Reabsorption and Secretion in the Loop of Henle

The thick ascending limb of the nephron loop has Na⁺–K⁺–2Cl⁻ symporters in the apical membranes of its cells. These symporters simultaneously reclaim one sodium ion, one potassium ion, and two chloride ions from the tubular fluid. Sodium ions are actively transported into the interstitial fluid at the base and sides of the cell, diffusing into the vasa recta. Chloride ions move through leakage channels in the basolateral membrane into the interstitial fluid and then into the vasa recta.

Interestingly, most potassium ions brought in by the symporters return to the tubular fluid through leakage channels in the apical membrane. This process primarily results in the reabsorption of sodium and chloride ions.

The movement of positively charged potassium ions into the tubular fluid leaves the interstitial fluid with more negative charges relative to the fluid in the lumen. This relative negativity promotes the reabsorption of Ca²⁺ and Mg²⁺ via the paracellular route.

The ascending limb of the nephron loop is impermeable to water. As a result, the osmolarity of the tubular fluid decreases progressively as solutes are absorbed in the ascending limb. In comparison, about 15% of the filtered water is reabsorbed in the descending limb of the nephron loop.