The Loop of Henle is a distinctive, hairpin-shaped segment of the kidney’s nephron, the microscopic structure responsible for filtering blood and forming urine. This U-shaped tubule extends from the proximal convoluted tubule, plunging deep into the kidney’s inner region, the renal medulla, before looping back toward the outer cortex. The primary role of this specialized segment is to create a powerful concentration gradient in the surrounding tissue of the renal medulla. This gradient allows the kidney to ultimately conserve water and produce urine that is far more concentrated than blood plasma.
Function of the Descending Limb
The descending limb carries the filtered fluid, or filtrate, from the cortex down into the increasingly salty environment of the medulla. Its cellular walls are highly permeable to water, largely due to the presence of aquaporin-1 channels. This segment is relatively impermeable to most solutes, including sodium chloride and urea.
As the filtrate travels downward, the concentration of solutes in the surrounding renal medulla steadily increases. This creates a powerful osmotic pressure difference across the tubule wall. Water is drawn passively out of the tubule and into the medullary tissue, following the concentration gradient. Since this movement is driven entirely by osmosis, the cells of the descending limb do not expend energy (ATP) for reabsorption. The continuous removal of water concentrates the remaining filtrate, which can reach an osmolarity of up to 1200 mOsm/L at the loop’s sharp bend.
Function of the Ascending Limb
The ascending limb carries the filtrate back toward the cortex and is functionally the opposite of the descending limb. The cells lining this segment are nearly completely impermeable to water, trapping it inside the tubule regardless of the surrounding osmotic pressure. This impermeability allows the ascending limb to remove solutes without simultaneously losing water, leading to fluid dilution.
The ascending limb is divided into a thin segment and a thick segment, both contributing to salt reabsorption. In the thin ascending limb, sodium and chloride ions move out passively, driven by the concentration gradient established by earlier water loss. The thick ascending limb is where active transport occurs. Specialized transport proteins, notably the Na+-K+-2Cl- cotransporter (NKCC2), actively pump these ions out of the filtrate and into the medullary tissue.
This active removal of salts requires the expenditure of metabolic energy (ATP) and continues to extract solutes even when the concentration outside the tubule is higher than inside. By the time the filtrate reaches the end of the ascending limb and enters the distal convoluted tubule, the continuous removal of salt without water reabsorption has made the fluid significantly dilute, typically around 100 mOsm/L. This segment is often referred to as the diluting segment of the nephron.
Establishing the Medullary Gradient
The opposing functions of the two limbs, with fluid flowing in opposite directions, work in concert to establish the necessary hyperosmotic environment in the kidney medulla, a mechanism known as the countercurrent multiplier. The active transport of salt out of the water-impermeable ascending limb is the fundamental action, known as the “single effect.” This action initially builds the solute concentration in the surrounding interstitial fluid, making the medullary tissue hypertonic.
The countercurrent flow then multiplies this single effect. As the hypertonic tissue draws water out of the descending limb, the filtrate becomes more concentrated as it moves deeper into the medulla. This highly concentrated fluid flows around the hairpin bend and into the ascending limb, providing a higher concentration of salt for the active transporters to remove. This continuous, cyclical process progressively raises the osmolarity from the outer medulla (around 600 mOsm/L) to the deepest part of the inner medulla (up to 1200 mOsm/L).
The establishment of this deep osmotic gradient is the ultimate achievement of the Loop of Henle. Without this gradient, the kidney would be unable to produce concentrated urine and conserve body water effectively. This steep difference in solute concentration provides the necessary osmotic force utilized by the collecting ducts, the final part of the nephron, to make fine adjustments to the body’s water balance.