Mammalian lungs are adapted for efficient gas exchange because their alveoli provide a large, moist surface, a short diffusion distance and steep concentration gradients. These features increase the rates of oxygen and carbon dioxide diffusion.
This is part of B3.1 Gas exchange and is required at both SL and HL.
How the Adaptations Work
Gas exchange occurs by simple diffusion: oxygen moves from alveolar air into the blood, while carbon dioxide moves from the blood into the alveoli. Each gas moves down its concentration gradient.
| Adaptation | How it increases gas exchange |
|---|---|
| Many alveoli | Provide a very large surface area, allowing more gas molecules to diffuse simultaneously. |
| Thin alveolar and capillary walls | Create a short diffusion distance, increasing diffusion rate. |
| Moist alveolar lining | Allows oxygen and carbon dioxide to dissolve before diffusing across cell membranes. |
| Permeable exchange surface | Allows dissolved gases to pass through the alveolar epithelium and capillary wall. |
| Extensive capillary beds | Provide a rich blood supply and place blood close to alveolar air. |
| Continuous blood flow | Removes oxygenated blood and brings carbon-dioxide-rich blood, maintaining steep concentration gradients. |
| Ventilation | Replaces alveolar air, maintaining a high oxygen concentration and a low carbon dioxide concentration in the alveoli. |
| Branched bronchioles | Distribute air throughout the lungs to the large number of alveoli. |
| Surfactant | Reduces surface tension in the moist lining, helping prevent alveolar collapse and maintaining the exchange surface. |
A common misconception is that oxygen is actively transported into the blood. It is not: both oxygen and carbon dioxide cross the gas-exchange surface by diffusion, without direct ATP use.
Exam Technique
For an explain question, link every structure to its effect: for example, “many alveoli provide a large surface area, increasing the rate of diffusion.” Do not merely list features. Examiners also expect concentration gradients to be linked separately to both ventilation and continuous blood flow.
