Reabsorption and Secretion in the Loop of Henle

Na+–K+–2Cl− symporters in the apical membranes simultaneously reclaim one sodium ion, one potassium ion, and two chloride ions from the tubular fluid. This active transport mechanism is essential for tubular reabsorption and secretion, enabling the thick ascending limb to selectively recover these critical ions from the filtrate.

Most potassium ions brought into cells by Na+–K+–2Cl− symporters leak back into the tubular fluid through leakage channels in the apical membrane. This recycling mechanism means the thick ascending limb primarily reabsorbs sodium and chloride ions rather than potassium, making potassium recycling a key feature of loop function.

Potassium ions returning to the tubular fluid create a relative negative charge in the interstitial fluid. This electrical gradient promotes the paracellular reabsorption of positively charged calcium and magnesium ions, demonstrating how ion recycling indirectly drives reabsorption of other essential minerals.

The thick ascending limb lacks aquaporin water channels, making it impermeable to water. This structural feature allows solutes to be reabsorbed without water following osmotically, causing the osmolarity of tubular fluid to decrease progressively as ions are absorbed in the ascending limb.

Sodium ions are actively transported into the interstitial fluid at the base and sides of cells, then diffuse into the vasa recta. Chloride ions move through leakage channels in the basolateral membrane into the interstitial fluid and subsequently into the vasa recta, facilitating ion recovery.

Approximately 15% of filtered water is reabsorbed in the descending limb of the nephron loop. In contrast, the ascending limb is impermeable to water, allowing it to concentrate solutes without water reabsorption, creating the osmotic gradient essential for formation of concentrated urine.

As sodium, potassium, and chloride ions are reabsorbed in the thick ascending limb without water following, the osmolarity of the tubular fluid decreases progressively. This selective solute removal creates a hypotonic fluid in the lumen while establishing the osmotic gradient necessary for subsequent reabsorption and secretion in the dct and collecting duct.