kidney loop of Henle long descending thin limb outer medulla

This is the segment of the long descending thin limb of the loop of Henle located within the outer medulla of the kidney. It connects the pars recta of the proximal tubule (descending from the cortex) to the inner medullary segment of the long descending thin limb, passing through the outer medullary zone.

The outer medullary segment of the long descending thin limb is highly permeable to water but has low permeability to solutes such as sodium and chloride. As tubular fluid passes through this segment, it equilibrates osmotically with the surrounding interstitium by losing water, progressively concentrating the intraluminal fluid as it descends toward the inner medulla.

The outer medulla is divided into inner and outer stripes and is characterized by active solute transport in the thick ascending limb of the loop of Henle, which creates the osmotic gradient that drives water reabsorption in descending limbs. The outer medullary interstitium is less hyperosmotic than the inner medulla, and urea contributes less to the osmotic gradient here compared to the inner medulla.

In the outer medulla, the long descending thin limbs of the loop of Henle are bundled together in vascular bundles with the descending and ascending vasa recta (the capillaries of the medulla). This close anatomical arrangement facilitates countercurrent exchange of solutes and water between the vessels and tubules, helping to maintain the medullary osmotic gradient.

Only nephrons with glomeruli in the juxtamedullary cortex (juxtamedullary nephrons, roughly 15% of all nephrons) have long loops that extend into the inner medulla. The outer medullary portion of these long descending thin limbs is essential for maximum urinary concentration. Damage to medullary structures in conditions such as sickle cell disease, analgesic nephropathy, or acute tubular necrosis can impair concentrating ability.