Diffusion distance

Definition

The distance particles must travel across an exchange surface, such as from air in an alveolus into the blood.

Used by Edexcel and OCR

Specification 8.4B · Paper 2

Particles cross more quickly when the diffusion distance is shorter. A thin barrier therefore increases the rate of diffusion compared with a thick one. Between air in an alveolus and blood in a capillary, both the alveolar wall and the capillary wall are thin, giving gases a short route to travel.
Revise: Factors Affecting Diffusion

Specification B2.2a · Paper 1 (Foundation), Paper 3 (Higher)

1
Do not confuse diffusion distance with the distance particles move overall: here it means the distance across the exchange surface.
2
Link each problem to its solution: a small surface area:volume ratio limits exchange at the exterior, while a long diffusion distance makes diffusion to internal cells too slow. Exchange surfaces and transport systems solve these related problems.

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Internal cells are farther from the outer surface, giving substances a longer diffusion distance.