Countercurrent Flow

"Direction and design of exchange tubes affect the efficiency of transfer regardless of whether the exchange involves heat, gases, ions, or other substances." - Kardong (2002), Vertebrates: Comparative Anatomy, Function, and Evolution.

Countercurrent flow, an arrangement in which two liquids or gases flow past each other in opposite directions, vastly increases the efficiency with which materials or energy can be exchanged between the passing substances. This arrangement is observed in many biological situations from 1) gas exchange in fish and shark gills (shown here) to 2) heat exchange in the feet of endothermic seabirds wading in cold seawater and 3) cooling of the blood before it reaches the brains of exercising mammals.

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