Davenport, J., Jones, T.T., Work, T.M., and Balazs, G.H. (2015). Topsy-turvy: turning the counter-current heat exchange of leatherback turtles upside down. Biology Letters. 11: 20150592. doi: 10.1098/rsbl.2015.0592.
Control of body temperature is essential to the complex biological machinery of an organism. An animal must manage the amount of heat in its body, so it stays in the “Goldilocks zone” of not too hot and not too cold. Counter-current heat exchange is a classic example of an elegant anatomical solution to this physiological problem. By allowing animals to retain heat in their body core at the expense of their less sensitive limbs, animals are able to more efficiently thermoregulate and save energy. A new paper out of Hawaii details how leatherback sea turtles (Dermochelys coriacea) do things a little bit differently, but first we need a quick primer on how diffusion and heat exchange works in animals.
Solutes always diffuse (move) from an area of high concentration to an area of low concentration. You can do this experiment at home if you don’t believe me: add some food colouring or drink mix (a “solute”) to a glass of water. Don’t stir, just let the powder slowly dissolve and mix with the entire glass of water.
The solute (e.g. dark purple food colouring) tries to spread out as evenly as possible – it wants to go to places where it is not highly concentrated (Fig. 2). This makes all the water in the beaker a light purple, as the solute divides evenly throughout the available space in the glass.
Biological Heat Exchange Systems
Transferring materials (ions, heat, gases) around the body works within the rules of diffusion, moving from areas of high concentration to areas of low concentration. When we think about exchanging between vessels with blood flowing through them, there are two possible arrangements: co-current and counter-current exchange. In a co-current system, blood in the two vessels flows in parallel. This differs from a counter-current system, where the blood in the two vessels flow in opposite directions (Fig. 3).
The direction of blood flow has a big impact on how effectively heat can transfer between the two vessels. A co-current system slowly loses its gradient (and driving force) as the two fluids become more similar to each other along the length of the vessel. In contrast, a counter-current system is more efficient because it maintains a concentration gradient over the entire length of the vessel.
Mammals and birds use special counter-current heat exchangers to avoid losing too much heat as blood moves through their limbs. These exchangers are made up of bundles of arteries and veins in complex arrangements called reta mirabila (“wonderful nets”). Heat travels from the warm arteries exiting the body core to the cool veins that have circulated through the limb (and close to the outside environment). This ensures that the body core is always relatively warm, and reduces excess heat loss as blood circulates through the limbs.
Cool Turtles Break the Rules
Dr. Davenport and his colleagues at the Honolulu NOAA Fisheries and USGS obtained six juvenile leatherback sea turtles collected as bycatch from longline fishing vessels. They performed routine necropsies on the carcasses, examining the blood vessel network of their flippers in detail. Surprisingly, they found the vessels were arranged in the wrong direction, acting to retain heat in the limb, not the body. But considering the life history of the leatherback turtle, this actually makes a lot of sense.
Despite their lethargy on land, leatherback sea turtles are excellent swimmers, and beat their long flippers continuously to power deep dives. All this work generates a lot of heat – so much that these heavily insulated, large turtles could easily become hyperthermic without some way of keeping cool. That’s where their “backwards” counter-current heat exchangers come in (Fig. 5).
During the breeding season, females use their hindlimbs to dig nests for their eggs, and are particularly vulnerable to overheating out of the cool seawater. Nesting females also take advantage of their plexus structures to hold heat in the flipper, where it is most easily lost to the air. Like overheated mammals and birds, blood vessels in the flippers with dilate, and the turtle will “flush” to try and get rid of excess heat.
Beyond Keeping Warm
While heat exchange is probably the most well-known example, countercurrent systems are involved in a host of biological systems beyond thermoregulation. Many desert and marine mammals use a counter-current systems to concentrate excess salts and conserve water. The kidney, which is responsible for reabsorbing many salts and ions into the body, uses a sophisticated counter-current multiplication loop to concentrate urine and retain the vast majority of salts and nutrients in the body. And giraffes depend upon a rete mirabile to prevent them from fainting when they raise their heads after drinking.
Header image from Wikimedia.