7.1 Specialised Exchange Surfaces

In organisms with a small SA:V ratio, metabolic demands are higher, and oxygen cannot be supplied (and carbon dioxide removed) quickly enough through diffusion to maintain these rates. Specialised exchange surfaces have evolved as a way to supply an organism with enough oxygen to meet its metabolic demands (and remove the resultant carbon dioxide).

• Good Blood Supply

means that a steep concentration gradient can be maintained, therefore increasing rate of diffusion, because oxygen is supplied and carbon dioxide removed rapidly.

• Ventilation

helps to maintain diffusion and concentration gradients

• Large SA

provides the space necessary for large amounts of gaseous exchange, overcoming the limits of organisms with a small SA:V ratio

• Thin Layers

ensures substances only have to diffuse short distances, increasing efficiency and speed of diffusion