# Explain how the tracheal system of insects achieves efficient delivery of oxygen directly to respiring cells, without relying on the circulatory system.

> IB DP Biology Higher Level (2023 syllabus) — B3.1 Gas exchange · Explain · 4 marks

## Mark scheme (4 marks)

1. Tracheae are air-filled tubes that branch repeatedly into smaller tracheoles, bringing oxygen close to every cell.
2. Oxygen diffuses directly from the tracheole tips to respiring cells along a short / steep concentration gradient, so a blood transport system is unnecessary.
3. Spiracles on the body surface can open and close to allow air entry whilst limiting water loss.
4. Active ventilation (e.g. body-wall muscle contractions) or fluid movement at tracheole tips during intense activity reduces the effective diffusion distance and increases the rate of oxygen delivery.

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