Within the

nephron The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structu ...

of the

kidney In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...

, the ascending limb of the loop of Henle is a segment of the heterogenous

loop of Henle In the kidney, the loop of Henle () (or Henle's loop, Henle loop, nephron loop or its Latin counterpart ''ansa nephroni'') is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after it ...

downstream of the descending limb, after the sharp bend of the loop. This part of the renal tubule is divided into a thin and thick ascending limb; the thick portion is also known as the distal straight tubule, in contrast with the distal convoluted tubule downstream.

Structure

The ascending limb of the loop of Henle is a direct continuation from the descending limb of loop of Henle, and one of the structures in the

of the kidney. The ascending limb has a thin and a thick segment. The ascending limb drains urine into the distal convoluted tubule. The thin ascending limb is found in the medulla of the kidney, and the thick ascending limb can be divided into a part that is in the

renal medulla The renal medulla (Latin: ''medulla renis'' 'marrow of the kidney') is the innermost part of the kidney. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, which ...

and a part that is in the

renal cortex The renal cortex is the outer portion of the kidney between the renal capsule and the renal medulla. In the adult, it forms a continuous smooth outer zone with a number of projections ( cortical columns) that extend down between the pyramids. I ...

. The ascending limb is much thicker than the descending limb. At the junction of the thick ascending limb and the distal convoluted tubule are a subset of 15–25 cells known as the macula densa that are part of

renal autoregulation Autoregulation is a process within many biological systems, resulting from an internal adaptive mechanism that works to adjust (or mitigate) that system's response to stimuli. While most systems of the body show some degree of autoregulation, i ...

through the mechanism of tubuloglomerular feedback.

Histology

As in the descending limb, the epithelium is

simple cuboidal epithelium Simple cuboidal epithelium is a type of epithelium that consists of a single layer of cuboidal (cube-like) cells which have large, spherical and central nuclei. Simple cuboidal epithelium is found on the surface of ovaries, the lining of nephro ...

Function

Thin ascending limb

The thin ascending limb is impermeable to water; but is permeable to ions allowing for some sodium reabsorption. Na/K-ATPase is expressed at very low levels in this segment and thus this reabsorption is likely through passive diffusion. Salt moves out of the tubule and into the interstitium due to osmotic pressure created by the countercurrent system.

Thick ascending limb

Functionally, the parts of the ascending limb in the medulla and cortex are very similar. The medullary ascending limb is largely impermeable to water.

Sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

(Na⁺),

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

(K⁺) and

chloride The term chloride refers to a compound or molecule that contains either a chlorine anion (), which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (). The pr ...

(Cl⁻) ions are reabsorbed by

active transport In cellular biology, active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellula ...

. The predominant mechanism of active transport in this segment is through the Na⁺/K⁺/Cl⁻ co-transporter NKCC2 as well as the sodium/hydrogen exchanger NHE3. In total this segment accounts for approximately 25–30% of total Na⁺ reabsorption along the nephron. This is of clinical importance since commonly used "loop diuretics" act by inhibiting the NKCC2. This active transport enables the kidney to establish an osmotic gradient that is essential to the kidneys ability to concentrate the urine past

isotonicity In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a partially-permeable cell membrane. Tonicity depends on the relative concentration of selective membra ...

. K⁺ is passively transported along its concentration gradient through a K⁺ leak channel in the apical aspect of the cells, back into the lumen of the ascending limb. This K⁺ "leak" generates a positive electrochemical potential difference in the lumen. This drives more paracellular reabsorption of Na⁺, as well as other

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s such as

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

(Mg²⁺) and importantly

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

Ca²⁺ due to charge repulsion. This is also the part of the tubule that generates Tamm–Horsfall protein. The function of this protein is not well understood, but is responsible for creating urinary casts.

Structure

Histology

Function

Thin ascending limb

Thick ascending limb

Clinical significance

See also

References

External links