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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
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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
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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
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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.
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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.
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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
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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.
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Countercurrent multiplication was originally studied as a mechanism whereby urine is concentrated in the nephron. Initially studied in the 1950s by
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pressure drawing water from the descending limb into the hyperosmolar medullary space, making the filtrate hypertonic (with a lower water potential).
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Since the descending limb of the loop of Henle consists of very leaky epithelium, the fluid inside the descending limb becomes hyperosmolar.
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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
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The descending limb of the loop of Henle is permeable to water but impermeable to solutes, due to the presence of
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360:; Mylle, M. (1958), "Evidence that the mammalian nephron functions as a countercurrent multiplier system",
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333:(7 ed.). New Delhi, India: Jaypee Brothers Medical Publishers (P) Ltd. pp. 328–333.
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Braun, Eldon (April 1998), "Comparative renal function in reptiles, birds, and mammals",
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The countercurrent flow within the descending and ascending limb thus increases, or
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is now "salty" or hypertonic, and will attract water as below. This constitutes the
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tubules running in opposite directions, separated by the interstitial space of the
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The ascending limb is impermeable to water (because of a lack of
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of these ions from the thick ascending limb creates an
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329:Sembulingam, K (2016).
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