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 mg/l, and in the buffer 1200 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 mg/l more than in the tube. thus when opposite the 1000 mg/l in the buffer liquid, the concentration in the tube is 800 and only 200 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 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. in the kidney nephron ion flow diagram 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 nephrons (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 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. proximal convoluted tubule: it then may reabsorb urea in the thick descending limb. 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. loop of henle descending: the liquid passes from the thin descending limb to the thick ascending limb. water is constantly released via osmosis. gradually there is a buildup of osmotic concentration, until 1200 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 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. loop of henle ascending: after the tip (or 'bend') of the loop, the liquid flows in the thin ascending limb. salt–sodium na+ and chloride cl− ions are pumped out of the liquid gradually lowering the concentration in the exiting liquid, but, using the 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 mosm inside the ascending limb to 1200 mosm outside it, with a 200 mosm across. pumps further up the thin ascending limb, pump out from 400 mosm into liquid at 600 mosm, so again the difference is retained at 200 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 distal convoluted tubule: once leaving the loop of henle the thick ascending limb can optionally reabsorb and re increase the concentration in the nephrons. collecting duct: the collecting duct receives liquid between 100 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. 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 and two of his former students who called the mechanism found in the loop of henle in mammalian kidneys a countercurrent multiplier and confirmed by laboratory findings in 1958 by professor carl w. gottschalk. 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. homer smith, a considerable contemporary authority on renal physiology, opposed the model countercurrent concentration for 8 years, until conceding ground in 1959. 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 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. 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 venae comitantes). 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. 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 lower legs, or tarsi, of a bird, for instance. when animals like the 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 flippers, tail flukes, and dorsal fins. 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. 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. another example is found in the legs of an 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 doesn't 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 −70 °c (−94 °f). 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. the salt secreting gland has been found in seabirds like pelicans, petrels, albatrosses, gulls, and terns. 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 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 doesn't 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. countercurrent exchange in industry and scientific research 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. evolving from craig's countercurrent distribution (ccd), the most widely used term and abbreviation is countercurrent chromatography or ccc, 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. 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 oils 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 autoclaves, 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. countercurrent furnace (kiln) heat exchange lime