IGCSE Biology glossary
Capillary
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Definition
The smallest type of blood vessel, with walls only one cell thick. Capillaries form dense networks in tissues, providing a large surface area and short diffusion distance for the exchange of substances (oxygen, glucose, carbon dioxide, urea) between blood and cells.
What to include in an exam answer
The two key adaptations: wall is one cell thick (short diffusion distance) and they form extensive networks (large surface area). Capillaries are where exchange actually happens.
Example in context
Explain how capillaries are adapted for exchange of substances (2 marks): Capillary walls are one cell thick, providing a very short diffusion distance. They form extensive networks close to cells, creating a large surface area for efficient exchange.
Related syllabus topic
Transport in animals