Counter-current exchange
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Counter-current exchange is the exchange of heat, gases or chemicals between two separated streams flowing in opposing directions. It is commonly used in the animal world to minimise heat loss from the circulatory system or to increase the uptake of oxygen in the gills of fish.
Counter-current exchange of heat
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 [link].
Counter-current exchange of gasses
Capilaries in fish are arranged in such a way that the blood flow through them is in an opposite direction to the stream of water passing through the gills. This enhances the efficiency and reduces the energy costof gas exchange. As blood moves through the gills it gains more and more oxygen, but due to the counter-current exchange, the water it encounters has higher oxygen levels too. This means that along the length of the capilaries, there is a diffusion gradient that is favouring the transfer of oxygen from the water to the blood stream.References
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