Ascending limb of loop of Henle

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Ascending limb of the loop of Henle is a segment of the nephron in the kidney. It is part of the loop of Henle, which plays a crucial role in the kidney's ability to produce concentrated urine. The ascending limb transports salts and water from the urine back into the blood, contributing to the regulation of blood pressure, blood volume, and electrolyte balance.

Structure

The ascending limb of the loop of Henle extends from the hairpin turn of the loop, ascending back towards the renal cortex. It is divided into two segments: the thin ascending limb and the thick ascending limb. The thin segment is permeable primarily to ions, whereas the thick segment, also known as the distal straight tubule, actively transports ions but is impermeable to water.

Thin Ascending Limb

The thin ascending limb is found in nephrons that extend deep into the renal medulla and is crucial for the concentration of urine. It allows passive transport of sodium and chloride ions out of the tubular fluid, contributing to the osmotic gradient in the medulla.

Thick Ascending Limb

The thick ascending limb (TAL) is characterized by its active transport of sodium, potassium, and chloride ions out of the urine. This segment utilizes a sodium-potassium-2chloride cotransporter (NKCC2) to move ions into the cell, and then these ions are transported into the bloodstream. The TAL is also responsible for the reabsorption of calcium and magnesium through paracellular pathways.

Function

The primary function of the ascending limb of the loop of Henle is to contribute to the concentration of urine and the maintenance of electrolyte balance. By actively transporting salts out of the urine, it creates an osmotic gradient that facilitates the passive reabsorption of water in the descending limb and collecting ducts. This process is essential for the production of concentrated urine, allowing the body to conserve water.

The ascending limb also plays a role in the countercurrent multiplication system, which enhances the kidney's ability to concentrate urine. As salt is actively transported out of the thick ascending limb, it increases the osmolality of the surrounding interstitial fluid, drawing water out of the descending limb and concentrating the urine.

Clinical Significance

Dysfunction or damage to the ascending limb of the loop of Henle can lead to various kidney diseases and disorders. For example, Bartter syndrome is a rare inherited condition that affects the thick ascending limb's ability to reabsorb sodium, leading to low potassium levels, alkalosis, and high blood calcium levels.

Diuretics that target the thick ascending limb, such as loop diuretics, are commonly used to treat conditions like hypertension and edema. By inhibiting the NKCC2 transporter, these drugs prevent sodium reabsorption, increasing urine output and reducing blood volume and pressure.

See Also

• Nephron
• Loop of Henle
• Renal physiology
• Diuretics

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