Countercurrent exchange
From Wikipedia, the free encyclopedia
Countercurrent exchange is a mechanism used to transfer some property of a fluid from one flowing current of fluid to another across a semipermeable barrier between them. The property transferred could be heat, concentration of a chemical substance, or others. Countercurrent exchange is used extensively in biological systems for a wide variety of purposes. For example, fish use it in their gills to transfer oxygen from the surrounding water into their blood, and birds use a countercurrent heat exchanger between blood vessels in their legs to keep heat concentrated within their bodies. In biology this is referred to as a Rete mirabile. The human kidneys use countercurrent exchange to remove water from urine so the body can retain water used to move the nitrogenous waste products. Countercurrent exchange is also a key concept in chemical engineering thermodynamics and manufacturing processes for example in extracting sucrose from sugar beet roots.
The diagram presents a generic representation of a countercurrent exchange system, with two parallel tubes containing fluid separated by a permeable barrier. The property to be exchanged, whose magnitude is represented by the shading, transfers across the barrier in the direction from greater to lesser according to the second law of thermodynamics. With the two flows moving in opposite directions, the countercurrent exchange system maintain a constant gradient between the two flows over their entire length. With a sufficiently long length and a sufficiently low flow rate this can result in almost all of the property being transferred.
By contrast, in the concurrent (or co-current, parallel) exchange system the two fluid flows are in the same direction. As the diagram shows, a concurrent exchange system has a variable gradient over the length of the exchanger and is only capable of moving half of the property from one flow to the other, no matter how long the exchanger is. It can't achieve more than 50%, because at that point, equilibrium is reached, and the gradient declines to zero.
[edit] Example
In a concurrent heat exchanger, the result is thermal equilibrium, with the hot fluid heating the cold, and the cold cooling the warm. Both fluids end up at around the same temperature, between the two original temperatures.
At the input end, we have a large temperature difference and lots of heat transfer; at the output end, we have a small temperature difference, and little heat transfer.
In a countercurrent heat exchanger, the hot fluid becomes cold, and the cold fluid becomes hot.
At the hot end, we have hot fluid coming in, warming further hot fluid which has been warmed through the length of the exchanger. Because the hot input is at its maximum temperature, it can warm the exiting fluid to near its own temperature.
At the cold end, because the cold fluid entering is still cold, it can extract the last of the heat from the now-cooled hot fluid, bringing its temperature down nearly to the level of the cold input.
[edit] Counter-current exchange of heat in organisms
Counter-current exchange is a highly efficient means of minimizing heat loss through the skin's surface because heat is recycled instead of being dissipated. This way, the heart does not have to pump blood as rapidly in order to maintain a constant body core temperature and thus, metabolic rate.
When animals like the leatherback turtle and dolphins are in colder water in which they are not acclimatized to, they use this CCHE mechanism. Counter current heat exchangers are made up of a complex network of peri-arterial venous plexuses that run from the heart and through the blubber to peripheral sites (i.e. the tail flukes, dorsal fin and pectoral fins). Each plexus consists of a singular artery containing warm blood from the heart surrounded by a bundle of veins containing cool blood from the body surface. As these fluids run past each other they create a heat gradient in which heat is transferred. The warm arterial blood transfers most of its heat to the cool venous blood in order to conserve heat by recirculating it back to the body core. Since the arteries are losing a good deal of their heat, by the time they reach the periphery surface, there will not be as much heat lost through convection [1].
