HENLE HANDLE: STRUCTURE, CHARACTERISTICS AND FUNCTION - BIOLOGY - 2024

The loop of Henle is a region in the nephrons of the kidneys of birds and mammals. This structure has a primary role in urine concentration and water reabsorption. Animals lacking this structure cannot produce hyperosmotic urine relative to blood.

In the mammalian nephron, the loop of Henle runs parallel to the collecting duct and reaches the papilla of the medulla (inner functional layer of the kidneys), causing the nephrons to be radially arranged in the kidney.

Source: Polish Wikipedia user Sati

Structure

The loop of Henle forms the U-shaped region of the nephrons. This region is formed by a set of tubules present in the nephron. Its constituent parts are the distal straight tubule, thin descending limb, thin ascending limb, and the proximal straight tubule.

Some nephrons have very short ascending and descending thin branches. Consequently, the loop of Henle is only formed by the distal rectus tubule.

The length of the thin branches can vary considerably between species and in the nephrons of the same kidney. This characteristic also makes it possible to differentiate two types of nephrons: cortical nephrons, with a short thin descending branch and without an ascending thin branch; and juxtaglomerular nephrons with long slender branches.

The length of the loops of Henle is related to the reabsorption capacity. In those mammals that live in deserts, such as kangaroo mice (Dipodomys ordii), the loops of Henle are considerably long, thus allowing maximum use of the water consumed and generating highly concentrated urine.

Tubule system

The proximal rectus tubule is the continuation of the proximal convoluted tubule of the nephron. This is in the medullary radius and descends towards the medulla. It is also known as the “thick descending limb of the loop of Henle”.

The proximal tubule continues in the thin descending branch that lies within the medulla. This portion describes a handle to return towards the bark, giving this structure the shape of a U. This branch continues in the thin ascending branch.

The distal rectus tubule is the thick ascending limb of the loop of Henle. This crosses the medulla upwards and enters the cortex in the medullary radius until it is very close to the renal corpuscle that originates it.

The distal tubule is continued, leaving the medullary radius and entering the vascular pole of the renal corpuscle. Finally, the distal tubule leaves the corpuscle area and becomes a convoluted tubule.

characteristics

The thin segments have thin epithelial membranes with cells that have few mitochondria and therefore low levels of metabolic activity. The thin descending limb has almost zero reabsorption capacity, while the thin ascending limb has a medium solute reabsorption capacity.

The thin descending limb is highly permeable to water and slightly permeable to solutes (such as urea and sodium Na ⁺). The ascending tubules, both the thin branch and the distal straight tubule, are practically impermeable to water. This characteristic is key to the concentration function of urine.

The thick ascending branch has epithelial cells that form a thick membrane, with a high metabolic activity and a high reabsorption capacity of solutes such as sodium (Na ⁺), chlorine (Cl ⁺) and potassium (K ⁺).

Function

The loop of Henle plays a fundamental role in the reabsorption of solutes and water, increasing the reabsorption capacity of nephrons through a countercurrent exchange mechanism.

The kidneys in humans have the capacity to generate 180 liters of filtrate per day, and this filtrate passes up to 1800 grams of sodium chloride (NaCl). However, the total urine output is around one liter and the NaCl that is discharged in the urine is 1 gram.

This indicates that 99% of the water and solutes are reabsorbed from the filtrate. Of this amount of reabsorbed products, about 20% of the water is reabsorbed in the loop of Henle, in the thin descending limb. Of the filtered solutes and charges (Na ⁺, Cl ^+, and K ⁺), about 25% is reabsorbed by the thick ascending tubule of the loop of Henle.

Other important ions such as calcium, bicarbonate and magnesium are also reabsorbed in this region of the nephrons.

Solute and water reabsorption

The reabsorption carried out by the loop of Henle occurs through a mechanism similar to that of the gills of fish for oxygen exchange and in the legs of birds for heat exchange.

In the proximal convoluted tubule, water and some solutes such as NaCl are reabsorbed, reducing the volume of the glomerular filtrate by 25%. However, the concentration of salts and urea remains at this point isosmotic with respect to the extracellular fluid.

As the glomerular filtrate passes through the loop, it reduces its volume and becomes more concentrated. The area of ​​highest concentration of urea is just below the loop of the thin descending limb.

The water moves out of the descending branches due to the high concentration of salts in the extracellular fluid. This diffusion occurs by osmosis. The filtrate passes through the ascending branch, while sodium is actively transported to the extracellular fluid, together with chlorine that diffuses passively.

The cells of the ascending branches are impervious to water so it cannot flow outside. This allows the extracellular space to have a high concentration of salts.

Countercurrent exchange

The solutes from the filtrate diffuse freely within the descending branches and then exit the loop in the ascending branches. This generates a recycling of solutes between the tubules of the loop and the extracellular space.

The countercurrent gradient of solutes is established because the fluids in the descending and ascending branches move in opposite directions. The osmotic pressure of the extracellular fluid is further increased by urea deposited from the collecting ducts.

Subsequently, the filtrate passes to the distal convoluted tubule, which empties into the collecting ducts. These ducts are permeable to urea, allowing its diffusion to the outside.

The high concentration of urea and solutes in the extracellular space, allow the diffusion by osmosis of the water, from the descending tubules of the loop towards said space.

Finally, the water diffused in the extracellular space is collected by the peritubular capillaries of the nephrons, returning it to the systemic circulation.

On the other hand, in the case of mammals, the resulting filtrate in the collecting ducts (urine) passes into a duct called the ureter and then into the urinary bladder. Urine leaves the body through the urethra, the penis, or the vagina.

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