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

Loop of Henle

In the kidney, the loop of Henle is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its discoverer F. G. J...

.

Physiology

The permeability is as follows:

Substance

Permeability >- | ions

Sodium Sodium Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...  and chloride Chloride The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...  ions do not easily pass through. >- | urea Urea Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....

>- | water Water Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

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

Osmosis

Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...

 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 of the Thick segment is low simple cuboidal epithelium. The epithelium of the Thin segment is simple squamous

Simple squamous epithelium

A simple squamous epithelium is a single layer of flat cells in contact with the basal lamina of the epithelium. This type of epithelium is often permeable and occurs where small molecules pass quickly through membranes via filtration or diffusion...

.

They can be distinguished from the vasa recta

Vasa recta

In the blood supply of the kidney, the vasa recta renis form a series of straight capillaries in the medulla...

 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

Thin ascending limb of loop of Henle

The thin ascending limb of loop of Henle is a sub-portion of the loop of Henle in the juxtamedullary nephron of the kidney. The thin ascending limb is impermeable to water, and is also permeable to ions in particular Na and Cl...

would be included.