Studying counter-current heat exchange is akin to a right of passage for budding young physiologists. After all, counter-current exchange systems are the quintessential example of an anatomical solution to a physiological challenge: elegant networks of interwoven vasculature whose arrangement alone maintains the internal milieu separate from a harsh and variable environment. The undergraduate physiology student may be bombarded with fascinating examples of heat exchangers: temperate ducks whose bodies stay toasty while their feet are on ice, tuna with red muscles keeping their core warm, etc. In these, as in most other examples, the counter-current heat exchanger serves to keep heat in the core of the body while letting the limbs cool down to the temperature of the surroundings.
Given our intimate acquaintance with the rete mirabile, the ‘wonderful net’, one would hardly imagine a case of counter-current heat exchange that would be surprising. However, leave it to the enigmatic leatherback sea turtle to provide us with an exception. John Davenport at University College Cork, Ireland, and colleagues at NOAA Fisheries and the US Geological Survey have uncovered a fascinating and unusual counter-current exchange system in these turtles and, frankly, it seems just a bit backward. A look at the turtle's biology suggests why that might be the case.
Leatherback sea turtles swim continuously and they often swim in cold temperate waters and dive down to water that is only just above freezing. The turtle's core temperature is warmer than the surrounding ocean, but its metabolic rate is low. Because of this low metabolism, there is good reason to suspect that the turtle depends on exercise – specifically, the heat produced as a by-product of muscular work – to keep itself warm. As a result, the temperature within the limbs is likely higher than the core temperature, and heat travels from the limbs to warm the core.
Davenport and his colleagues noticed that there were networks of blood vessels – venous retes – at the base of each of the turtle's limbs and examined the pelvic retes in detail. But the vessels seemed to run counter to what one would expect from previous examples, where heat would be kept out of the limbs to heat the core of the body. Instead, the turtle's rete appear to act to retain heat within the limb, because of the heat gradient between the warm limb and the cooler body. Our hypothetical undergraduate physiologist may now be puzzled as to how this arrangement makes sense. But recall that the leatherback depends on swimming for warmth. The researchers suspect that the rete functions to maintain high limb muscle temperature for swimming in chilly water, while the core depends on insulation and thermal inertia to retain heat.
The authors also note that hyperthermia, overheating, is a true risk for leatherbacks, in particular when nesting; turtles exercise their hindlimbs while moving over the sand and digging, which could potentially cause them to overheat. In this case, counter-current exchange would serve to protect the body from excessive heat generated by the limbs.
The leatherback's unusual employment of counter-current heat exchange is a welcome reminder that the solutions to physiological challenges are just as varied as the challenges themselves – even when they superficially look alike.