Respiration and Circulation
2. Gas Exchange in the Lungs
Learning outcomes
- I can describe the structure of the alveoli.
- I can explain how oxygen enters the bloodstream in the lungs.
- I can explain how carbon dioxide leaves the bloodstream.
- I can describe diffusion as the process responsible for gas exchange.
- I can explain why the alveoli are adapted for efficient gas exchange.
Introduction
Every time you breathe in, your lungs fill with air containing oxygen. This oxygen is needed by every cell in your body for cellular respiration, the process that releases energy from food. At the same time, your cells produce carbon dioxide as a waste product, which must be removed from the body.
Gas exchange takes place inside millions of tiny air sacs called alveoli. These specialised structures allow oxygen to move into the bloodstream while carbon dioxide moves out of the blood to be exhaled. This exchange occurs by a simple process called diffusion, making the lungs one of the body's most efficient organs.
Where Does Gas Exchange Occur?
Gas exchange takes place in the alveoli (singular: alveolus).
The alveoli are tiny air sacs found at the ends of the bronchioles inside the lungs.
Each lung contains approximately 300–500 million alveoli, providing an enormous surface area for gas exchange.
The alveoli are surrounded by tiny blood vessels called capillaries.
Together, the alveoli and capillaries form the gas exchange surface.
Figure 1. Gas exchange occurs in the alveoli, which are surrounded by networks of capillaries.
Structure of the Alveoli
Each alveolus has several important features.
It has:
- A very thin wall (only one cell thick).
- A moist inner surface.
- A rich supply of blood capillaries.
- Elastic fibres that allow it to expand and recoil during breathing.
These adaptations make gas exchange rapid and efficient.
Oxygen Enters the Bloodstream
When you inhale:
- Air enters the alveoli.
- The air contains a high concentration of oxygen.
At the same time:
- Blood arriving at the alveoli has a lower concentration of oxygen.
Because of this concentration difference:
- Oxygen diffuses through the thin alveolar wall.
- Oxygen passes through the capillary wall.
- Oxygen enters the bloodstream.
Inside the blood, oxygen binds to haemoglobin in red blood cells.
The circulatory system then transports oxygen to body tissues.
Figure 2. Oxygen diffuses from the alveoli into the blood because of a concentration gradient.
Carbon Dioxide Leaves the Bloodstream
Blood returning from body tissues contains:
- A high concentration of carbon dioxide.
- A lower concentration of oxygen.
At the alveoli:
- Carbon dioxide diffuses from the blood into the alveolar air.
- During exhalation, carbon dioxide leaves the body.
This constant exchange helps maintain healthy levels of oxygen and carbon dioxide in the blood.
What Is Diffusion?
Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration.
Gas exchange depends on diffusion.
Oxygen diffuses:
- From the alveoli (high concentration)
- Into the blood (low concentration)
Carbon dioxide diffuses:
- From the blood (high concentration)
- Into the alveoli (low concentration)
Diffusion continues until the concentration difference is reduced.
Why Are the Alveoli So Efficient?
The alveoli have several adaptations that maximise gas exchange.
Large Surface Area
Millions of alveoli provide an enormous surface area.
A larger surface allows more gas exchange to occur at the same time.
Thin Walls
The walls of both the alveoli and capillaries are only one cell thick.
This creates a very short distance for gases to diffuse.
Moist Surface
Oxygen dissolves in the thin layer of moisture covering the alveoli before diffusing into the blood.
Rich Blood Supply
A dense network of capillaries constantly brings blood to the alveoli.
This maintains a concentration gradient for diffusion.
Continuous Ventilation
Breathing continually replaces the air inside the alveoli.
Fresh oxygen enters while carbon dioxide is removed, maintaining the concentration difference needed for diffusion.
Figure 3. The alveoli are highly adapted for rapid gas exchange.
Gas Exchange and the Circulatory System
The respiratory and circulatory systems work closely together.
The respiratory system:
- Supplies oxygen.
- Removes carbon dioxide.
The circulatory system:
- Delivers oxygen to body cells.
- Returns carbon dioxide to the lungs.
Without both systems working together, efficient gas exchange would not be possible.
Why Gas Exchange Is Important
Gas exchange allows cells to:
- Carry out cellular respiration.
- Produce energy (ATP).
- Grow and repair tissues.
- Maintain normal body functions.
Removing carbon dioxide is equally important because excess carbon dioxide can change the acidity (pH) of the blood.
Figure 4. The respiratory and circulatory systems work together to transport oxygen and carbon dioxide.
Summary of Gas Movement
| Gas | Moves From. | Moves To. | Process |
|---|---|---|---|
| Oxygen | Alveoli | Blood capillaries. | Diffusion |
| Carbon dioxide. | Blood capillaries. | Alveoli | Diffusion |
The direction of movement is determined by concentration gradients.
Figure 5. Oxygen and carbon dioxide diffuse in opposite directions across the alveolar wall.
Worked Example
Question
Explain why oxygen moves from the alveoli into the blood but carbon dioxide moves from the blood into the alveoli.
Solution
The air inside the alveoli contains a higher concentration of oxygen than the blood arriving from the body, so oxygen diffuses into the blood.
The blood contains a higher concentration of carbon dioxide than the air inside the alveoli, so carbon dioxide diffuses into the alveoli and is removed when you exhale.
Both gases move by diffusion, from areas of higher concentration to areas of lower concentration.
Real-World Connection
When you exercise, your muscles use oxygen more quickly and produce more carbon dioxide. Your breathing rate and heart rate both increase. This brings more oxygen to the alveoli and moves more blood past them, allowing gas exchange to occur more rapidly so your muscles receive the oxygen they need while excess carbon dioxide is removed efficiently.
Did You Know?
If all the alveoli in an adult's lungs were spread out flat, they would cover an area of approximately 70 square metres—about the size of a singles tennis court. This enormous surface area allows enough oxygen to diffuse into the bloodstream to meet the body's needs, even during vigorous exercise.
Key Terms
Alveolus (alveoli) – A tiny air sac in the lungs where gas exchange occurs.
Capillary – A tiny blood vessel with walls only one cell thick.
Diffusion – The movement of particles from an area of higher concentration to an area of lower concentration.
Gas exchange – The movement of oxygen into the blood and carbon dioxide out of the blood.
Haemoglobin – A protein in red blood cells that transports oxygen.
Respiratory membrane – The thin barrier formed by the walls of the alveolus and capillary through which gases diffuse.
Key Takeaways
- Gas exchange occurs in the alveoli, tiny air sacs surrounded by capillaries.
- Oxygen diffuses from the alveoli into the bloodstream, while carbon dioxide diffuses from the blood into the alveoli.
- Gas exchange occurs by diffusion, moving gases from areas of higher concentration to areas of lower concentration.
- The alveoli are adapted for efficient gas exchange because they have a large surface area, thin walls, a moist lining, a rich blood supply, and are continuously ventilated.
- Oxygen carried by the blood is delivered to body cells for cellular respiration, while carbon dioxide is transported back to the lungs for removal.
- The respiratory and circulatory systems work together to maintain the body's oxygen supply and remove carbon dioxide.