kidney loop of Henle long descending thin limb inner medulla

This structure is the segment of the long descending thin limb of the loop of Henle that traverses the inner medulla of the kidney. It forms the descending portion of the hairpin loop that extends deep into the renal medulla, and it is specifically the segment located within the innermost zone of the medulla.

The inner medullary segment of the long descending thin limb is highly permeable to water but relatively impermeable to sodium and urea. As tubular fluid descends through the hyperosmotic inner medullary interstitium, water is drawn out by osmosis, concentrating the tubular fluid. This progressive water removal contributes to the countercurrent multiplier system that generates the medullary osmotic gradient.

The inner and outer medullary segments of the long descending thin limb differ in their transport properties. The inner medullary segment has high water permeability and low solute transport activity. The outer medullary segment also has high water permeability. As the limb transitions to the ascending thin limb in the inner medulla, the tubule becomes relatively water-impermeable but permeable to sodium chloride.

Urea is a key component of the inner medullary osmotic gradient. The collecting duct in the inner medulla is permeable to urea under the influence of antidiuretic hormone, allowing urea to accumulate in the inner medullary interstitium. The long descending thin limb in the inner medulla is exposed to this high urea concentration, which contributes to the osmotic equilibration of tubular fluid as it descends.

At the hairpin bend at the tip of the loop of Henle, the descending thin limb transitions to the ascending thin limb of the loop. This transition occurs at the deepest point of penetration into the inner medulla. At this bend, tubular fluid has reached its maximum concentration and minimum volume, and the subsequent ascending segment begins the process of actively diluting the tubular fluid as it returns toward the cortex.