Explain how you would explain counter-current gas exchange in fish gills. in the context of Exchange substances official content.

A strong answer should clearly explain counter-current gas exchange in fish gills. using accurate biological vocabulary and direct links to Gas exchange.

For this question, start with the A-Level concept in Gas exchange, then connect it to the scenario using gas exchange, gill and a clear cause-and-effect chain. This exam-style question is aligned to the approved learning objective for Gas exchange and expects a structured biological explanation rather than a single fact. Distractors usually fail because they reverse the mechanism, use the wrong scale of organisation, or describe a related topic without answering the exact objective.