Explain how countercurrent heat exchange in the flippers of marine mammals such as dolphins reduces heat loss to the surrounding water.

IB DP Biology Standard 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).

Marine mammals maintain a constant core body temperature despite living in cold ocean water. Their flippers, which lack significant insulating blubber, contain a specialised arrangement of blood vessels that helps conserve body heat.

Model answer (4 marks)

Arteries carrying warm blood from the core run alongside veins returning cold blood from the flipper surface.
Heat is transferred from the warmer arterial blood to the cooler venous blood across the vessel walls, down a temperature gradient.
Because the blood flows in opposite directions, a temperature gradient is maintained along the entire length of the exchanger, maximising heat transfer.
The venous blood is pre‑warmed before it returns to the core, so less metabolic energy is required to maintain core temperature, reducing overall heat loss.

Examiner tips

Use the term "countercurrent" and explain the opposite flow of arteries and veins.
Show that heat moves from warm to cool across vessel walls.
Mention the maintained temperature gradient and its effect on heat transfer.
Explain the energetic benefit of pre‑warming venous blood.

Common mistakes

Confusing the direction of blood flow or using “concurrent” instead of “countercurrent”.
Failing to state that heat transfer occurs across vessel walls.
Not linking the pre‑warming of venous blood to reduced metabolic energy.

Mark scheme (4 marks)

Arteries carrying warm blood from the core run alongside veins returning cold blood from the flipper surface
Heat transfers from the warmer arterial blood to the cooler venous blood across the vessel walls, down a temperature gradient
Because blood flows in opposite directions, a temperature gradient is maintained along the entire length of the exchanger, maximising heat transfer
Venous blood is pre-warmed before returning to the core, so less metabolic energy is needed to maintain core temperature, reducing overall heat loss

Key terms in this question

countercurrent heat exchange