Editing Countercurrent exchange (section)

==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>