Descending limb of loop of Henle

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The descending limb of loop of Henle is the portion of the renal tubule constituting the first part of the loop of Henle.

Physiology

The permeability is as follows:

Substance	Permeability
ions	Low permeability. Sodium and chloride ions do not easily pass through.
urea	Moderate permeability.
water	Highly permeable. Water is readily reabsorbed from the descending limb by osmosis.

Also, the medullary interstitium is highly concentrated (because of the activity of the ascending limb), leading to a strong osmotic gradient from the descending limb to the medulla.

Because of these factors, the concentration of the urine increases dramatically in the descending limb. Osmolality can reach up to 1400 mOsmol/kg by the end of the descending limb.

Histology

The epithelium is low simple cuboidal.

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 epithelium.

Nomenclature

Like the ascending limb, the descending limb has thick and thin portions. However, this distinction is not as important physiologically as in the ascending limb, so often the two are treated as one structure. The thick descending limb is less important than the thin descending limb, so often the terms "descending limb" and "thin descending limb" are used interchangeably.

Some sources simply refer to a "thin limb". In this context, the thin ascending limb of loop of Henle would be included.

Additional images

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  Longitudinal section of descending limb of Henle’s loop. a. Membrana propria. b. Epithelium.