Countercurrent Heat Exchange

With the arterial and venous blood flows positioned within a few cells layers of each other in the narrow petiole, the opportunity for heat exchange is great. The arterial blood from the abdomen is cooler because the abdomen does not require such a warm temperature as the thorax. The thorax needs to be warmer to initiate flight. The flight muscles located in the thorax warm it up when they contract (Roberts and Harrison,1998).

As the venous blood leaves the flight muscles in the thorax, it carries heat that the muscles produce. So that the precious heat from the thorax is not dumped wastefully into the abdomen, the venous blood transfers its heat to the cooler arterial blood as it runs through the petiole. This method helps retain the heat that the flight muscles are producing in the thorax because, by the time the arterial blood reaches the thorax, it is the same temperature as the rest of the thorax (Roberts and Harrison, 1998).

The countercurrent heat exchange aids the process of warming up before flight and the muscles do not have to work as hard to reach 30 degrees C if the heat is recycled through the countercurrent heat exchange (Roberts and Harrison, 1998). 

Numbers represent degrees Celsius. Adapted from diagram in Schmidt-Nielsen, 1997.