A countercurrent mechanism system is a mechanism that expends energy to create a

concentration gradient Molecular diffusion, often simply called diffusion, is the thermal motion of all (liquid or gas) particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size (mass) o ...

. It is found widely in nature and especially in mammalian organs. For example, it can refer to the process that is underlying the process of

urine Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excreted from the body through the urethra. Cellular ...

concentration, that is, the production of

hyperosmotic In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a partially-permeable cell membrane. Tonicity depends on the relative concentration of selective membrane- ...

urine by the

mammalian Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...

kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blo ...

. The ability to concentrate urine is also present in

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

s. Countercurrent multiplication is frequently mistaken for

countercurrent exchange Countercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some chemical, between two flowing bodies flowing in opposite directions to each ...

, a similar but different mechanism where gradients are maintained, but not established.

Physiological principles

The term derives from the form and function of the

loop of Henle In the kidney, the loop of Henle () (or Henle's loop, Henle loop, nephron loop or its Latin counterpart ''ansa nephroni'') is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its ...

, which consists of two parallel limbs of

renal The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; bloo ...

tubules running in opposite directions, separated by the interstitial space of the

renal medulla The renal medulla is the innermost part of the kidney. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, which then splits up to form the segmental arteries whi ...

. * The descending limb of the loop of Henle is permeable to water but impermeable to solutes, due to the presence of

aquaporin 1 Aquaporin 1 (AQP-1) is a protein that in humans is encoded by the ''AQP1'' gene. AQP-1 is a widely expressed water channel, whose physiological function has been most thoroughly characterized in the kidney. It is found in the basolateral and api ...

in its tubular wall. Thus, water moves across the tubular wall into the medullary space, making the filtrate hypertonic (with a lower water potential). This is the filtrate that continues to the ascending limb. * The ascending limb is impermeable to water (because of a lack of

aquaporin Aquaporins, also called water channels, are channel proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells, mainly facilitating transport of water between cells. The cell membranes of a ...

, a common transporter protein for water channels in all cells except the walls of the ascending limb of the

) but permeable to solutes, but here Na⁺, Cl⁻, and K⁺ are actively transported into the medullary space, making the filtrate hypotonic (with a higher water potential). The interstitium is now "salty" or hypertonic, and will attract water as below. This constitutes the ''single effect'' of the countercurrent multiplication process. *

Active transport In cellular biology, ''active transport'' is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellul ...

of these ions from the thick ascending limb creates an

osmotic Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region o ...

pressure drawing water from the descending limb into the hyperosmolar medullary space, making the filtrate hypertonic (with a lower water potential). * The countercurrent flow within the descending and ascending limb thus increases, or ''multiplies'' the osmotic gradient between tubular fluid and interstitial space.

Details

Countercurrent multiplication was originally studied as a mechanism whereby urine is concentrated in the nephron. Initially studied in the 1950s by

Gottschalk Gottschalk or Godescalc (Old High German) is a male German name that can be translated literally as " servant of God". Latin forms include ''Godeschalcus'' and ''Godescalcus''. Given name * Godescalc of Benevento, 8th-century Lombard duke *Godescal ...

and Mylle following Werner Kuhn's postulations, this mechanism gained popularity only after a series of complicated micropuncture experiments.. The proposed mechanism consists of pump, equilibration, and shift steps. In the proximal tubule, the osmolarity is isomolar to plasma (300 mOsm/L). In a hypothetical model where there was no equilibration or pump steps, the tubular fluid and interstitial osmolarity would be 300 mOsm/L as well. Pump: The Na⁺/K⁺/2Cl⁻ transporter in the ascending limb of the loop of Henle helps to create a gradient by shifting Na⁺ into the medullary interstitium. The thick ascending limb of the loop of Henle is the only part of the nephron lacking in aquaporin—a common transporter protein for water channels. This makes the thick ascending limb impermeable to water. The action of the Na⁺/K⁺/2Cl⁻ transporter therefore creates a hypoosmolar solution in the tubular fluid and a hyperosmolar fluid in the interstitium, since water cannot follow the solutes to produce osmotic equilibrium. Equilibration: Since the descending limb of the loop of henle consists of very leaky epithelium, the fluid inside the descending limb becomes hyperosmolar. Shift: The movement of fluid through the tubules causes the hyperosmotic fluid to move further down the loop. Repeating many cycles causes fluid to be near isosmolar at the top of Henle's loop and very concentrated at the bottom of the loop. Animals with a need for very concentrated urine (such as desert animals) have very long loops of Henle to create a very large osmotic gradient. Animals that have abundant water on the other hand (such as beavers) have very short loops. The vasa recta have a similar loop shape so that the gradient does not dissipate into the plasma. The mechanism of counter current multiplication works together with the vasa recta's counter current exchange to prevent the wash out of salts and maintain a high osmolarity at the inner medulla.

References

The Loop of Henle: Concentration
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University of Liverpool , mottoeng = These days of peace foster learning , established = 1881 – University College Liverpool1884 – affiliated to the federal Victoria Universityhttp://www.legislation.gov.uk/ukla/2004/4 University of Manchester Act 200 ...

Countercurrent Multiplier Animation
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University of Colorado The University of Colorado (CU) is a system of public universities in Colorado. It consists of four institutions: University of Colorado Boulder, University of Colorado Colorado Springs, University of Colorado Denver, and the University o ...

{{DEFAULTSORT:Countercurrent Multiplication Membrane biology