The loop of Henle (; also known as Henle's loop, Henle loop, nephron loop) is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its discoverer, the German anatomist Friedrich Gustav Jakob Henle, the loop of Henle's main function is to create a concentration gradient in the medulla of the kidney. The loop is also sometimes called the nephron loop.

Blood supply

thumb|Counter current multiplier diagram

The loop of Henle is supplied by blood in a series of straight capillaries descending from the cortical efferent arterioles. These capillaries (called the vasa recta; recta is from the Latin for "straight") also have a countercurrent multiplier mechanism that prevents washout of solutes from the medulla, thereby maintaining the medullary concentration. As water is osmotically driven from the descending limb into the interstitium, it readily enters the capillaries. The low bloodflow through the vasa recta allows time for osmotic equilibration, and can be altered by changing the resistance of the vessels' efferent arterioles.

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.

The main function of the loop of Henle is to absorb water and sodium chloride from the filtride which preserving water and produces highly concentrated urine.

Physiology

The descending loop of Henle receives isotonic (300 mOsm/L) fluid from the proximal convoluted tubule (PCT). The fluid is isotonic because as ions are reabsorbed by the gradient time system, water is also reabsorbed maintaining the osmolarity of the fluid in the PCT. Substances reabsorbed in the PCT include urea, water, potassium, sodium, chloride, glucose, amino acids, lactate, phosphate, and bicarbonate. Since water is also reabsorbed the volume of fluid in the loop of Henle is less than the PCT, approximately one-third of the original volume.

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 effect.

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. 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.

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. This permits that the urine of these rodents can reach 9000 mOsm, in other words, a highly concentrated urine.

Additional images

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Image:Gray1133.png|Transverse section of pyramidal substance of kidney of an adult pig, the bloodvessels of which are injected.

Image:Kidney nephron molar transport diagram.svg| Diagram of physiological functions of nephron, including the loop of Henle.

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References

Further reading

  • Loop of Henle physiology

fr:Rein#Anse de Henle