Introduction to Gas Exchange
Gas exchange is the process by which organisms obtain oxygen for cellular respiration and expel carbon dioxide as a waste product. In humans, this occurs in the lungs; in plants, primarily in the leaves. Efficient gas exchange requires:
- A large surface area to maximise the rate of diffusion
- A thin barrier to minimise diffusion distance
- A moist surface so gases can dissolve before crossing membranes
- A concentration gradient maintained by continuous blood flow (animals) or ventilation
These four factors are formalised in Fick's Law of Diffusion, which states:
Every structural adaptation of a gas exchange surface can be explained by how it maximises the numerator (surface area × concentration gradient) or minimises the denominator (diffusion distance).
Gas exchange is driven entirely by passive diffusion , no energy is directly used to move gases across exchange surfaces. However, energy is used to ventilate the lungs and circulate blood, which maintains the concentration gradients needed for diffusion.
Applying Fick's Law: The alveoli of the human lung have a combined surface area of ~70 m² and walls only 0.1–0.2 μm thick. Substituting these into Fick's Law shows that both the enormous surface area and the vanishingly small diffusion distance act together to produce an extremely high rate of O₂ and CO₂ diffusion.
Alveolar Structure: The Site of Gas Exchange
The alveoli (singular: alveolus) are the primary sites of gas exchange in the mammalian lung. The human lungs contain approximately 300 million alveoli, giving a combined surface area of around 70 m² , roughly the size of a tennis court.
Each alveolus is surrounded by a dense network of capillaries, creating the thin interface across which oxygen and carbon dioxide diffuse.
Alveolus: A tiny, spherical air sac in the lung, surrounded by capillaries, where gas exchange between air and blood occurs by diffusion.
The airways leading to the alveoli form a branching system:
- Trachea → Bronchi → Bronchioles → Alveoli
This branching reduces air speed as it approaches the alveoli, allowing more time for gas exchange to occur.
