Explain how countercurrent heat exchange in the limbs of an Arctic mammal reduces heat loss to the environment.

IB DP Biology Higher Level (2023 syllabus) — B4.1 Adaptation to environment · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Arctic mammals such as the Arctic fox and caribou maintain core body temperature close to 37 °C even when air temperatures fall below −30 °C. Blood flowing to the extremities passes through a rete mirabile — a network of interdigitated arteries and veins — before reaching the foot or flipper.

Model answer (4 marks)

Warm arterial blood flowing towards the extremity runs adjacent to cool venous blood returning from the extremity, so the two flows are in opposite directions (counter‑current arrangement). Heat is transferred by conduction from the warmer arterial blood to the cooler venous blood across the vessel walls, cooling the artery progressively as it approaches the extremity. Blood reaching the extremity is therefore much cooler than core temperature, reducing the temperature gradient between the limb surface and the cold environment, so less heat is lost to the surroundings. The venous blood is warmed by the arterial blood before returning to the core, conserving heat within the body and maintaining core homeothermy.

Examiner tips

Show the counter‑current arrangement first, then explain heat transfer by conduction, then link to reduced temperature gradient and heat conservation.
Use the exact terms: arterial, venous, conduction, temperature gradient, core homeothermy.
Keep the answer concise – 4 points, no extra explanation.

Common mistakes

Confusing the direction of blood flow or saying the artery is warmed instead of cooled.
Failing to mention the reduced temperature gradient or the conservation of heat in the core.
Using vague terms like ‘heat loss’ without explaining the mechanism of conduction.

Mark scheme (4 marks)

Warm arterial blood flowing towards the extremity runs adjacent/parallel to cool venous blood returning from the extremity, so the two flows are in opposite directions (countercurrent arrangement).
Heat is transferred by conduction from the warmer arterial blood to the cooler venous blood across the vessel walls, so the artery is progressively cooled as it approaches the extremity.
Blood reaching the extremity is therefore much cooler than core temperature, reducing the temperature gradient between the limb surface and the cold environment, so less heat is lost to the surroundings.
Venous blood is warmed by the arterial blood before returning to the core, conserving heat within the body and maintaining core homeothermy.

Key terms in this question

countercurrent heat exchange