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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 well.{Respicius Rwehumbiza, 2010}
This allows for a countercurrent exchange system whereby the medulla becomes increasingly concentrated, but at the same time setting up an osmotic gradient for water to follow should the aquaporins of the collecting duct be opened by ADH.
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 ...
In other words, the length of the loop of Henle limits the concentration of the gradient, i.e., the longer the loop, the greater the osmotic gradient. Thus, longer loops would allow for steeper gradients and greater capacity to concentrate urine. Through the countercurrent multiplier the loop of Henle increases the osmolarity of the medulla.
The presence of aquaporin-1 channels in the thin segment facilitates high water permeability, crucial for water reabsorption as part of the kidney's countercurrent exchange mechanism. [ 4 ] [ 5 ] They can be distinguished from the vasa recta by the absence of blood, and they can be distinguished from the thick ascending limb by the thickness of ...
The ability of mammals and some birds to concentrate wastes into a volume of urine much smaller than the volume of blood from which the wastes were extracted is dependent on an elaborate countercurrent multiplication mechanism. This requires several independent nephron characteristics to operate: a tight hairpin configuration of the tubules ...
A countercurrent system in the renal medulla provides the mechanism for generating a hypertonic interstitium, which allows the recovery of solute-free water from within the nephron and returning it to the venous vasculature when appropriate. Some diseases of the nephron predominantly affect either the glomeruli or the tubules.
The predominant mechanism of active transport in this segment is through the Na + /K + /Cl − co-transporter NKCC2 as well as the sodium/hydrogen exchanger NHE3. [4] In total this segment accounts for approximately 25–30% of total Na + reabsorption along the nephron.