You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
Gaseous exchange is the process by which oxygen moves from the air in the alveoli into the blood, and carbon dioxide moves from the blood into the alveoli to be breathed out. This lesson covers the mechanism of gaseous exchange, the six key features of the alveoli that make it efficient, the role of haemoglobin, and how gaseous exchange changes during exercise. This is one of the most frequently examined topics within the AQA GCSE PE specification (3.1.1.2).
Gaseous exchange is the process by which gases move between the air in the alveoli and the blood in the surrounding capillaries. It occurs by diffusion — the movement of particles from an area of high concentration to an area of low concentration, down the concentration gradient.
Two gases are exchanged:
| Gas | Direction of Movement | Reason |
|---|---|---|
| Oxygen (O₂) | From the alveoli into the blood | Oxygen concentration is higher in the alveolar air than in the blood arriving from the body |
| Carbon dioxide (CO₂) | From the blood into the alveoli | Carbon dioxide concentration is higher in the blood (having been produced by cells during respiration) than in the alveolar air |
Exam Tip: Always describe gaseous exchange in terms of concentration gradients. Oxygen diffuses from high concentration in the alveoli to low concentration in the blood. Carbon dioxide diffuses from high concentration in the blood to low concentration in the alveoli. Using the term "concentration gradient" will gain you marks.
graph LR
A[Alveolar Air] -->|O₂ diffuses into blood| B[Capillary Blood]
B -->|CO₂ diffuses into alveoli| A
B --> C[O₂ binds to haemoglobin]
C --> D[Oxyhaemoglobin formed]
D --> E[Carried to body tissues via pulmonary veins and heart]
style A fill:#27ae60,color:#fff
style B fill:#e74c3c,color:#fff
style C fill:#3498db,color:#fff
style D fill:#2980b9,color:#fff
style E fill:#f39c12,color:#fff
AQA expects you to know six features of the alveoli that make gaseous exchange efficient. This is one of the most commonly set questions in the exam.
There are approximately 300–500 million alveoli in the lungs. Their combined surface area is approximately 70 square metres — roughly the size of half a tennis court. This enormous surface area means that a very large volume of gas can be exchanged at any one time.
The walls of the alveoli are only one cell thick (approximately 0.2 micrometres). The adjacent capillary walls are also one cell thick. This means that gases only need to diffuse across two very thin layers of cells — a total distance of less than 1 micrometre. The shorter the diffusion distance, the faster the rate of diffusion.
The inner surface of each alveolus is coated with a thin layer of moisture. Gases must dissolve in this moisture before they can diffuse across the alveolar wall. The moist lining ensures that oxygen can dissolve quickly and diffuse into the blood efficiently.
Each alveolus is wrapped in a dense network of tiny capillaries. This ensures that there is always a large volume of blood flowing past the alveolar surface, ready to pick up oxygen and release carbon dioxide.
The continuous flow of blood through the capillaries means that deoxygenated blood is constantly being brought to the alveoli, and oxygenated blood is constantly being carried away. This maintains a steep concentration gradient for both oxygen (high in alveoli, low in blood) and carbon dioxide (high in blood, low in alveoli). A steeper gradient means faster diffusion.
The concentration gradient is the difference in concentration of a gas between two areas. At the alveoli:
The gradient is maintained by continuous ventilation (breathing in fresh air and exhaling stale air) and continuous blood flow past the alveoli.
| Feature | How It Aids Gaseous Exchange |
|---|---|
| Large surface area | More space for diffusion to occur simultaneously |
| Thin walls | Short diffusion distance = faster diffusion |
| Moist lining | Gases dissolve in moisture, aiding diffusion across the membrane |
| Rich capillary network | Large volume of blood exposed to alveolar air |
| Good blood supply | Maintains steep concentration gradient by constantly refreshing blood |
| Concentration gradient | Drives the diffusion of O₂ into the blood and CO₂ out of the blood |
Exam Tip: For a 6-mark question on alveolar adaptations, you should aim to name all six features and explain how each one specifically assists gaseous exchange. Simply listing the features without linking each one to efficient exchange will not earn full marks.
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.