Exchange of Gases in the Lungs (AQA A-Level Biology): Revision Notes
Exchange of Gases in the Lungs
Location and structure of gas exchange
Gas exchange in mammals takes place at the epithelium of the alveoli. These are tiny spherical air sacs with diameters ranging from 100-300 μm, located deep within the lungs. To maintain a continuous oxygen supply throughout the body, a concentration gradient must be preserved at the alveolar surface.
The alveolar epithelium represents one of the most sophisticated biological exchange surfaces, perfectly adapted for its role in sustaining life through efficient gas transfer.
The positioning of alveoli deep within the lungs provides essential protection for these delicate structures, while the branching bronchial tree ensures air can reach every alveolus effectively.
Essential features of exchange surfaces
Effective gas exchange requires specialised surfaces with specific characteristics that have evolved to maximise diffusion efficiency. Understanding these features helps explain why the alveolar system is so remarkably effective.
Critical Features for Efficient Gas Exchange:
All effective exchange surfaces must be:
- Thin and partially permeable - allowing gases to pass through easily
- Large in surface area - providing maximum space for diffusion
- Well-ventilated - ensuring movement of the external medium (air) and internal medium (blood) to maintain concentration gradients
Since these delicate exchange surfaces can be easily damaged, they are positioned inside the organism for protection. However, this internal location means the organism needs mechanisms to move the external medium across the surface - in mammals, this occurs through breathing, which functions as a form of mass transport.
Structure of alveoli and capillary networks
Each human lung contains approximately 300 million alveoli, providing a combined surface area of around 70 m² - roughly half the size of a tennis court. This enormous surface area is packed into the relatively small space of your lungs, demonstrating remarkable biological engineering.
The structural features that make this system highly efficient include:
- Alveolar walls composed of a single layer of epithelial cells, measuring just 0.05-0.3 μm in thickness
- Dense networks of pulmonary capillaries surrounding each alveolus
- Capillary walls made from a single layer of thin endothelial cells (0.04-0.2 μm thick)
- Extremely narrow capillaries (7-10 μm wide) that force red blood cells to slow down and flatten against the walls
Structural Adaptation in Action:
Consider a single alveolus: It's surrounded by a network of capillaries so dense that the capillary surface area actually exceeds the alveolar surface area. When a red blood cell (diameter ~7 μm) enters a pulmonary capillary (7-10 μm wide), it must deform and slow down, spending approximately 0.75 seconds in close contact with the alveolar wall - enough time for complete gas exchange to occur.
Why gas exchange is highly efficient in alveoli
The remarkable efficiency of alveolar gas exchange results from multiple complementary adaptations working together. The diffusion of gases between alveolar air and blood occurs exceptionally quickly, allowing the human body to meet its high metabolic oxygen demands.
Key Factors Enabling Rapid Gas Exchange:
- Reduced red blood cell speed - as red blood cells squeeze through narrow pulmonary capillaries, they slow down and flatten against capillary walls, allowing more time for gas exchange
- Minimal diffusion distance - the combined thickness of the alveolar wall and capillary wall creates an extremely short pathway for gas molecules
- Massive surface area - the 300 million alveoli provide enormous space for simultaneous gas exchange
- Continuous ventilation - breathing movements constantly refresh the air in alveoli, maintaining steep concentration gradients
- Constant blood circulation - heart action ensures continuous blood flow through pulmonary capillaries, preventing equilibrium and maintaining gradients
These adaptations work synergistically - removing any one factor would significantly reduce the system's efficiency and potentially compromise oxygen delivery to tissues.
The diffusion process
The actual mechanism of gas exchange relies on the fundamental principle of diffusion down concentration gradients. This process occurs continuously and simultaneously for both oxygen and carbon dioxide, but in opposite directions.
Oxygen moves from the alveolar air space (high concentration) across the thin respiratory membrane into the blood plasma and red blood cells (lower concentration). Simultaneously, carbon dioxide follows the reverse pathway, diffusing from the blood (high CO₂ concentration) into the alveolar space (low CO₂ concentration) where it can be exhaled.
This process relies on simple diffusion down concentration gradients, with no energy expenditure required. The continuous movement of air through ventilation and blood through circulation ensures these gradients are maintained, allowing efficient gas exchange to continue without the cell having to actively transport gases.
The beauty of this system lies in its passive nature - as long as breathing and circulation continue, gas exchange occurs automatically and efficiently.
Key Points to Remember:
- Gas exchange occurs at the alveolar epithelium - thin-walled air sacs surrounded by capillary networks
- Exchange surfaces must be thin, permeable, large in area, and well-ventilated to maintain gradients
- Each lung contains ~300 million alveoli with a total surface area of ~70 m²
- Rapid diffusion results from short distances, large surface areas, slow red blood cell movement, and continuous ventilation/circulation
- Oxygen and carbon dioxide move by simple diffusion down their concentration gradients between alveolar air and blood