Countercurrent exchange is one of my favorite physiology tidbits. Creatures across the animal kingdom use countercurrent exchange to increase the efficiency of functions ranging from a fish collecting oxygen in its gills to a wolf conserving valuable body heat in cold weather. We humans also employ countercurrent exchange in architecture and engineering – it’s one way to make a building’s heating and air conditioning systems more efficient.
So, what is countercurrent exchange? Imagine that you have any two fluids (either liquid or gas) flowing through two adjacent conduits, and you want to transfer either molecules or heat energy from one fluid to the other as they flow alongside each other. Flow in the same direction is called concurrent exchange; flow in opposite directions is called countercurrent exchange. For reasons that I’ll explain below, the transfer of molecules or heat is much more efficient if the fluids are flowing in opposite directions than in the same direction.
But how does it work? Let’s start with a few basic concepts:
First of all, molecules and energy like to spread out, and will tend to move from a more crowded area to a less crowded area. A drop of bright red food coloring will spread out to color a whole glass of water pink; an ice cube dropped in a hot cup of tea will melt and make the whole cup of tea cooler (and weaker); and a dollop of spaghetti sauce on your plate will somehow find a way to stain the tablecloth, the placemat, and the white shirt you’re wearing (or maybe that’s just me.)
Why does this happen? Keep in mind that molecular movement is very haphazard, and molecules crash into and bounce off each other pretty much at random. Now, say you have two “rooms” with a door connecting them, like in the picture on the right; one room contains a lot of high-energy molecules and the other just a few. In the crowded room, molecules are bouncing around a lot and will likely shove a number of molecules over into the emptier room. Meanwhile, very few of the molecules from the empty room are likely to get pushed back into the already-crowded room. This process continues until eventually, both rooms contain approximately the same number of molecules, although they are still passing from one room to the other. At this point, the two rooms have reached a state of “dynamic equilibrium”.
The tendency of molecules to move from more crowded to less crowded is a useful property when transferring molecules (or energy) from one substance to another. Imagine a fish moving oxygen from oxygen-rich surrounding water to the oxygen-starved blood in its gills: the oxygen already has a tendency to move from the water to the blood, as long as it can pass through whatever barrier lies in between, such as the permeable walls of blood capillaries in a fish’s gills.
The more efficient the transfer of oxygen from water to gills, the better for the fish. Generally speaking, organisms that spend less energy to do the same things will out-compete their less efficient rivals. They will be more likely to survive, produce more offspring, and be overall more successful; this leads to huge evolutionary pressure in favor of efficiency. Thus, the high-efficiency benefits of countercurrent exchange have led to its widespread adoption in biological systems.
So how exactly do concurrent and counter-current systems work? Let’s look at them in the model of a fish gill:
On a basic level, fish gills contain a lot of small, thin blood vessels. Deoxygenated blood is pumped in after circulation through the body to be re-oxygenated by the surrounding water that gets pushed through the gills. The freshly oxygenated blood then continues on to bring oxygen to the rest of the body.
In a concurrent exchange setup, the blood vessels are oriented in the same direction as the flow of the water through the gills. This is not a very efficient setup – only half of the concentration of oxygen at most can transfer from the water to the blood. For simplicity’s sake, let’s stay that the fish starts out with an adjacent stream of 100% oxygen-saturated water flowing through the gills containing 0% oxygen-saturated blood, and that both water and blood can hold equal amounts of oxygen. Oxygen will tend to flow from the 100% oxygen-saturated water into the deoxygenated blood when they first come into contact with each other. Further down the line, as the oxygen continues to transfer, you might get 90% saturated water, 10% saturated blood; then 70% saturated water, 30% saturated blood. So far, so good – oxygen is still flowing from water to blood. The transfer of oxygen continues until you finally have 50% oxygen-saturated water and 50% saturated blood – and now, no more oxygen can transfer from water to blood. True, the fish has gained oxygen in its bloodstream, but half of the oxygen remains in the water flowing out of the gills again. All that oxygen, wasted! If only there were a way for gills to harvest almost all of the oxygen from the water passing through…
And that’s where countercurrent exchange comes in to play. Simply by switching the direction of blood flow so that the water and the blood are flowing in opposite directions, efficiency of oxygen transfer increases dramatically. How? Well, may seem counterintuitive, but here’s an image to illustrate the basic difference between countercurrent and concurrent exchange:
In countercurrent exchange, the water and blood are flowing in opposite directions. At the point where the blood first begins to flow alongside the water (at the right side of the figure above), the water has already traveled the length of the section where water and blood are adjacent. As you can see in the image, there is a higher concentration of oxygen in the water than in the blood for almost the entire length where they interact, which means that there is a consistent tendency for oxygen to move from the water into the blood. Even at the far right end of the image, when there is hardly any oxygen remaining in the water, this flow from water to blood still continues; although there is very little oxygen remaining in the water, there is none at all in the deoxygenated blood entering the gills.
In short, transfer of oxygen in a countercurrent exchange system can be almost 100% efficient – twice the maximum efficiency of a concurrent exchange system – with a simple reversal of the direction of flow for one of the fluids.
Countercurrent exchange is used for more than just oxygenation of gills. Many endothermic (“warm-blooded”) animals use it to prevent heat from their extremities by conserving heat in their core body.
Imagine a wolf walking in the snow: with body-temperature blood continuously flowing through thin vessels in the wolf’s legs and feet, a lot of body heat will be lost unnecessarily. Fortunately, countercurrent exchange prevents some of this heat loss – in fact, much of the heat carried through the bloodstream will never reach the foot. The artery that carries blood to the foot runs right next to the vein that carries blood back from the foot, allowing countercurrent exchange of heat along the length of the leg. As warm blood travels down the artery, heat continuously leaches over into the cooler blood carried back up by the vein, which is thus warmed up as it travels back to the body. As a result, by the time the blood in the artery reaches the wolf’s foot, it is considerably cooler, and therefore less body heat is lost from the foot in the snow. Interestingly, the foot operates at a lower temperature than the core of the wolf’s body.
So how is a fish gill like an HVAC system? Many air conditioning and heating systems use the same principle of countercurrent exchange when ventilating buildings with outside air. The air intake and output pipes run adjacent to each other, in opposite directions, so as outside air is brought into the building, it is made warmer or cooler by capturing heat from, or dumping heat into, the building-temperature air flowing out the ventilation system. Countercurrent exchange can save us energy, too!
Countercurrent exchange is a simple and elegant solution for a variety of problems in nature and engineering. What other applications can you think of?