Alveoli
Exam-ready definition
The tiny air sacs in the lungs where gas exchange takes place. Oxygen diffuses into the blood and carbon dioxide diffuses out.
How this is marked
Adaptation questions are the staple: large total surface area, thin walls (one cell thick. Short diffusion distance), good blood supply (maintains concentration gradient), moist lining. Tie every feature to HOW it speeds up diffusion, or the marks halve.
In an exam answer
Linking adaptation to function: 'The alveoli have a large surface area and walls one cell thick, so oxygen diffuses quickly into the blood over a short distance.'
Full topic guide: Respiration
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