Countercurrent multiplication
Encyclopedia
A countercurrent multiplier system is a mechanism that expends energy to create a concentration gradient.
It is found widely in nature and especially in mamalian organs. For example, it can refer to the process underlying the process of urine
concentration, that is, the production of hyperosmotic urine by the mammalian kidney
.
The ability to concentrate urine is also present in bird
s, but involves another mechanism which is not comparable.
Countercurrent multiplication is frequently mistaken for Countercurrent exchange
a similar but different mechanism where gradients are maintained, but not established.
, which consists of two parallel limbs of renal tubules running in opposite directions, separated by the interstital space of the renal medulla
.
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). In a hypothetical model where there was no equilibration or pump steps, the tubular fluid and interstitial osmolarity would be 300 mOsm 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. Interestingly, 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.
It is found widely in nature and especially in mamalian organs. For example, it can refer to the process underlying the process of urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
concentration, that is, the production of hyperosmotic urine by the mammalian kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
.
The ability to concentrate urine is also present in bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s, but involves another mechanism which is not comparable.
Countercurrent multiplication is frequently mistaken for Countercurrent exchange
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 component, between two flowing bodies flowing in opposite directions to each other. The flowing bodies can be liquids, gases, or...
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 HenleLoop of Henle
In the kidney, the loop of Henle is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its discoverer F. G. J...
, which consists of two parallel limbs of renal tubules running in opposite directions, separated by the interstital space of the renal medulla
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 arcuate arterioles. The arcuate arterioles each in turn branch into...
.
- Water flows from the tubular fluid of the descending limb of the loop of Henle into the medullary space.
- The ascending limb is impermeable to water (because of a lack of aquaporinAquaporinAquaporins are proteins embedded in the cell membrane that regulate the flow of water.Aquaporins are integral membrane proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells....
, a common transporter protein for water channels in all cells except the walls of the ascending limb of the loop of HenleLoop of HenleIn the kidney, the loop of Henle is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its discoverer F. G. J...
), but here Na+, Cl-, and K+ are actively transported into the medullary space, making the filtrate hypotonic (with a higher water potential). This constitutes the single effect of the countercurrent multiplication process. - Active transportActive transportActive transport is the movement of a substance against its concentration gradient . In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine...
of these ions from the thick ascending limb creates an osmotic 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 describing the mechanism whereby urine is concentrated in the nephron. Initially studied in the 1950s by GottschalkCarl W. Gottschalk
Carl William Gottschalk was the Kenan Professor and Distinguished Research Professor of Medicine at the University of North Carolina at Chapel Hill...
and Mylle following Werner Kuhn
Werner Kuhn
Werner Kuhn is a Swiss physical chemist who developed the first model of the viscosity of polymer solutions using statistical mechanics. He is known for being the first to apply Boltzmann's entropy formula:S = k \log W \!...
'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). In a hypothetical model where there was no equilibration or pump steps, the tubular fluid and interstitial osmolarity would be 300 mOsm 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. Interestingly, 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.