Countercurrent exchange

Countercurrent exchange is transfer between adjacent flows moving in opposite directions. In biological heat exchangers, warm arterial blood transfers heat to cooler returning blood, conserving warmth in the body’s core.

Heat exchanger: A device or biological structure that transfers heat between fluids without necessarily mixing them. Countercurrent exchange is a flow arrangement used to improve heat exchange.
Thermoregulation: The regulation of an organism’s body temperature within a viable range. Countercurrent heat exchange is one strategy for controlling heat loss.
Artery: A blood vessel that carries blood away from the heart. Arteries deliver warm blood toward exposed limbs and extremities.
Cocurrent exchange: Heat or substance transfer between adjacent fluids flowing in the same direction. Unlike counterflow, cocurrent streams progressively approach the same temperature.
Thermal conduction: Transfer of thermal energy through matter by microscopic interactions, without bulk movement of the material. Heat crosses the thin barrier between adjacent blood vessels by conduction.
Pinniped: A member of the marine mammal group that includes seals, sea lions, and walruses. Pinnipeds use countercurrent exchangers in flippers and limbs to reduce heat loss.
Vein: A blood vessel that returns blood toward the heart. Veins carry cooled blood back beside incoming arterial blood.
Crosscurrent exchange: Exchange between flows that cross one another, usually at roughly right angles. Crossflow has a different spatial pattern from parallel streams moving in opposite directions.
Temperature gradient: The rate and direction of temperature change across distance. A sustained temperature difference drives heat from arterial to venous blood.
Penguin: A flightless seabird adapted to swimming and life in the Southern Hemisphere. Blood vessels in penguin legs can exchange heat countercurrently and limit heat loss.

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