Gaseous Exchange in Humans
Gaseous exchange in humans happens by diffusion across the walls of the alveoli, where oxygen moves from air into blood and carbon dioxide moves from blood into air, down their concentration gradients.
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Where gaseous exchange happens
Gaseous exchange takes place in the lungs, at the alveoli, tiny air sacs found in clusters at the end of the smallest bronchioles. Each lung contains millions of alveoli, and each alveolus is surrounded by a dense network of blood capillaries.
Together they form a huge, thin, moist surface where blood and air are brought very close together.
The diffusion pathway
- The concentration of oxygen in the air inside an alveolus is higher than in the blood flowing through the surrounding capillaries.
- Oxygen diffuses down this concentration gradient, across the thin alveolar wall and the thin capillary wall, both of which are only one cell thick.
- Oxygen dissolves in the moisture lining the alveolus before it diffuses, then enters the blood plasma and passes into red blood cells.
- Inside red blood cells, oxygen binds to haemoglobin to form oxyhaemoglobin, which is carried around the body.
- Carbon dioxide is more concentrated in the blood than in the alveolar air, so it diffuses in the opposite direction, from the blood into the alveolus, and is removed from the body during exhalation.
Adaptations of the alveolus
- A very large total surface area, from the millions of alveoli in both lungs, allows a large amount of gas to diffuse at any moment.
- Walls that are only one cell thick, for both the alveolus and the surrounding capillary, keep the diffusion distance extremely short.
- A moist lining allows oxygen and carbon dioxide to dissolve, since gases diffuse faster in solution.
- A dense network of capillaries maintains a steep concentration gradient by constantly bringing in blood low in oxygen and high in carbon dioxide, and carrying away blood that is now rich in oxygen.
How it is examined
Questions on this standard often show a diagram of an alveolus and a capillary and ask you to label the structures, state the direction gases move, or explain how a named feature speeds up diffusion. You may also be asked to link fast breathing or heart rate during exercise to the need for more efficient gaseous exchange.
Worked exam-style question
Question. A diagram shows an alveolus surrounded by a network of blood capillaries, with air on one side of the alveolar wall and blood flowing through the capillary on the other side. (a) State the direction oxygen moves between the air in the alveolus and the blood, and explain why it moves in this direction.
(b) Explain why the wall of the alveolus and the wall of the capillary are each only one cell thick. (c) Explain why the alveolus needs a dense network of surrounding capillaries rather than just one capillary.
(d) During exercise, both breathing rate and heart rate increase. Explain how this helps maintain efficient gaseous exchange.
Model answer. (a) Oxygen moves from the air in the alveolus into the blood, because the concentration of oxygen in the alveolar air is higher than in the blood, so oxygen diffuses down its concentration gradient. (b) A wall only one cell thick gives the shortest possible diffusion distance, allowing gases to diffuse across quickly.
(c) A dense capillary network constantly brings fresh blood low in oxygen past the alveolus and carries away blood that is now rich in oxygen, which keeps the concentration gradient steep so diffusion continues rapidly; a single capillary would quickly reach equilibrium and diffusion would slow down. (d) Faster breathing brings more fresh air with a high oxygen concentration into the alveoli more often, and a faster heart rate circulates blood more quickly past the alveoli and to respiring muscles, together maintaining a steep concentration gradient and meeting the muscles' increased demand for oxygen during exercise.
Practice question
Try this. A patient with a lung disease has some alveoli whose walls have thickened and lost part of their surrounding capillary network. Explain how each of these two changes would affect the rate of gaseous exchange.
Exam tip
Key terms
These Chapter 8 terms are essential for describing gaseous exchange precisely:
- Alveolus, the tiny air sac in the lung where gaseous exchange occurs.
- Gaseous exchange, the diffusion of oxygen and carbon dioxide between air and blood.
- Diffusion, the net movement of particles from a region of higher to lower concentration.
- Haemoglobin, the protein in red blood cells that binds oxygen to form oxyhaemoglobin.
- Trachea, the airway that carries air towards the bronchi and, eventually, the alveoli.
Source:SRC-DSKP-EN
Frequently asked questions
What adaptations make the alveolus efficient for gaseous exchange?
How does oxygen move from the air in an alveolus into a red blood cell?
Why does breathing rate increase during exercise?
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