Editing Countercurrent exchange

{{Short description|Mechanism occurring in nature and mimicked in industry and engineering}}
{{More citations needed|date=May 2020}}

[[File:Échange de chaleur contre-courant.svg|thumb|263x263px|Counter heat current exchange: Note the gradually declining differential and that the once hot and cold streams exit with a reversed temperature difference; the hotter entering stream becomes the exiting cooler stream and vice versa.]]
'''Countercurrent exchange''' is a mechanism between two flowing bodies flowing in opposite directions to each other, in which there is a transfer of some property, usually heat or some chemical. The flowing bodies can be liquids, gases, or even solid powders, or any combination of those. For example, in a [[distillation column]], the vapors bubble up through the downward flowing liquid while exchanging both heat and mass. It occurs in nature and is mimicked in industry and engineering. It is a kind of exchange using counter [[flow arrangement]].

The maximum amount of heat or mass transfer that can be obtained is higher with countercurrent than co-current (parallel) exchange because countercurrent maintains a slowly declining difference or [[gradient]] (usually temperature or concentration difference). In cocurrent exchange the initial gradient is higher but falls off quickly, leading to wasted potential. For example, in the adjacent diagram, the fluid being heated (exiting top) has a higher exiting temperature than the cooled fluid (exiting bottom) that was used for heating. With cocurrent or parallel exchange the heated and cooled fluids can only approach one another. The result is that countercurrent exchange can achieve a greater amount of heat or mass transfer than parallel under otherwise similar conditions.

Countercurrent exchange when set up in a circuit or loop can be used for building up concentrations, heat, or other properties of flowing liquids. Specifically when set up in a loop with a buffering liquid between the incoming and outgoing fluid running in a circuit, and with [[active transport]] pumps on the outgoing fluid's tubes, the system is called a [[countercurrent multiplication|countercurrent multiplier]], enabling a multiplied effect of many small pumps to gradually build up a large concentration in the buffer liquid.

Other countercurrent exchange circuits where the incoming and outgoing fluids touch each other are used for retaining a high concentration of a dissolved substance or for retaining heat, or for allowing the external buildup of the heat or concentration at one point in the system.

Countercurrent exchange circuits or loops are found extensively in [[nature]], specifically in [[biology|biologic systems]]. In vertebrates, they are called a [[rete mirabile]], originally the name of an organ in fish [[gills]] for absorbing oxygen from the water. It is mimicked in industrial systems. Countercurrent exchange is a key concept in [[chemical engineering]] [[thermodynamics]] and manufacturing processes, for example in extracting [[sucrose]] from [[sugar beet]] roots.

[[Countercurrent multiplication]] is a similar but different concept where liquid moves in a loop followed by a long length of movement in opposite directions with an intermediate zone. The tube leading to the loop passively building up a gradient of heat (or cooling) or solvent concentration while the returning tube has a constant small pumping action all along it, so that a gradual intensification of the heat or concentration is created towards the loop. Countercurrent multiplication has been found in the kidneys<ref>Both countercurrent exchange and countercurrent multiplication systems have been found in the kidneys. The latter in the loop of Henle, the first in the [[Straight arterioles of kidney|vasa recta]]</ref> as well as in many other biological organs.

== Three current exchange systems ==
[[Image:Heat exchanger.svg|thumb|right|320px|Three topologies of countercurrent exchange systems]]
Countercurrent exchange and cocurrent exchange are two mechanisms used to transfer some property of a [[fluid]] from one flowing current of fluid to another across a barrier allowing one way flow of the property between them. The property transferred could be [[heat]], [[concentration]] of a [[chemical substance]], or other properties of the flow.

When heat is transferred, a thermally-conductive membrane is used between the two tubes, and when the concentration of a chemical substance is transferred a [[semipermeable membrane]] is used.

=== Cocurrent flow—half transfer ===
[[Image:Comparison of con- and counter-current flow exchange.jpg|thumb|right|400px|A comparison between the operations and effects of a cocurrent and a countercurrent flow exchange system is depicted by the upper and lower diagrams respectively. In both it is assumed (and indicated) that red has a higher value (e.g. of temperature) than blue and that the property being transported in the channels therefore flows from red to blue. Channels are contiguous if effective exchange is to occur (i.e. there can be no gap between the channels).]]
In the cocurrent flow exchange mechanism, the two fluids flow in the same direction.

As the cocurrent and countercurrent exchange mechanisms diagram showed, a cocurrent 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 when the point of equilibrium is reached, and the gradient has declined to zero. In the case of unequal flows, the equilibrium condition will occur somewhat closer to the conditions of the stream with the higher flow.

====Cocurrent flow examples====
[[Image:Delta T 1.svg|thumb|right|200px|Cocurrent and countercurrent heat exchange]]
A cocurrent heat exchanger is an example of a cocurrent flow exchange mechanism. Two tubes have a liquid flowing in the same direction. One starts off hot at {{Convert|60|°C|abbr=on}}, the second cold at {{Convert|20|°C|abbr=on}}. A thermoconductive membrane or an open section allows heat transfer between the two flows.

The hot fluid heats the cold one, and the cold fluid cools down the warm one. The result is thermal equilibrium: Both fluids end up at around the same temperature: {{Convert|40|°C|abbr=on}}, almost exactly between the two original temperatures ({{Convert|20|°C|abbr=on}} and {{Convert|60|°C|abbr=on}}). At the input end, there is a large temperature difference of {{Convert|40|°C|abbr=on}} and much heat transfer; at the output end, there is a very small temperature difference (both are at the same temperature of {{Convert|40|°C|abbr=on}} or close to it), and very little heat transfer if any at all. If the equilibrium—where both tubes are at the same temperature—is reached before the exit of the liquid from the tubes, no further heat transfer will be achieved along the remaining length of the tubes.

A similar example is the cocurrent concentration exchange. The system consists of two tubes, one with brine (concentrated saltwater), the other with freshwater (which has a low concentration of salt in it), and a [[semi permeable membrane]] which allows only water to pass between the two, in an [[osmosis|osmotic process]]. Many of the water molecules pass from the freshwater flow in order to dilute the brine, while the concentration of salt in the freshwater constantly grows (since the salt is not leaving this flow, while water is). This will continue, until both flows reach a similar dilution, with a concentration somewhere close to midway between the two original dilutions. Once that happens, there will be no more flow between the two tubes, since both are at a similar dilution and there is no more [[osmotic pressure]].

=== Countercurrent flow—almost full transfer ===
[[Image:Spiral-heat-exchanger-schematic-workaround.svg|thumb|right|130px|Spiral counter-current heat exchange schematic]]
In countercurrent flow, the two flows move in opposite directions.

Two tubes have a liquid flowing in opposite directions, transferring a property from one tube to the other. For example, this could be transferring heat from a hot flow of liquid to a cold one, or transferring the concentration of a dissolved solute from a high concentration flow of liquid to a low concentration flow.

The counter-current exchange system can maintain a nearly constant [[gradient]] between the two flows over their entire length of contact. With a sufficiently long length and a sufficiently low flow rate this can result in almost all of the property transferred. So, for example, in the case of heat exchange, the exiting liquid will be almost as hot as the original incoming liquid's heat.

==== Countercurrent flow examples ====

In a '''countercurrent heat exchanger''', the hot fluid becomes cold, and the cold fluid becomes hot.

In this example, hot water at {{Convert|60|°C|abbr=on}} enters the top pipe. It warms water in the bottom pipe which has been warmed up along the way, to almost {{Convert|60|°C|abbr=on}}. A minute but existing heat difference still exists, and a small amount of heat is transferred, so that the water leaving the bottom pipe is at close to {{Convert|60|°C|abbr=on}}. Because the hot input is at its maximum temperature of {{Convert|60|°C|abbr=on}}, and the exiting water at the bottom pipe is nearly at that temperature but not quite, the water in the top pipe can warm the one in the bottom pipe to nearly its own temperature. At the cold end—the water exit from the top pipe, because the cold water entering the bottom pipe is still cold at {{Convert|20|°C|abbr=on}}, it can extract the last of the heat from the now-cooled hot water in the top pipe, bringing its temperature down nearly to the level of the cold input fluid ({{Convert|21|°C|abbr=on}}).

The result is that the top pipe which received hot water, now has cold water leaving it at {{Convert|20|°C|abbr=on}}, while the bottom pipe which received cold water, is now emitting hot water at close to {{Convert|60|°C|abbr=on}}. In effect, most of the heat was transferred.

==== Conditions for higher transfer results ====
Nearly complete transfer in systems implementing countercurrent exchange, is only possible if the two flows are, in some sense, "equal".

For a maximum transfer of substance concentration, an equal flowrate of [[solvent]]s and [[Solution (chemistry)|solution]]s is required. For maximum heat transfer, the average [[specific heat capacity]] and the mass flow rate must be the same for each stream. If the two flows are not equal, for example if heat is being transferred from water to air or vice versa, then, similar to cocurrent exchange systems, a variation in the gradient is expected because of a buildup of the property not being transferred properly.<ref>The specific heat capacity should be calculated on a mass basis, averaged over the temperature range involved. This is in keeping with the second law of thermodynamics</ref>

==Countercurrent exchange in biological systems==
[[Image:Circulus arteriosus schaf.jpg|right|thumb|120px|[[Rete mirabile]] = RM]]

Countercurrent exchange is used extensively in biological systems for a wide variety of purposes. For example, [[fish]] use it in their [[gill]]s to transfer oxygen from the surrounding water into their blood, and [[bird]]s use a countercurrent [[heat exchanger]] between blood vessels in their legs to keep heat concentrated within their bodies. In vertebrates, this type of organ is referred to as a [[rete mirabile]] (originally the name of the organ in the fish gills). Mammalian [[kidney]]s use countercurrent exchange to remove water from urine so the body can retain water used to move the nitrogenous waste products (see [[#Countercurrent multiplier|countercurrent multiplier]]).

=== Countercurrent multiplication loop ===
[[File:Countercurrentmultiplier.jpg|thumb|right|250 px|Counter current multiplication loop diagram]]
A countercurrent multiplication loop is a system where fluid flows in a loop so that the entrance and exit are at similar low concentration of a dissolved substance but at the far end of the loop there is a high concentration of that substance. A buffer liquid between the incoming and outgoing tubes receives the concentrated substance. The incoming and outgoing tubes do not touch each other.

The system allows the buildup of a high concentration gradually, by allowing a natural buildup of concentration towards the tip inside the in-going tube, (for example using osmosis of water out of the input pipe and into the buffer fluid), and the use of many [[active transport]] pumps each pumping only against a very small gradient, during the exit from the loop, returning the concentration inside the output pipe to its original concentration.

The incoming flow starting at a low concentration has a [[semipermeable membrane]] with water passing to the buffer liquid via [[osmosis]] at a small gradient. There is a gradual buildup of concentration inside the loop until the loop tip where it reaches its maximum.

Theoretically a similar system could exist or be constructed for heat exchange.

In the example shown in the image, water enters at 299&nbsp;mg/L (NaCl / H<sub>2</sub>O). Water passes because of a small [[osmotic pressure]] to the buffer liquid in this example at 300&nbsp;mg/L (NaCl / H<sub>2</sub>O). Further up the loop there is a continued flow of water out of the tube and into the buffer, gradually raising the concentration of NaCl in the tube until it reaches 1199&nbsp;mg/L at the tip. The buffer liquid between the two tubes is at a gradually rising concentration, always a bit over the incoming fluid, in this example reaching 1200&nbsp;mg/L. This is regulated by the pumping action on the returning tube as will be explained immediately.

The tip of the loop has the highest concentration of salt (NaCl) in the incoming tube—in the example 1199&nbsp;mg/L, and in the buffer 1200&nbsp;mg/L. The returning tube has active transport pumps, pumping salt out to the buffer liquid at a low difference of concentrations of up to 200&nbsp;mg/L more than in the tube. Thus when opposite the 1000&nbsp;mg/L in the buffer liquid, the concentration in the tube is 800 and only 200&nbsp;mg/L are needed to be pumped out. But the same is true anywhere along the line, so that at exit of the loop also only 200&nbsp;mg/L need to be pumped.

In effect, this can be seen as a gradually multiplying effect—hence the name of the phenomena: a 'countercurrent multiplier' or the mechanism: Countercurrent multiplication, but in current engineering terms, countercurrent multiplication is any process where only slight pumping is needed, due to the constant small difference of concentration or heat along the process, gradually raising to its maximum. There is no need for a buffer liquid, if the desired effect is receiving a high concentration at the output pipe.<ref>{{cite journal|doi=10.1021/ac00126a070 | volume=58 | issue=13 | title=Current multiplier for use with ultramicroelectrodes | year=1986 | journal=Analytical Chemistry | pages=2889–2891 | author=Hsuan Jung Huang, Peixin He, Faulkner Larry R}}</ref>

==== In the kidney ====
[[File:Kidney nephron molar transport diagram.svg|right|400px|Nephron Ion flow diagram]]
[[File:Gray1128.png|thumb|right|400px|Loop of Henle (''[[Gray's Anatomy]]'' book)]]
A circuit of fluid in the [[loop of Henle]]—an important part of the kidneys—allows for gradual buildup of the concentration of urine in the kidneys, by using [[active transport]] on the exiting [[nephron]]s (tubules carrying liquid in the process of gradually concentrating the urea). The active transport pumps need only to overcome a constant and low gradient of concentration, because of the countercurrent multiplier mechanism.<ref>See the [http://www.colorado.edu/intphys/Class/IPHY3430-200/countercurrent_ct.swf countercurrent multiplier animation] {{Webarchive|url=https://web.archive.org/web/20110606095648/http://www.colorado.edu/intphys/Class/IPHY3430-200/countercurrent_ct.swf |date=2011-06-06 }} at the [[University of Colorado]] website.</ref>

Various substances are passed from the liquid entering the nephrons until exiting the loop (See the nephron flow diagram). The sequence of flow is as follows:
* [[Renal corpuscle]]: Liquid enters the nephron system at the [[Bowman's capsule]].<ref>Beginning with the [[afferent arteriole]], a [[blood vessel]] leading to the [[Glomerulus]], filtered blood is passed to the nephrons in the Bowman's capsule which surrounds the Glomerulus. (The blood leaves the Glomerulus in the [[efferent arteriole]]).</ref>
* [[Proximal convoluted tubule]]: It then may reabsorb urea in the thick [[descending limb of loop of henle|descending limb]].<ref>The liquid from the Bowman's capsule reaches the thick descending limb. [[Urea]] may be reabsorbed into the low (300 [[mOsm]]) osmotic concentration in the limb nephrons. The urea absorption in the thick descending limb is inhibited by [[Sartan]]s and catalyzed by [[Lactic acid|lactates]] and [[ketone]]s.</ref> Water is removed from the nephrons by [[osmosis]] (and glucose and other ions are pumped out with [[active transport]]), gradually raising the concentration in the nephrons.<ref>[[Glucose]], [[amino acid]]s, various [[ions]] and organic material leave the limb, gradually raising the concentration in the nephrons. [[Dopamin]] inhibits the secretion from the thick descending limb, and [[Angiotensin II]] catalyzes it</ref>
* Loop of Henle Descending: The liquid passes from the thin descending limb to the thick ascending limb. Water is constantly released via osmosis.<ref>The semipermeable membrane of the thin descending limb does not permit passage of ions or large dissolved molecules</ref>{{citation needed|date=August 2015}} Gradually there is a buildup of osmotic concentration, until 1200&nbsp;mOsm is reached at the loop tip, but the difference across the membrane is kept small and constant.
:For example, the liquid at one section inside the thin descending limb is at 400&nbsp;mOsm while outside it is 401. Further down the descending limb, the inside concentration is 500 while outside it is 501, so a constant difference of 1 mOsm is kept all across the membrane, although the concentration inside and outside are gradually increasing.{{citation needed|date=August 2015}}
* Loop of Henle Ascending: after the tip (or 'bend') of the loop, the liquid flows in the ''thin'' [[Thin ascending limb of loop of Henle|ascending limb]].<ref>The thin ascending limb's membrane does not permit free passage of any substance including water.</ref>{{citation needed|date=August 2015}} Salt–[[sodium]] Na<sup>+</sup> and [[chloride]] Cl<sup>−</sup> ions are pumped out of the liquid<ref>[[Furosemide]] inhibits salt secretion from the thin ascending limb, while [[aldosterone]] catalyzes the secretion.</ref>{{citation needed|date=August 2015}} gradually lowering the concentration in the exiting liquid, but, using the [[#Countercurrent multiplier|countercurrent multiplier]] mechanism, always pumping against a constant and small osmotic difference.
:For example, the pumps at a section close to the bend, pump out from 1000&nbsp;mOsm inside the ascending limb to 1200&nbsp;mOsm outside it, with a 200&nbsp;mOsm across. Pumps further up the thin ascending limb, pump out from 400&nbsp;mOsm into liquid at 600&nbsp;mOsm, so again the difference is retained at 200&nbsp;mOsm from the inside to the outside, while the concentration both inside and outside are gradually decreasing as the liquid flow advances.

:The liquid finally reaches a low concentration of 100 mOsm when leaving the ''thin'' ascending limb and passing through the ''thick'' one<ref>Water or liquid with very low osmotic concentration leaving the nephrons is reabsorbed in the [[Peritubular capillaries]] and returned to the blood.</ref>
* [[Distal convoluted tubule]]: Once leaving the loop of Henle the thick ascending limb can optionally reabsorb and re increase the concentration in the nephrons.<ref>Reabsorbing and increasing the concentration is done by optionally absorbing [[potassium]] (K<sup>+</sup>) and [[hydrogen]] (H<sup>+</sup>) cations, while releasing water and the continued pumping out of calcium (Ca<sup>+</sup>) and salt (Na<sup>+</sup> and Cl<sup>−</sup> ions). The repeated concentration by secretion of calcium and salt ions is inhibited by [[thiazides]] and catalyzed by [[Aantidiuretic hormone]] and [[aldosterone]]</ref>
* [[Collecting duct]]: The collecting duct receives liquid between 100&nbsp;mOsm if no re-absorption is done, to 300 or above if re-absorption was used. The collecting duct may continue raising the concentration if required, by gradually pumping out the same ions as the Distal convoluted tubule, using the same gradient as the ascending limbs in the loop of Henle, and reaching the same concentration.<ref>[[Atrial natriuretic peptide]] and [[urodilatin]] inhibit water salt and calcium secretion from the collecting duct, while antidiuretic hormone and aldosterone catalyze it.</ref>
* Ureter: The liquid urine leaves to the [[ureter]].
* Same principle is used in hemodialysis within artificial kidney machines.

==== History ====
Initially the countercurrent exchange mechanism and its properties were proposed in 1951 by professor [[Werner Kuhn (chemist)|Werner Kuhn]] and two of his former students who called the mechanism found in the [[loop of Henle]] in mammalian [[kidneys]] a Countercurrent multiplier<ref>The original lecture was published in 1951 in German. [http://www.isbnlib.com/preview/1859734219/ According to a book on Jewish scientists under the Reich] Kuhn theorized and studied this mechanism already in the early 1940s. This was confirmed in 2001 in [http://jasn.asnjournals.org/content/12/7/1566.full.pdf the translation to the original lecture] published with remarks by Professor Bart Hargitay, then one of the two former student aids. Harbitay says: Before settling in Basel, Kuhn did some very fundamental work in Kiel, separating isotopes in a centrifuge. This caused him to be fascinated with the effect of countercurrents in multiplying a very small single effect to significant separations. (Journal of the American Society of Nephrology website)</ref> and confirmed by laboratory findings in 1958 by Professor [[Carl W. Gottschalk]].<ref>{{citation|last1=Gottschalk|first1=C. W.|author1-link=Carl W. Gottschalk|first2=M.|last2=Mylle|title=Evidence that the mammalian nephron functions as a countercurrent multiplier system|journal=Science|volume=128|issue=3324|year=1958|page=594|doi=10.1126/science.128.3324.594|pmid=13580223|bibcode=1958Sci...128..594G|s2cid=44770468}}.</ref> The theory was acknowledged a year later after a meticulous study showed that there is almost no osmotic difference between liquids on both sides of nephrons.<ref>{{citation|last1=Gottschalk|first1=C. W.|author1-link=Carl W. Gottschalk|first2=M.|last2=Mylle|title=Micropuncture study of the mammalian urinary concentrating mechanism: evidence for the countercurrent hypothesis|journal=American Journal of Physiology|volume=196|issue=4|pages=927–936|year=1959|pmid=13637248|doi=10.1152/ajplegacy.1959.196.4.927|doi-access=}}. See also [http://www.nature.com/ki/journal/v31/n2/abs/ki198729a.html History of the urinary concentrating mechanism] an article in 'Kidney'—the ''Journal of International Society of Nephrology'', where Prof. Gottschalk points to the heated debate prior to the acceptance of the theory of the countercurrent multiplier action of the kidney</ref> [[Homer W. Smith|Homer Smith]], a considerable contemporary authority on renal physiology, opposed the model countercurrent concentration for 8 years, until conceding ground in 1959.<ref name= HomerSmith>Smith, Homer W., The fate of sodium and water in the renal tubules, Bull. New York Academy of Medicine 35:293–316, 1959.</ref> Ever since, many similar mechanisms have been found in biologic systems, the most notable of these: the [[rete mirabile]] in fish.

=== Countercurrent exchange of heat in organisms ===

[[File:Arm counter-current flow.jpg|thumb|The arterial and deep vein blood supply to the human arm. The superficial (subcutaneous) veins are not shown. The deep veins are wrapped round the arteries, and the consequent counter-current flow allows the hand to be cooled down considerably without loss of body heat, which is short-circuited by the counter current flow.<ref name =knut>{{cite journal |last1=Schmidt-Nielsen |first1=Knut |title=Countercurrent systems in animals |journal=Scientific American | date=1981 |volume=244 |issue=May |pages=118–128 |doi=10.1038/scientificamerican0581-118 |pmid=7233149 |bibcode=1981SciAm.244e.118S }}</ref><ref name=grays>{{cite book |last1=Williams |first1=Peter L. |last2=Warwick |first2=Roger |last3=Dyson|first3=Mary |last4=Bannister |first4=Lawrence H. |title=Gray's Anatomy| pages=691–692, 791, 10011–10012 |location=Edinburgh |publisher=Churchill Livingstone | edition=Thirty-seventh |date=1989|isbn= 0443-041776 }}</ref>]]

In cold weather the blood flow to the limbs of birds and mammals is reduced on exposure to cold environmental conditions, and returned to the trunk via the deep veins which lie alongside the arteries (forming [[vena comitans|venae comitantes]]).<ref name=grays /><ref name=scholander>{{cite journal |last1=Scholander |first1=P. F. |title=The wonderful net |journal=Scientific American | date=1957 |volume=196 |issue=April |pages=96–110 |doi=10.1038/scientificamerican0457-96 |bibcode=1957SciAm.196d..96S }}</ref><ref>{{cite book |last1=Gilroy |first1=Anne M. |last2=MacPherson |first2=Brian R. |last3=Ross|first3=Lawrence M. |title=Atlas of Anatomy| pages=318, 349  |location=Stuttgart|publisher=Thieme Medical Publishers|date=2008|isbn= 978-1-60406-062-1 }}</ref> This acts as a counter-current exchange system which short-circuits the warmth from the arterial blood directly into the venous blood returning into the trunk, causing minimal heat loss from the extremities in cold weather.<ref name =knut /><ref name=grays /> The subcutaneous limb veins are tightly constricted, thereby reducing heat loss via this route, and forcing the blood returning from the extremities into the counter-current blood flow systems in the centers of the limbs. Birds and mammals that regularly immerse their limbs in cold or icy water have particularly well developed counter-current blood flow systems to their limbs, allowing prolonged exposure of the extremities to the cold without significant loss of body heat, even when the limbs are as thin as the [[Bird anatomy#Skeletal system|lower legs, or tarsi]], of a bird, for instance.<ref name=scholander />

When animals like the [[leatherback sea turtle|leatherback turtle]] and [[dolphins]] are in colder water to which they are not acclimatized, they use this CCHE mechanism to prevent heat loss from their [[Flipper (anatomy)|flippers]], tail flukes, and [[dorsal fin]]s. Such CCHE systems are made up of a complex network of peri-arterial venous [[plexuses]], or venae comitantes, that run through the blubber from their minimally insulated limbs and thin streamlined protuberances.<ref name=scholander /> Each plexus consists of a central artery containing warm blood from the heart surrounded by a bundle of veins containing cool blood from the body surface. As these fluids flow past each other, they create a heat gradient in which heat is transferred and retained inside the body. The warm arterial blood transfers most of its heat to the cool venous blood now coming in from the outside. This conserves heat by recirculating it back to the body core. Since the arteries give up a good deal of their heat in this exchange, there is less heat lost through [[convection]] at the periphery surface.<ref name=knut />

Another example is found in the legs of an [[Arctic fox#Adaptations|Arctic fox]] treading on snow. The paws are necessarily cold, but blood can circulate to bring nutrients to the paws without losing much heat from the body. Proximity of arteries and veins in the leg results in heat exchange, so that as the blood flows down it becomes cooler, and does not lose much heat to the snow. As the (cold) blood flows back up from the paws through the veins, it picks up heat from the blood flowing in the opposite direction, so that it returns to the torso in a warm state, allowing the fox to maintain a comfortable temperature, without losing it to the snow. This system is so efficient that the Arctic fox does not begin to shiver until the temperature drops to {{convert|-70|°C}}.

=== Countercurrent exchange in sea and desert birds to conserve water ===
Sea and desert birds have been found to have a [[salt gland]] near the nostrils which concentrates brine, later to be "sneezed" out to the sea, in effect allowing these birds to drink seawater without the need to find freshwater resources. It also enables the seabirds to remove the excess salt entering the body when eating, swimming or diving in the sea for food. The kidney cannot remove these quantities and concentrations of salt.<ref>{{Cite journal|last1=Schmidt-Nielsen|first1=Knut|last2=Fange|first2=Ragnar|date=July 1958|title=The Function of the Salt Gland in the Brown Pelican|journal=The Auk|volume=75|issue=3|pages=282–289|doi=10.2307/4081974|jstor=4081974 |issn=0004-8038|doi-access=free}}</ref><ref>{{Cite journal|last=Schmidt-Nielsen|first=Knut|date=1959|title=SALT GLANDS|url=https://www.jstor.org/stable/24944892|journal=Scientific American|volume=200|issue=1|pages=109–119|doi=10.1038/scientificamerican0159-109 |jstor=24944892 |pmid=13624738 |bibcode=1959SciAm.200a.109S |issn=0036-8733|url-access=subscription}}</ref>

The salt secreting gland has been found in seabirds like [[pelican]]s, [[petrel]]s, [[albatross]]es, [[gull]]s, and [[tern]]s. It has also been found in Namibian ostriches and other desert birds, where a buildup of salt concentration is due to dehydration and scarcity of drinking water.

In seabirds the salt gland is above the beak, leading to a main canal above the beak, and water is blown from two small nostrils on the beak, to empty it. The salt gland has two countercurrent mechanisms working in it:

a. A salt extraction system with a countercurrent multiplication mechanism, where salt is actively pumped from the blood 'venules' (small veins) into the gland tubules. Although the fluid in the tubules is with a higher concentration of salt than the blood, the flow is arranged in a countercurrent exchange, so that the blood with a high concentration of salt enters the system close to where the gland tubules exit and connect to the main canal. Thus, all along the gland, there is only a small gradient to climb, in order to push the salt from the blood to the salty fluid with [[active transport]] powered by [[Adenosine triphosphate|ATP]].

b. The blood supply system to the gland is set in countercurrent exchange loop mechanism for keeping the high concentration of salt in the gland's blood, so that it does not leave back to the blood system.

The glands remove the salt efficiently and thus allow the birds to drink the salty water from their environment while they are hundreds of miles away from land.<ref>{{cite book|last1=Proctor|first1=Noble S.|last2=Lynch|first2=Patrick J.|title=Manual of Ornithology|year=1993|publisher=Yale University Press}}</ref><ref>{{cite web|last=Ritchison|first=Gary|title=Avian osmoregulation|url=http://people.eku.edu/ritchisong/bird_excretion.htm|access-date=16 April 2011|archive-date=19 December 2019|archive-url=https://web.archive.org/web/20191219202005/http://people.eku.edu/ritchisong/bird_excretion.htm|url-status=dead}}</ref>

== Countercurrent exchange in industry and scientific research ==
[[Image:Hardendale Lime Works - geograph.org.uk - 73044.jpg|thumb|right|240px|Hardendale Lime Works in the UK using countercurrent kilns to reach high temperatures]]
Countercurrent Chromatography is a method of separation, that is based on the differential partitioning of analytes between two immiscible liquids using countercurrent or cocurrent flow.<ref>{{cite web|title=TheLiquidPhase|url=http://www.theliquidphase.org/|access-date=16 April 2011|url-status=dead|archive-url=https://web.archive.org/web/20080905223925/http://theliquidphase.org/|archive-date=5 September 2008}}</ref> Evolving from Craig's Countercurrent Distribution (CCD), the most widely used term and abbreviation is CounterCurrent Chromatography (CCC),<ref>{{cite web|title=Countercurrent Chromatography|url=http://tigger.uic.edu/~gfp/countercurrent/index2.htm|publisher=University of Illinois at Chicago|access-date=16 April 2011}}</ref> in particular when using hydrodynamic CCC instruments. The term partition chromatography is largely a synonymous and predominantly used for hydrostatic CCC instruments.
* [[Distillation]] of chemicals such as in petroleum refining is done in towers or columns with perforated trays. Vapor from the low boiling fractions bubbles upward through the holes in the trays in contact with the down flowing high boiling fractions. The concentration of low boiling fraction increases in each tray up the tower as it is "stripped". The low boiling fraction is drawn off the top of the tower and the high boiling fraction drawn from the bottom. The process in the trays is a combination of [[heat transfer]] and [[mass transfer]]. Heat is supplied at the bottom, known as a "reboiler" and cooling is done with a condenser at the top.
[[Image:Coflore ACX.png|thumb|left|120px|Counter flow in [[liquid–liquid extraction]]]]
* [[Liquid–liquid extraction]] (also called 'solvent extraction' or 'partitioning') is a common method for extracting a substance from one liquid into another liquid at a different 'phase' (such as "slurry"). This method, which implements a countercurrent mechanism, is used in [[nuclear reprocessing]], [[ore]] processing, the production of fine organic compounds, the processing of [[perfumes]], the production of [[vegetable oil]]s and [[biodiesel]], and other industries.
* [[Gold]] can be separated from a [[cyanide]] solution with the [[Merrill–Crowe process]] using Counter Current Decantation (CCD). In some mines, [[nickel]] and [[cobalt]] are treated with CCD, after the original ore was treated with concentrated [[sulfuric acid]] and steam in [[titanium]] covered [[autoclave]]s, producing nickel cobalt slurry. The nickel and cobalt in the slurry are removed from it almost completely using a CCD system exchanging the cobalt and nickel with [[flash steam]] heated water.

[[File:Countercurrent furnace.svg|thumb|300px|Countercurrent furnace (kiln) heat exchange]]
* [[Lime (substance)|Lime]] can be manufactured in countercurrent [[Metallurgical furnace|furnace]]s allowing the heat to reach high temperatures using low cost, low temperature burning fuel. Historically this was developed by the Japanese in certain types of the [[Anagama kiln]]. The kiln is built in stages, where fresh air coming to the fuel is passed downwards while the smoke and heat is pushed up and out. The heat does not leave the kiln, but is transferred back to the incoming air, and thus slowly builds up to {{Convert|3000|°C|abbr=on}} and more.

[[Image:CemKilnKiln.jpg|thumb|right|220px|Cement counter-current rotary kiln]]
* [[Cement]] may be created using a countercurrent kiln where the heat is passed in the cement and the exhaust combined, while the incoming air draft is passed along the two, absorbing the heat and retaining it inside the furnace, finally reaching high temperatures.
* [[Gasification]]: the process of creating [[methane]] and [[carbon monoxide]] from organic or fossil matter, can be done using a [[Gasification#Counter-current fixed bed ("up draft") gasifier|counter-current fixed bed ("up draft") gasifier]] which is built in a similar way to the Anagama kiln, and must therefore withstand more harsh conditions, but reaches better efficiency.
* In nuclear power plants, water leaving the plant must not contain even trace particles of Uranium. Counter Current Decantation (CCD) is used in some facilities to extract water, totally clear of Uranium.
[[Image:MULTI-STk.jpg|thumb|350px|Exchange current decantation depicted in centrifugal extractors as 1st stage]]
* [[Zippe-type centrifuge]]s use countercurrent multiplication between rising and falling convection currents to reduce the number of stages needed in a cascade.
* Some [[Centrifugal extractor]]s use counter current exchange mechanisms for extracting high rates of the desired material.
* Some [[protein skimmer]]s (devices used to clean saltwater pools and fish ponds of organic matter) use [[protein skimmer#Counter-current flow systems|counter current technologies]].
* Countercurrent processes have also been used to study the behavior of small animals and isolate individuals with altered behaviors due to genetic mutations.<ref>{{cite journal | author = Benzer Seymour | year = 1967 | title = Behavioral Mutants Of Drosophila Isolated By Countercurrent Distribution | url = http://authors.library.caltech.edu/5225/1/BENpnas67.pdf| journal = Proceedings of the National Academy of Sciences USA | volume = 58 | issue = 3| pages = 1112–1119 | doi=10.1073/pnas.58.3.1112| pmid = 16578662 | pmc = 335755 | bibcode = 1967PNAS...58.1112B | doi-access = free }}</ref><ref>{{cite journal | author = Dusenbery David B | year = 1973 | title = Countercurrent separation: A new method for studying behavior of small aquatic organisms | journal = Proceedings of the National Academy of Sciences USA | volume = 70 | issue = 5| pages = 1349–1352 | doi=10.1073/pnas.70.5.1349| pmid = 4514305 | pmc = 433494| bibcode = 1973PNAS...70.1349D | doi-access = free }}</ref><ref>{{cite journal | author = Dusenbery David B., Sheridan Robert E., Russell Richard L. | year = 1975 | title = Chemotaxis-Defective Mutants of the Nematode ''Caenorhabditis elegans'' | journal = Genetics | volume = 80 | issue = 2| pages = 297–309 | doi = 10.1093/genetics/80.2.297 | pmid = 1132687 | pmc = 1213328 }}</ref>

== See also ==
* [[Anagama kiln]]
* [[Bidirectional traffic]]
* [[Economizer]]
* [[Heat recovery ventilation]]
* [[Regenerative heat exchanger]]
* [[Countercurrent multiplier]]

== References ==
{{reflist|30em}}

== External links ==
* [https://web.archive.org/web/20110606095648/http://www.colorado.edu/intphys/Class/IPHY3430-200/countercurrent_ct.swf Countercurrent multiplier animation] from Colorado University.
* [https://cob.silverchair-cdn.com/cob/content_public/journal/jeb/113/1/10.1242_jeb.113.1.447/4/jexbio_113_1_447.pdf?Expires=1724002965&Signature=dcyAoYwlDu4xcdOgLg-dKWEM65MidurECC3AF-eD3QFsPhOTsOgw8siFsiy9M7-nAJ6fvTmwupOd1mab5h24yIu-eJ0pFLY~3STm~xMUdYnBd6GR5UKIN7YFOrHB~xs4MczW79QXfgxAHO3UGb~5K4fipIngGZtVyyIDq2cPFPEe5nWam-DRb5ej~oVZ~xLBcFiZUU80Spa9pV36dNgVxGPdNh8o~3uhnwtA2LZRGLcxSy08hfpsYmWBMNYwgnm8mVvuHLj2JuJpeoUlurct9fIQ6oyk6gQItUCkAuYhRU7Y4aFo11cf1VZd6IdsxFtir0MGr2rtCyvwTpVIy-1vrw__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA Research about elephant seals] using countercurrent heat exchange to keep heat from leaving their body while breathing out, during [[hibernation]].
* [https://patents.google.com/patent/US4520509 Patent for a snow mask with a removable countercurrent exchange module] which keeps the warmth from leaving the mask when breathing out.

[[Category:Chemical process engineering]]
[[Category:Industrial processes]]
[[Category:Animal anatomy]]
[[Category:Renal physiology]]
[[Category:Heat transfer]]