Lung structure and function
Air passes through the trachea, bronchi and bronchioles to reach millions of alveoli, whose large surface area, thin walls, moist lining and rich blood supply make gaseous exchange efficient.
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The lungs are the main organs of the respiratory system. Air travels through a branching system of airways before reaching the alveoli, where gaseous exchange with the blood takes place.
Parts and functions
| Part | Function |
|---|---|
| Larynx | Voice box at the top of the trachea; contains the vocal cords |
| Trachea | Windpipe held open by rings of cartilage; lined with cilia and mucus that trap dust and microbes |
| Bronchi | Branches of the trachea, one leading into each lung |
| Bronchioles | Smaller branching tubes that carry air deeper into the lung |
| Alveoli | Tiny air sacs where gaseous exchange between air and blood takes place |
| Diaphragm | Sheet of muscle below the lungs that flattens to draw air in during inhalation |
| Intercostal muscles | Muscles between the ribs that move the rib cage during breathing |
| Pleural membranes | Thin membranes around the lungs; the fluid between them reduces friction as the lungs move |
How structure suits function
Alveoli are adapted to make gaseous exchange as efficient as possible. There are millions of them in each lung, giving a very large total surface area for diffusion.
Each alveolus wall is only one cell thick, so oxygen and carbon dioxide have only a short distance to diffuse across. The inner surface of an alveolus is moist, which allows gases to dissolve before diffusing through the wall, and each alveolus is surrounded by a dense network of capillaries, which constantly carries blood away, maintaining a steep concentration gradient that keeps diffusion moving quickly.
- There are millions of alveoli in each lung, giving a very large total surface area for gases to diffuse across.
- Each alveolus wall is one cell thick and the capillary wall beside it is also one cell thick, so the diffusion distance for oxygen and carbon dioxide is very short.
- The inner surface of the alveolus is moist, so gases dissolve before they diffuse across the wall.
- A dense network of capillaries with a constant blood flow keeps carrying oxygen away and bringing carbon dioxide, maintaining a steep concentration gradient.
- The alveolus walls are elastic and recoil during exhalation, helping to push air back out of the lungs.
Related processes
The lungs are involved in three linked ideas that are easy to mix up. Breathing, or ventilation, is the physical movement of air in and out of the lungs, driven by the diaphragm and intercostal muscles changing the volume and pressure of the chest.
Gaseous exchange is the diffusion of oxygen from the alveoli into the blood and carbon dioxide from the blood into the alveoli. Cellular respiration is separate again: it happens inside every body cell, where oxygen is used to release energy from glucose as ATP, producing the carbon dioxide that the blood carries back to the lungs.
Breathing supplies the oxygen and removes the carbon dioxide that make cellular respiration possible.
Common labelling errors
Worked exam-style question
Question. The diagram shows an alveolus lying next to a blood capillary. (a) Name the gas that diffuses from the alveolus into the blood, and the gas that diffuses in the opposite direction.
(b) Explain two features of the alveolus that make gaseous exchange rapid. (c) Explain how the blood flow through the capillary helps oxygen keep diffusing into the blood.
(d) State one difference between breathing and respiration.
Model answer. (a) Oxygen diffuses from the alveolus into the blood, and carbon dioxide diffuses from the blood into the alveolus. (b) The alveolus wall is only one cell thick, giving a short diffusion distance, and there are millions of alveoli giving a large total surface area; its moist lining also lets gases dissolve before crossing (any two).
(c) The blood flowing past constantly carries away the oxygen that has just diffused in and brings carbon dioxide, so the concentration gradient of oxygen between the alveolus and the blood stays steep, keeping diffusion fast. (d) Breathing is the physical movement of air in and out of the lungs, while respiration is the chemical release of energy from glucose inside cells (breathing is not respiration).
Source:SRC-DSKP-EN
Frequently asked questions
What adaptations make alveoli efficient for gaseous exchange?
What is the role of the cartilage rings in the trachea?
What is the difference between breathing and respiration?
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