29.15
Reabsorption and Secretion in the DCT and Collecting Duct
The early phase of the DCT manages the reabsorption of approximately 10-15% of filtered water, 5–10% of filtered sodium, and 5–10% of filtered chlorid…
The early part of the distal convoluted tube, or DCT, reabsorbs around 10 to 15% of the filtered water and 5% of the filtered sodium and chloride ions.
Sodium and chloride are reabsorbed via Na+– Cl− symporters in the apical membranes. These ions then move into the peritubular capillaries through sodium-potassium pumps and Cl− leakage channels.
The early DCT is also responsible for reabsorbing the majority of calcium ions in response to parathyroid hormone.
Two types of cells — principal and intercalated cells — are present in the distal part of the DCT and throughout the collecting ducts.
The principal cells reabsorb sodium ions via sodium leakage channels and secrete potassium ions.
The intercalated cells reabsorb bicarbonate ions and secrete hydrogen ions, thereby regulating blood pH.
Under circumstances like dehydration or low blood volume, the hormone ADH makes the cells in the distal part of the DCT and the collecting ducts permeable to water.
The adrenal cortex releases aldosterone to upregulate sodium reabsorption. As a result, water follows sodium along the osmotic gradients.
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Q1: How much water and electrolytes does the early DCT reabsorb?
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.
Q2: What role does parathyroid hormone play in the DCT?
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.
Q3: What are the functions of principal and intercalated cells in the collecting duct?
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.
Q4: How does ADH increase water reabsorption in the DCT and collecting duct?
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.
Q5: When is ADH released and what triggers its secretion?
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.
Q6: What is the relationship between sodium reabsorption and water reabsorption in the collecting duct?
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.
Q7: How do aquaporin channels differ between the apical and basolateral membranes?
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.