Henle’s Loop: Structure, Components, and Functions

The Henle loop is a part of a nephron whose function is to reabsorb water and sodium chloride from the filtrate. The Henle loop is a U-shaped tubule part of the nephron which is responsible to conduct urine within each nephron in reptiles, birds, and mammals.

• In simpler terms, it balances the intake of water removes excess water in the form of concentrated urine, and stores what amount is needed for the body.
• Hence it absorbs the amount that is required for survival.
• So, the desert animal has a highly efficient Henle loop than the animal found in tropical forests and has sufficient availability of water.

Key Terms: Nephron, Kidney, Urine, Renal Tubule, Glomerulus, Tubule, Henles’s Loop.

Read Also: Disorders of Excretory System

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Where is it Located? 

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A human excretory system has many parts including Kidney. A kidney is reddish-brown bean-shaped which outer part is a tough capsule. The inner part of the kidney has two zones one is the cortex and the medulla(a few conical masses). Medulla has around a million nephrons. Each nephron has two parts one Glomerulus(which is a tuft of capillaries) while the other is Bowman’s capsule.

• Bowman’s capsule continues to form a Proximal convoluted tubule(PCT) and the next part of this tubule is the Henle loop.
• In most of the nephrons, the Henle loop is too short and extends only very little into the medulla called cortical nephrons.
• On the other hand, in some of the nephrons, the loop of Henle is very long and present deep into the medulla termed as juxtamedullary.
• The minute vessel which is parallel to the Henle loop forms a U - shaped structure called Vasa Recta.

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Components of Henle Loop 

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Anatomically, the Henle loop is divided into three components thin ascending limb, thin descending limb, and thick ascending limb. The thick ascending limb is sometimes also known as a diluting segment.

• The liquid that enters the Henle loop is a solution of salt, urea, and other substances which as passed by the Proximal convoluted tubule(PCT).
• When the liquid was passed through PCT, PCT is responsible for the reabsorption of most dissolved components into the blood that is necessary for a body like glucose, amino acids, and sodium bicarbonate.
• The liquid that reaches the bend of the loop is richer in salt and urea than the blood plasma is, and the initial segment of the loop, the thin descending limb, is permeable to water.
• Sodium chloride diffuses out of the tubule and into the surrounding tissue, where its concentration is lower when the liquid returns through the thin ascending limb.
• The tubule wall in the third segment of the loop, the thick ascending leg, has the capacity to further remove salt in an active-transport process that requires the use of energy, even when the concentration gradient is present.

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Descending Limb 

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The thin descending limb is permeable to the water but almost impermeable to electrolytes.

• The liquid that reaches the bend is much richer in salt and urea even than blood plasma.
• Reabsorption happens through AQPI channels.
• Small quantities of urea Na+ and other ions are reabsorbed during this process.

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Ascending Limb 

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Ascending limb is a limb that is impermeable to water but allows the transport of electrolytes.

• It is very crucial as it is responsible for maintaining the high osmolarity of medullary interstitial fluid.
• When the liquid returns to the thin ascending limb sodium chloride diffuse out of the tubule into the surrounding tissues where concentration is lower.
• After this when it reaches the thick ascending limb further removal of salt occurs if required.

Read More: Difference Between Left and Right Kidney

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Counter Current of Henle loop and Vasa Recta 

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In a healthy person, the reabsorption of salt from urine maintains the bodily requirement. During periods of low intake of salt virtually none is allowed to escape in urine, but in periods of high salt intake the excess is excreted.

• It is the ability of mammals they can produce concentrated urine.
• Not only Henleloop is crucial in this process but also vasa recta are equally important.
• The flow of the filtrate in the two limbs of the Henle loop is in the opposite directions which are responsible for counter current.
• Similarly, the Vasa recta (U-shaped) has two limbs, and the direction of the flow of the filtrate through these two limbs is in the opposite or counter direction.
• This is the reason for calling it counter current mechanism.
• The proximity between these two loops is responsible for maintaining the increasing osmolarity towards the inner medullary interstitium.
• The osmolarity gradient is mainly caused by NaCl and urea.

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The Counter-Current Mechanism 

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In the actual mechanism, the absorption of water takes place in descending limb which helps to increase the osmotic gradient. At the same time, the active transport of NaCl occurs into the interstitium with the help of water-impermeable cells of ascending limb to help in maintaining the gradient. It also ensures that concentration is declining as the fluid passes toward the DCT(Dictal Convalent tubule).

• The Henle loop is supplied by 2 vasa recta the two straight tubules.
• Because they are situated along with each other.
• They are responsible for the counter-current mechanism.
• These carry current in opposite direction hence absorb water on one side and solute on another side.
• This is known as the counter-multiplier system, which aids to maintain the osmotic medullary gradient in outer medullary tissue.
• This process makes certain of vasopressin-driven reabsorption of water.
• The thick ascending limb in this system expresses co-transporter sodium-potassium-chloride and assists in the reabsorption of one-third of the filtered sodium and chloride from the fluid in the tubular lumen into the blood. 

Read More: Kidney Failure Symptoms

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Function of Henle’s Loop 

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A few functions of Henle’s loop are— 

1. The homeostatic mechanism which aids in regulating extracellular fluid volume.
2. Regulating potassium, calcium, and magnesium discharge at the lowest en costs.
3. Homeostasis of the acid-base balance through bicarbonate as well as ammonia discharge
4. Regulating the make-up of urinary protein.

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Things to Remember

• The Henle loop is a part of a nephron whose function is to reabsorb water and sodium chloride from the filtrate.
•  It is present in the kidney.
• Vasa recta (U-shaped) has two limbs, and the direction of the flow of the filtrate through these two limbs is in the opposite or counter direction.
• The Henle loop is supplied by 2 vasa recta the two straight tubules.
• The Henle loop is divided into three components thin ascending limb, thin descending limb, and thick ascending limb.
• It is the ability of mammals they can produce concentrated urine. Not only Henleloop is crucial in this process but also vasa recta are equally important.
• The flow of the filtrate in the two limbs of the Henle loop is in the opposite directions which are responsible for counter current.