Applied physiology: The renal tubule

SummaryThis article provides an update on some of the most important transport functions of the kidney—the control of water, sodium and potassium excretion. Water re-absorption by the kidney requires the presence in the nephron epithelium of water channels—aquaporins—which are present in the apical and basolateral membranes of the tubule cells, and are responsible for the ‘obligatory’ water re-absorption of the proximal tubule, and the ‘facultative absorption’ (under the control of vasopressin) in the distal nephron. The latter is an important determinant of renal concentrating ability. Sodium and potassium homeostasis are maintained by complex interlinking of the control of transport of these ions along the nephron. Physical factors, and the renin–angiotensin system, are responsible for the fine control of sodium and potassium re-absorption in the proximal tubule, but the same hormones (renin–angiotensin–aldosterone) in the distal nephron promote sodium re-absorption but potassium secretion. This spatially separated arrangement provides the mechanism that enables sodium and potassium homeostasis to be effectively regulated independently.