Gas Exchange

All aerobic organisms need oxygen for cellular respiration and must remove the carbon dioxide it produces. Gas exchange is the movement of these gases between an organism and its environment across a specialized surface. Whether the organism is a fish, a mammal, or a plant, gas exchange always relies on diffusion down concentration gradients, and the exchange surfaces share a common set of properties.

Properties of an Efficient Exchange Surface

Diffusion is described by Fick's law, which states that the rate of diffusion is proportional to the surface area and the concentration difference, and inversely proportional to the diffusion distance:

$$\text{rate} \propto \frac{\text{surface area} \times \text{concentration difference}}{\text{diffusion distance}}$$

From this relationship we can pr