Worked example 1
Explain how the structure of the alveoli is adapted for efficient gas exchange.
Show solution outline
Here's how to structure your answer for full marks:
- Large Surface Area: There are millions of alveoli in the lungs, providing a vast total surface area (approximately 70m²) over which gases can diffuse. This maximises the rate of exchange.
- Thin Walls: Both the alveolar epithelium and the capillary endothelium are only one cell thick. This creates an extremely short diffusion pathway (around 0.5 µm) for oxygen and carbon dioxide, allowing for rapid movement.
- Rich Blood Supply: Each alveolus is surrounded by a dense network of capillaries. This ensures a constant supply of deoxygenated blood and rapid removal of oxygenated blood, maintaining steep concentration gradients for both oxygen (high in alveoli, low in blood) and carbon dioxide (high in blood, low in alveoli).
- Moist Surface: The inner surface of the alveoli is covered by a thin layer of fluid. Respiratory gases like oxygen must first dissolve in this moisture before they can diffuse across the alveolar and capillary membranes, which is essential for efficient transport.