Gas exchange and exchange surfacesSpec B3.1.1, B3.1.2, B3.1.3
In short
Gas exchange is the diffusion of oxygen into an organism and carbon dioxide out of it across a gas-exchange surface. As organisms get larger their surface area-to-volume ratio falls and diffusion distances rise, so large organisms need surfaces that are permeable, thin, moist and large, with steep gradients kept up by blood flow and ventilation.
Every organism must take in oxygen for aerobic respiration and remove carbon dioxide, and photosynthesising organisms do the reverse in the light. Gas exchange happens by diffusion, so it depends on surface area, distance and the concentration gradient.
Why size makes it harder
- As an organism gets bigger, its volume rises faster than its surface area, so the surface area-to-volume ratio decreases. There is less surface for each unit of respiring tissue.
- The distance from the centre of the organism to its exterior also increases, and diffusion is only fast over very short distances.
- Small organisms (for example a unicellular organism or a flatworm) can exchange gases across their whole body surface. Large, active organisms need a specialised gas-exchange surface and a transport system to carry gases to and from every cell.
Properties of gas-exchange surfaces
| Property | Why it helps |
|---|---|
| Permeable | Oxygen and carbon dioxide can pass through the cells and membranes freely. |
| Thin tissue layer | A short diffusion distance makes diffusion fast. |
| Moist | Gases dissolve in the film of water before they diffuse across the surface. |
| Large surface area | More gas can diffuse across at the same time. |
Keeping concentration gradients steep in animals
- Dense networks of blood vessels (capillaries) lie right next to the exchange surface.
- Continuous blood flow carries oxygenated blood away and brings deoxygenated blood with a high carbon dioxide concentration, so the gradient never levels out.
- Ventilation replaces the medium next to the surface: air is pumped in and out of lungs, and water is pumped over the gills of fish.
Linking question: how do multicellular organisms solve the problem of access to materials for all their cells? A good answer pairs a large exchange surface with a transport system that delivers to every cell.
Written and checked against the IB Biology SL specification · Updated October 2026