Loop of Henle

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In the kidney, the loop of Henle is a hairpin-shaped tube (similar to the letter "U") being a structure found in nephrons. It is the portion of the 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 is forked into the renal medulla, so that the first part (the descending limb) goes down the cortex to the medulla, and the second (the ascending limb) goes back up the cortex.

Based on the length of the loop of Henle, there are two types of nephrons:

Cortical nephrones, with a short Henle handle, which drops only to the outer cord.
Yuxtamedular nephronas, with a long Henle handle, which goes down to the inner core, reaching to the end of the papilla.

The ability to concentrate urine depends fundamentally on the length of the loop of Henle. For this reason, animals that live in environments with great water scarcity, which need to concentrate their urine as much as possible, have a large number of juxtamedullary nephrons (for example, dromedaries).

Function

Identification of renal flow along the nephrene.

Its function is to provide the adequate osmotic environment for the nephron to concentrate urine, through a countercurrent multiplying mechanism that uses ionic pumps in the medulla to reabsorb ions from the urine. The water present in the filtrate flows through channels of aquaporin (AQP), leaving the tube passively in favor of the concentration gradient created by the ionic pumps.

The primary filtrate passes to the proximal convoluted tubule, which connects with the descending limb of the loop of Henle (with a wide cortical zone and a narrow medullary zone), which has low permeability to ions and urea, but it is very permeable to water, since it presents type 1 aquaporin (AQP1) channels, with constitutive expression, both on the apical and basolateral sides. In this zone, 20% of the filtered water is reabsorbed.

Below is the ascending limb of the loop of Henle, with a narrow internal medullary zone, a wide external medullary zone, and a wide cortical zone. This segment is impermeable to water and permeable to ions. In the ascending limb of the loop there are Na+-K+-2Cl- ion channels (NKCC2), specific to this area, on the apical side of the epithelium, which reabsorb sodium (Na⁺), potassium (K⁺) and chloride (Cl^-) from urine by active transport, associated with the activity of the Na+-K+ pump present on the basolateral side. In this area, the reabsorption of 25% of the filtered Na⁺ occurs in the glomerulus. The reabsorbed K⁺ re-exits into the lumen of the loop of Henle, which is important for maintaining the function of the Na⁺-K^{+-2Cl^-, in addition to generating a positive electrochemical potential in the lumen, which favors the paracellular reabsorption of important cations, such as sodium (Na⁺), potassium (K⁺), magnesium (Mg²⁺) and calcium (Ca²⁺).}

The impermeability to water of the ascending limb of the loop of Henle is associated with the patency of the descending loop. The reabsorption of water in the descending loop occurs thanks to the accumulation of sodium chloride and urea in the medulla, which generates an ionic gradient necessary to be able to reabsorb water. In turn, the concentration of sodium chloride in the medullary interstitium is due to the joint action of the cotransporter Na⁺-K⁺-2Cl^- (NKCC2) and the Na⁺-K⁺ pump of the ascending branch, which transfer sodium chloride from the tube lumen to the medullary interstitium.

As the water is withdrawn from the lumen of the descending loop, the filtrate inside the tube is increasingly concentrated in sodium chloride, so that it is reabsorbed to a greater extent in the ascending loop, which increases the osmolarity of the interstitium: a countercurrent multiplier effect is therefore produced. Since the flow of the descending loop and the ascending loop are in opposite directions, osmotic stratification occurs: at the beginning of the descending loop (at the junction between the cortex and the medulla), the concentration of the interstitial medium is approximately 300 mOsm/L, while going down the loop of Henle, the osmolarity increases gradually, reaching a maximum of 1200 mOsm/L in the papilla area (in a juxtamedullary nephron, when the system is operating at its maximum efficiency).).

In turn, water exiting the descending loop does not dilute the gradient of the medullary interstitium because it is immediately absorbed by the ascending vasa recta (see "Blood supply").

NKCC2 is blocked by furosemide and other loop diuretics, causing bulkier, more dilute urine by increasing the amount of sodium excreted in the urine, which carries water with it.

The ascending zone of the loop of Henle is continuous with the distal convoluted tubule, where reabsorption and secretion of ions occurs again, to further concentrate the urine.

Finally, the distal convoluted tube connects to the collecting duct, which is common to several nephrons. Its function is to determine the final concentration of the urine through the hormones aldosterone and vasopressin (AVP or ADH).

Blood Supply

The loop of Henle is supplied with blood by a series of straight capillaries that descend from the cortical efferent arterioles. These capillary tubes, called vasa recta (Latin for vasa recta), also have a countercurrent exchange mechanism that prevents the loss of solutes from the medulla, thereby maintaining medullary concentration. As water is osmotically driven from the descending limb into the interstitium, it readily enters the vasa recta. The flow of blood through the vasa recta is low, to allow time for osmotic equilibrium to occur, and can be altered by changing the resistance of the efferent arterioles of the vessels.

In addition, the vasa recta contain the large proteins and ions that were not filtered through the glomerulus, which provide an oncotic pressure that allows ions to enter the vasa recta from the interstitium.

Additional images

Cross section of the pyramidal substance of the pig kidney, the blood vessels of which it is injected.

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