What do the great white shark, the bluefin tuna, a goose’s foot, a whale’s tongue and the future of regenerative energy have in common? The answer is a countercurrent heat exchange system.
If a warmer liquid flows in an opposite direction past a colder liquid, some of its heat is absorbed by the colder liquid. Let’s think for a moment about the pipes in a living animal: Blood vessels are made of a tough layer of connective tissue and several layers of smooth muscle cells. These can allow the transfer of heat, gases and certain other ions. The principle is simple. If there is a gradient in temperature or gas content between one side of the cell wall and the other, heat or gas will transfer from the higher towards the lower concentration and the two liquids will eventually equalize.
If a shark or a tuna swims at high speed along the watery highways, the work in their muscles creates heat and warms their blood. Thus venous blood, which comes from the hard working muscles and is on its way back to the gills, has picked up some heat. Arterial blood is colder. It is coming from the gills, where it has just picked up oxygen from the seawater.
Because the seawater is colder, the blood must cool down when it flows through the gills. Here is where the rete mirabile or miracle network comes in. Instead of loosing all this heat to the ocean, shark, tuna and mackarel send the cold arteries through a web of small venous blood vessels, warming the blood before it gets to the muscles. With this trick they maintain a body temperature much warmer than the surrounding water. Even though they are still fish and not warm-blooded, this allows them to save energy for their costly lifestyle as fast hunters.
Sharks also have other amazing adaptations to life in the cold ocean. A couple of weeks ago a few students and I dissected a spiny dogfish, which is a small shark. The most striking organ in these animals are the two large lobes of liver. Sharks can make a special oil in their liver which helps with buoyancy.
Before I get sidetracked talking about the amazing features of sharks, I am sure you are dying to read how and why the goose’s foot and the whale’s tongue do countercurrent heat exchange.
There is a German children’s song that comes to mind about the goose’s foot. It goes something like this: A male voice asks “Susan, dear Susan, what is the rustle in the straw?” And then a female voice answers, “It is the dear goslings, they have no shoes. The shoemaker has leather but no thread, so the dear goslings have to go without shoes.”
As a kid I always wondered about why geese needed shoes and what they would look like if they had shoes. It turns out they don’t need shoes, because nature has given them the means to keep their feet warm, even if they stand on ice. The goose employs the same principle of countercurrent heat exchange to cool off the blood before it enters the foot and save the heat, so that the bird does not continuously get chilled to the bone. Cold feet don’t seem to bother it as long as the rest of the goose is warm in its down coat.
Years ago, I had the questionable honor of trying to obtain a biological tissue sample from a dead humpback whale. The poor animal had managed to get its fluke wrapped in the buoy line of a setnet site and drowned. The carcass floated lifeless at the surface and we carefully approached with our work vessel.
For a while, I could not quite figure out what I was seeing; there was the whale’s body, its throat ridges were wide and bulking, but what was the large, shapeless mass that was wobbling next to it? My coworker explained that it was the whales’ tongue (it was huge).
Among the baleen whales there are the gulpers, skimmers and scoopers. Gulpers open their mouth wide when they encounter a swarm of krill or a school of small fish and gulp down a big mouthful. The powerful tongue then presses the water past their baleen out of the mouth while the food remains caught behind and in the baleen. Humpback whales are gulpers, thus the big tongue.
Now imagine how much heat a warm-blooded animal would lose when that oversized tongue, which consists mostly of one strong muscle, with every gulp gets exposed to all that cold water. So, with the same countercurrent heat exchange principle as in the goose’s foot ,the whale takes the heat out of the blood before it enters the tongue and warms it back up when it flows back to the heart.
Bioengineers have been looking into ways to use countercurrent heat exchange in modern energy-saving devices. It can also be used to extract heat from the differential between ocean surface water and bottom water.
Who knows? If we develop this technology further, perhaps someone can find some benefit in the warming of the ocean surface temperatures. A miracle network using the heat in the ocean might just give energy recycling a new twist.