Figure 1: Fick's law

There are four characteristics of an efficient gas exchange system:

• large surface area : volume ratio - diffusion is greater when the surface area of the specialised respiratory surface is large.

• thin - diffusion is greater when the distance travelled across the specialised respiratory surface is small.

• maintaining a concentration gradient - diffusion is greater when concentration gradient of O2 / CO2 across the specialised respiratory surface is high.

• moist - O2 and CO2 must be dissolved in water before it can diffuse across a semipermeable cell membrane, therefore gas exchange requires a moist respiratory surface.

Note: In mammals, there is a fifth requirement for an efficient gas exchange system, and that is a transport system. The circulatory system consists of the heart, blood vessels and blood - which work together to transport respiratory gases (O2 / CO2) throughout the body. 

The effect of body size and shape on diffusion

The rate of gas exchange depends on the organism’s surface area. This means the ability to exchange substances depends on the surface area : volume ratio. As the organism gets larger, their volume and surface area also get larger. However, volume increases much more than surface area.

Small animals have a very large surface area : volume ratio

Gas exchange and niche

A wide variety of structures have evolved to suit the environment and niche of the animal (Figure 2). Complex gas exchange systems and breathing movements have evolved to increase the uptake of O2 to meet the demands of their lifestyles.

Gas exchange systems include gills, tracheae, and lungs. These surfaces are branched and folded, and have thin membranes that provide a larger area for diffusion.