Secondary active transport is the mechanism by which a solute is moved against its own concentration gradient without direct expenditure of ATP, by coupling its movement to the downhill flux of a second ion whose gradient was itself established by a primary, ATP-consuming pump. In the nephron this principle is realized by the Na⁺/K⁺-ATPase on the basolateral membrane of tubular epithelial cells, which hydrolyzes ATP to export sodium and import potassium, lowering intracellular sodium and creating an inward sodium gradient across the apical membrane; apical carrier proteins then exploit that gradient, either as symporters (cotransporters), which carry sodium and the coupled solute in the same direction, or as antiporters (exchangers), which move them in opposite directions. The concept belongs to renal and cell physiology at the intersection of membrane transport theory, and it supplies the general explanation for selective reabsorption of solutes from the tubular filtrate into the peritubular capillaries throughout every segment of the nephron.
Secondary active transport is the mechanism by which a solute is moved against its own concentration gradient without direct expenditure of ATP, by coupling its movement to the downhill flux of a sec…