Database results:
examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Gas Exchange by Diffusion
Every cell in your body needs oxygen to release energy from food and produces carbon dioxide as a waste product. Gas exchange is the process where oxygen from the air enters your bloodstream and carbon dioxide leaves it. This happens in your lungs through a process called diffusion.
Key Definitions:
Diffusion is a passive process that requires no energy. Gases move from areas of high concentration to areas of low concentration. In the lungs:
Diffusion works effectively for gas exchange because:
The respiratory system is specially designed to maximise gas exchange. Air enters through your nose or mouth, passes down the trachea (windpipe), through the bronchi and bronchioles and finally reaches the alveoli in the lungs.
Your lungs contain about 300 million alveoli, giving them a total surface area roughly the size of a tennis court! This enormous surface area is crucial for efficient gas exchange.
Alveoli are tiny balloon-like sacs with very thin walls (just one cell thick). They are surrounded by networks of capillaries, also with walls just one cell thick.
The lungs contain millions of alveoli, creating a huge surface area for gas exchange - about 70-100 square metres in adults!
Alveoli are moist, which helps gases dissolve so they can diffuse across the membranes more easily.
The alveoli have several special features that make them perfect for gas exchange:
Once gases have diffused across the alveolar membrane, they need to be transported around the body:
Oxygen is transported in two ways:
Each haemoglobin molecule can carry four oxygen molecules, making it very efficient.
Carbon dioxide is transported in three ways:
Marine mammals like whales and dolphins are excellent examples of adapted gas exchange systems. They can hold their breath for up to 2 hours! This is possible because:
These adaptations show how gas exchange systems can evolve to suit different environments and lifestyles.
Several factors can speed up or slow down the rate at which gases diffuse across the alveolar membrane:
A larger difference in concentration means faster diffusion. This is why breathing faster during exercise helps - it maintains a high oxygen concentration in the alveoli.
Larger surface area allows more diffusion to occur simultaneously. Damage to the alveoli (as in emphysema) reduces surface area and impairs gas exchange.
Shorter distance means faster diffusion. Conditions that cause thickening of the alveolar walls (like pulmonary fibrosis) slow down gas exchange.
Several conditions can affect the efficiency of gas exchange:
In asthma, the airways become inflamed and narrow, making it harder to breathe. This reduces the amount of air reaching the alveoli, affecting gas exchange. During an asthma attack:
These changes reduce airflow and impair gas exchange.
Emphysema is often caused by smoking and involves the destruction of alveolar walls. This:
People with emphysema struggle to get enough oxygen into their blood.
Your respiratory system is constantly working at an impressive rate:
All of this happens through the simple process of diffusion - no energy required!
Gas exchange in the lungs is a remarkable example of how structure is perfectly matched to function. The alveoli provide an enormous surface area with very thin walls, allowing oxygen and carbon dioxide to diffuse quickly between the air and blood. This process is passive, requiring no energy input, yet it's essential for all the energy-requiring processes in your body.
Understanding gas exchange helps explain why smoking damages the lungs, why exercise makes you breathe faster and how conditions like asthma affect breathing. It's a perfect example of how physics (diffusion) and biology work together in your body.
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