Countercurrent multiplication

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The term countercurrent multiplication refers to the process underlying the process of urine concentration, that is, the production of hyperosmolar urine by the mammalian kidney. The ability to concentrate urine is also present in birds, but involves another mechanism which is not comparable.

[edit] Physiological principles

The term derives from the form and function of Henle's loop, which consists of two parallel limbs of renal tubules running in opposite directions, separated by the interstital space of the renal medulla.

• Water flows from the tubular fluid of the descending limb of Henle's loop into the medullary space.
• The ascending limb is impermeable to water, but here NaCl is actively reabsorbed into the medullary space, effectively diluting the tubular fluid. This constitutes the single effect of the countercurrent multiplication process.
• Reabsorbtion of salt from the ascending loop creates an osmotic force drawing water from the descending limb into the hyperosmolar medullar space, further concentrating tubular fluid.
• The countercurrent flow within the descending and ascending limb thus increases, or multiplies the osmotic gradient between tubular fluid and interstitial space.

[edit] Details

Countercurrent multiplication is a hypothesis describing the mechanism whereby urine is concentrated in the nephron. Initially proposed in the 1950s by Gottschalk and Mylle^[1], this mechanism gained popularity only after a series of complicated micropuncture experiments that listed as reference 2^[2]. The proposed mechanism consists of pump, equilibration, and shift steps. In the proximal tubule, the osmolarity is isosmolar 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 interstitum. Since the ascending limb of the loop of Henle consists of epithelium containing many tight junctions impermeable to water, this creates a hypoosmolar solution in the tubular fluid and a hyperosmolar fluid in the interstitium. Equilibration: Since the descending limb of the loop of henle consists of very leaky epithelium, the fluid inside the descending limb becomes hyperosmolar as well. 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.

[edit] References

• The Loop of Henle: Concentration at University of Liverpool