Countercurrent exchange
Countercurrent exchange is a mechanism used to transfer some property of a fluid from one flowing current of fluid to another across a Semipermeable membrane or thermally-conductive material 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. Mammalian 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 semipermeable or thermoconductive membrane. 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 can maintain a nearly 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.  It is important to note that such nearly complete transfer is only possible if the two flows are, in some sense, "equal".  If we are talking about mass transfer and measuring concentration by the quantity of solute per unit quantity of solvent, not per unit quantity of solution, (We could measure concentration in molality, for example), then "equal" will simply mean that the solvent flow rates are equal in the two tubes (It would also be acceptable to measure concentration as amount of solute per unit mass or per mole of solution,as is done with mass fractions or mole fractions, in which case the flows would be considered equal if they had equal flowrates of solution.  However, the same could not be said for concentration measured as quantity of solute per unit volume of solution, like molarity, since the solute can alter the volume of different solutions in different ways if it has a different partial molar volume in the two solutions.)  .  If we are talking about heat transfer, then the product of the average specific heat capacity (on a mass basis, averaged over the temperature range involved) and the mass flow rate must be the same for each stream.  If the two flows were not equal in this sense, then conservation of mass or energy would require that the streams leave with different concentrations or temperatures than those indicated in the diagram.
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.  With equal flows in the two tubes, this method of exchange is only capable of moving half of the property from one flow to the other, no matter how long the exchanger is. If each stream changes its property to be 50% closer to that of the opposite stream's inlet condition, exchange will stop because at that point equilibrium is reached, and the gradient has declined to zero.  In the case of unequal flows, the equillibrium condition will occur somewhat closer to the conditions of the stream with the higher flow.

## 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.

# 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 to which they are not acclimatized, 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 .