Reabsorption and Secretion in the DCT and Collecting Duct

The early distal convoluted tubule reabsorbs approximately 10-15% of filtered water and 5% of filtered sodium and chloride ions. Sodium and chloride move via Na+–Cl− symporters in apical membranes, then through sodium-potassium pumps and chloride leakage channels into peritubular capillaries. This selective reabsorption helps maintain electrolyte balance and blood osmolarity.

Parathyroid hormone stimulates calcium reabsorption in the early distal convoluted tubule based on the body's calcium requirements. This hormone-regulated process allows the kidney to adjust calcium levels in the blood independently of other ion reabsorption mechanisms, maintaining proper calcium homeostasis for muscle and bone function.

Principal cells reabsorb sodium ions via sodium leakage channels and secrete potassium ions to regulate electrolyte balance. Intercalated cells reabsorb bicarbonate ions and secrete hydrogen ions, thereby regulating blood pH. Together, these two cell types maintain acid-base balance and electrolyte homeostasis in the distal nephron.

Antidiuretic hormone binds to receptors on principal cells, triggering insertion of aquaporin-2 water channels into the apical membrane. These pre-formed channels mobilize from cytoplasmic vesicles to the cell surface, allowing water to pass from the tubular lumen into cells. Water then exits through basolateral aquaporins into the bloodstream, concentrating urine and restoring blood volume.

The hypothalamus triggers ADH release from the posterior pituitary gland during dehydration or low blood volume, when plasma osmolarity rises. This response makes the distal DCT and collecting duct permeable to water, allowing increased reabsorption. Aldosterone from the adrenal cortex simultaneously upregulates sodium reabsorption, and water follows along osmotic gradients.

Sodium reabsorption by principal cells creates an osmotic gradient that drives water reabsorption through aquaporin channels. When aldosterone increases sodium reabsorption, water follows passively along the osmotic gradient established by the reabsorbed sodium. This coupling ensures that water and electrolyte reabsorption remain coordinated for effective blood volume regulation.

Aquaporin-2 channels in the apical membrane are regulated by ADH and mobilize from intracellular vesicles to allow water entry from the tubular lumen. Aquaporin-3 and aquaporin-4 channels in the basolateral membrane are constitutively present and allow water exit into the interstitial space and bloodstream. This asymmetric distribution enables directional water transport across principal cells.