Loop of Henle

Loop of Henle
Scheme of renal tubule and its vascular supply (loop of Henle visible center-left)
Details
Precursor	Metanephrogenic blastema
Identifiers
Latin	ansa nephroni
MeSH	D008138
FMA	17718 17698, 17718
Anatomical terminology [edit on Wikidata]

In the

• Thin descending limb of loop of Henle

The thin descending limb has low permeability to ions and urea while being highly permeable to water. The loop has a sharp bend in the renal medulla going from descending to ascending thin limb.

• Thin ascending limb of loop of Henle

The thin ascending limb is impermeable to water, but it is permeable to ions.

• Ascending limb of loop of Henle

Sodium (Na⁺),

cations such as magnesium (Mg²⁺) and calcium

(Ca²⁺).

The cortical thick ascending limb drains urine into the distal convoluted tubule.^[3]

The tissue type of the loop is simple

The loop of Henle is supplied by blood in a series of straight capillaries descending from the cortical efferent arterioles. These capillaries (called the

As well, blood in the vasa recta still has large proteins and ions which were not filtered through the glomerulus. This provides an oncotic pressure for ions to enter the vasa recta from the interstitium.^{[citation needed]}

The main function of the loop of Henle is to set up a concentration gradient.^{[citation needed]}

Physiology

The descending loop of Henle receives

The interstitium of the kidney increases in osmolarity outside as the loop of Henle descends from 600 mOsm/L in the outer medulla of the kidney to 1200 mOsm/L in the inner medulla. The descending portion of the loop of Henle is extremely permeable to water and is less permeable to ions, therefore water is easily reabsorbed here and solutes are not readily reabsorbed. The 300 mOsm/L fluid from the loop loses water to the higher concentration outside the loop and increases in tonicity until it reaches its maximum at the bottom of the loop. This area represents the highest concentration in the nephron, but the collecting duct can reach this same tonicity with maximum ADH^[clarification needed] effect.^[3]

The ascending limb of the loop of Henle receives an even lower volume of fluid and has different characteristics compared to the descending limb. In the ascending portion, the loop becomes impermeable to water and the cells of the loop actively reabsorb solutes from the luminal fluid; therefore water is not reabsorbed and ions are readily reabsorbed. As ions leave the lumen via the Na–K–Cl symporter and the Na–H antiporter, the concentration becomes more and more hypotonic until it reaches approximately 100–150 mOsm/L. The ascending limb is also called the diluting segment of the nephron because of its ability to dilute the fluid in the loop from 1200 mOsm/L to 100 mOsm/L.^[3]

Flow of the fluid through the entire loop of Henle is considered slow. As flow increases, the ability of the loop to maintain its osmolar gradient is reduced. The vasa recta (capillary loops) also have a slow flow as well. Increases in vasa recta flow wash away metabolites and cause the medulla to lose osmolarity as well. Increases in flow will disrupt the kidney's ability to form concentrated urine.^[3]

Overall the loop of Henle reabsorbs around 25% of filtered ions and 20% of the filtered water in a normal kidney. These ions are mostly Na⁺, Cl⁻, K⁺, Ca²⁺ and HCO₃⁻. The powering force is the

The loop of Henle is always a U-shaped tubule, with an descending limb and an ascending limb, however its length varies among different vertebrates. This is associated with the fact that it has two functions; whilst the first is to clean wastes, the second is to maintain a balance between ions and H₂O. This allows to balance the blood pressure, blood pH and membrane potentials. To achieve such balance between water and ions, the loop of Henle coordinates its function with the collecting duct to regulate the amount of water to reabsorb or to excrete. While the loop of Henle makes the medulla of the kidney salty, the collecting duct regulates the permeability of water that could be reabsorbed to such salty environment. The saltier the medulla, the more water can be reabsorbed from the pre-urine in collecting duct; before it becomes urine. ^[5] Aquaporin-2 (AQ2) sits in collecting duct and is selectively inserted into cell membranes, according to the body's needs, to reabsorb water to create that balance.^[6]

Vertebrates who live in the desert do not have access to lots of water. Therefore, some of them have a longer loop of Henle which creates a saltier medulla, leading them to reabsorb more water from the pre-urine. For example, the urine concentration in humans can be as concentrated as 1400 mOsm which is limited by the length of our loop of Henle, i.e., 2.2 mm. While a camel's loop of Henle, which is around 4.1 mm, can reach 2800 mOsm. Another example is the Australian mouse whose loop of Henle, 5.2 mm, can make the medulla as salty as 9000 mOsm.^[7] This permits that the urine of these rodents can reach 9000 mOsm, in other words, a highly concentrated urine.

Additional images

• 
  Transverse section of pyramidal substance of kidney of an adult pig, the bloodvessels of which are injected.
• 
  Diagram of physiological functions of nephron, including the loop of Henle.

References

Further reading

• Douglas C. Eaton; John Pooler (2004). Vander's Renal Physiology (6th ed.). McGraw-Hill Medical.
  ISBN 0-07-135728-9
  .
• Lote, Christopher J. (2000). "The loop of Henle, distal tubing and collecting duct". Principles of Renal Physiology. Springer. p. 70.
  ISBN 978-0-7923-6178-7
  .

• Loop of Henle physiology