Several substances are transported from the peritubular capillaries into the interstitial fluid and then to the tubular lumen via the tubular epithelial cells.
Antiport of H+ along with Na2+ reabsorption in the proximal tubule and distal tubule. H+ is secreted throughout the nephron.
K+ transport is unique, in that it is reabsorbed in some parts of the tubule and secreted in others. It is reabsorbed in the convoluted portion of proximal tubule and secreted in the straight portion of proximal tubule. It is secreted and reabsorbed in the distal nephron.
Ammonium ions are synthesised in the epithelial cells and diffuse into the tubular fluid.
