The lungs are adapted for efficient gas exchange in several ways.
Firstly, there are millions of alveoli, which provide a very large surface area for gases to diffuse across.
Secondly, the walls of the alveoli and the capillaries are both only one cell thick. This creates a very short diffusion distance for oxygen and carbon dioxide to travel, which speeds up the rate of diffusion.
Thirdly, the lungs have a rich blood supply from the capillary network surrounding the alveoli. This constantly transports oxygen away from the lungs and brings carbon dioxide to the lungs, maintaining a steep concentration gradient for both gases.
Finally, breathing movements (ventilation) ensure that the air in the alveoli is constantly refreshed. This also helps to maintain a steep concentration gradient by bringing in air with a high oxygen concentration and removing air with a high carbon dioxide concentration.
The rate of diffusion is maximised when there is a large surface area, a short diffusion pathway, and a steep concentration gradient. The human lungs are adapted to provide all three.
1. **Large Surface Area:** The lungs contain millions of tiny air sacs called alveoli. This structure, resembling a bunch of grapes, creates an enormous total surface area (around 70m² in an adult) for gas exchange to occur simultaneously.
2. **Short Diffusion Pathway:** The walls of the alveoli are extremely thin, only one cell thick. The capillaries that surround the alveoli also have walls that are one cell thick. This means that oxygen and carbon dioxide only have to diffuse across two single layers of cells, creating a very short distance between the air in the lungs and the blood.
3. **Steep Concentration Gradient:**
* **Good Blood Supply:** A dense network of capillaries constantly flows around the alveoli. This blood continuously transports oxygen away from the lungs to the body's cells and brings carbon dioxide from the cells to the lungs. This ensures that the blood arriving at the alveoli is always low in oxygen and high in carbon dioxide, maintaining a steep gradient for diffusion.
* **Ventilation:** The process of breathing in and out constantly replaces the air inside the alveoli. This brings in fresh air with a high concentration of oxygen and removes stale air with a high concentration of carbon dioxide, again ensuring the concentration gradient is as steep as possible.
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