of higher concentration to an area of lower concentration.
The concentration gradient is the difference in concentration between two areas. The steeper the gradient, the faster the rate of diffusion. The alveoli maintain a steep concentration gradient through:
Breathing (ventilation) is the mechanical process of moving air in and out of the lungs. It is NOT the same as respiration (respiration is a chemical reaction in cells).
Breathing involves two sets of muscles:
| What happens | Effect |
|---|---|
| Diaphragm contracts and flattens (moves down) | Volume of thorax (chest cavity) increases |
| External intercostal muscles contract | Ribs move up and out |
| Volume of thorax increases | Pressure inside the lungs decreases (below atmospheric pressure) |
| Air is drawn in through the mouth/nose to equalise pressure |
| What happens | Effect |
|---|---|
| Diaphragm relaxes and domes upward (moves up) | Volume of thorax decreases |
| External intercostal muscles relax | Ribs move down and in |
| Volume of thorax decreases | Pressure inside the lungs increases (above atmospheric pressure) |
| Air is pushed out through the mouth/nose |
Exam tip: Exhalation at rest is a passive process — the muscles relax and the elastic lungs recoil. During exercise, the internal intercostal muscles contract to actively pull the ribs down and in, forcing air out faster.
| Gas | Inhaled air (%) | Exhaled air (%) | Reason for difference |
|---|---|---|---|
| Oxygen | ~21% | ~16% | Some oxygen is absorbed into the blood for respiration |
| Carbon dioxide | ~0.04% | ~4% | CO₂ produced by respiration is released into the alveoli |
| Nitrogen | ~78% | ~78% | Nitrogen is not used by the body |
| Water vapour | Variable | Higher | Water evaporates from the moist alveolar surface |
During exercise:
This ensures the concentration gradient at the alveoli remains steep, maximising gas exchange.
Asthma is a condition where the bronchioles become inflamed and constricted:
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