Medullary Collecting Ducts

Physiology/Transport: Together with the DCT, the collecting ducts reabsorb about 7% of the filtered NaCl and about 17% of the total reabsorbed water. The two also secrete different amounts of K+ and H+. The medullary collecting duct, unlike its cortical counterpart, is permeable to urea. Its permeability to water, though low, can be increased by antidiuretic hormone (ADH).

As explained in the Histology section, the collectings ducts (and last segment of the distal tubule) are made up of two cells types: principal cells and intercalated cells. The action of principal cells is largely achieved by the basolateral Na+-K+-ATPase that maintains a low [K+]intracellular.



Na+ enters the principal cell by a specific channel. The departure of Na+ from the lumen results in a negative potential there, facilitating the paracellular reabsorption of Cl-. K+, meanwhile, has channels on both the apical and basolateral membrane. Thus, the primary actions of the principal cell are to reabsorb Na+ and water and to excrete K+.

The intercalated cells, meanwhile, secrete H+ (reabsorb HCO3-) or secrete HCO3-, thus regulating acid-base balance. They also reabsorb K+.