Thin Limbs of the Loop of Henle

The thin descending limb of the loop of Henle begins in the outer medulla as a gradual transition from the pars recta and ends with a hairpin turn at the tip of the loop of Henle. In general, thin limbs from juxtamedullary nephrons descend deeply into the medulla, while thin limbs from more superficial nephrons descend only superficially into the medulla. After the hairpin turn, the ascending limb begins. While juxtamedullary nephrons have well developed thin ascending limbs that end at the junction of the inner and outer medulla, more superficial nephrons lack thin ascending limbs.

Transport in the thin limbs is mostly passive. As fluid descends in the thin descending limb, it is exposed to an increasingly hypertonic interstitium. Because the water permeability of descending segments is relatively high, and solute permeabilities are relatively low, most of the osmotic equilibration occurs by water absorption. This leads to a fluid that arrives at the tip of the loop of Henle that is almost isosmotic with the interstitium, but is very different in com-

The Loop Henule Pics

thin descending limb, luminal NaCl and urea are concentrated by water abstraction. The thin ascending limb has a high NaCl permeability and low permeabilities to urea and water. As the tubular fluid travels along the thin ascending limb, there is passive NaCl absorption. Although there is a large gradient for urea secretion, permeability is low.

thin descending limb, luminal NaCl and urea are concentrated by water abstraction. The thin ascending limb has a high NaCl permeability and low permeabilities to urea and water. As the tubular fluid travels along the thin ascending limb, there is passive NaCl absorption. Although there is a large gradient for urea secretion, permeability is low.

position. Such luminal fluid has significantly less urea, but significantly higher concentrations of NaCl than are present in the medullary interstitium (Fig. 6).

The thin ascending limb has a very low water permeability and very high permeabilities to Na+ and Cl". CI" permeability is two to four times that of Na+ permeability. While there is some permeability to urea, it is significantly lower than NaCl permeability. As fluid passes along the thin ascending limb, the transepithelial NaCl concentration gradient (lumen greater than interstitium) leads to significant passive NaCl absorption. While there is also a passive gradient for urea to enter the lumen, the lower urea permeability limits this flux. Because the Cl ~ permeability is significantly larger than the Na + permeability, passive NaCl absorption leads to a lumen-positive voltage. This lumenpositive voltage serves as a passive driving force for absorption of all cations, including K + , Ca2+, and Mg2+. While in theory, Na + and Cl " absorption could occur across the paracellular pathway, significant evidence suggests that at least part of Cl " reabsorption is transcellular and takes place across apical and basolateral membrane Cl ~ conductances.

It should be noted that all of the driving forces for passive transport are established by passive H20 absorption in the thin descending limb. Thus, any condition which alters medullary interstitial osmolality will also affect passive H20 absorption in the thin descending limb and secondarily regulate passive solute fluxes in the thin ascending limb. This effect is most pronounced with loop diuretics which almost completely inhibit the generation of a hypertonic medullary interstitium.

Water abstraction in the thin descending limb causes a significant increase in the concentration of HC03" and pH in the luminal fluid. Under most conditions, there is no significant transepithelial flux of NaHC03 in the thin limbs. One exception is in the presence of carbonic anhydrase inhibitors. Here, decreased rates of proximal tubule HC03" absorption and normal rates of water abstraction in the thin descending limb combine to generate very high luminal concentrations of HC03" and H2C03, which provides a driving force for significant passive absorption. While the specific ion species transported have not been established, the low ionic permeability of the thin descending limb and the limited ability to form C02 in the absence of carbonic anhydrase suggest that the transported species is H2C03.

Driven by the high medullary interstitial K + concentration, K+ diffuses passively into the thin descending limb. Most of this K + is reabsorbed in the thin and thick ascending limbs and the medullary collecting duct. The physiologic purpose of this "K + recycling" is presently not clear. Possibly, the accumulation and high concentration of K + in the medullary interstitium may limit the passive loss of K + from the collecting ducts. In the thin ascending limb, K + absorption is passive, driven by the lumen positive voltage.

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Responses

  • sean
    Are aquaporins present in thin descending limb of Henle?
    11 months ago
  • caitlin wilson
    Which is hyeophobic to water ascending limb or descending limb?
    10 months ago

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