Overview
This lecture covers the structure and functions of the distal convoluted tubule (DCT) and collecting duct in kidney nephrons, emphasizing their roles in ion transport, acid-base balance, and water reabsorption.
Structure of the Nephron
- Each nephron consists of a glomerulus and renal tubule (proximal tubule, loop of Henle, distal tubule, collecting duct).
- The distal convoluted tubule has early and late segments; the collecting duct has cortical and medullary regions.
Early Distal Convoluted Tubule (DCT)
- Early DCT is continuous with the loop of Henle and resembles its thick ascending limb.
- It is impermeable to water and urea, but permeable to solutes.
- Sodium-chloride co-transporter on the luminal side brings Na+ and Cl– into the cell.
- Sodium-potassium ATPase on the basolateral side maintains the gradient for Na+ reabsorption.
- Site of action for thiazide diuretics, which inhibit sodium-chloride reabsorption.
- Genetic defect in sodium-chloride channel causes Gitelman syndrome (autosomal recessive).
- Parathyroid hormone (PTH) increases Na+/Ca2+ exchange, boosting calcium reabsorption.
Late DCT and Cortical Collecting Duct
- Have two cell types: principal cells (regulate Na+/K+) and intercalated cells (regulate acid-base).
- Principal cells: Sodium reabsorbed via epithelial sodium channels; potassium secreted into tubular fluid.
- Aldosterone increases sodium-potassium ATPase activity, enhancing Na+ reabsorption and K+ secretion.
Intercalated Cells
- Type A (alpha): Secrete H+ and reabsorb bicarbonate (manage acidosis); H+ secretion is active (ATP-dependent).
- Type B (beta): Reabsorb H+ and secrete bicarbonate (manage alkalosis); have reversed transporter placement.
- Aldosterone increases H+ ATPase activity in alpha cells.
Water Permeability and ADH
- Late DCT and cortical collecting duct are impermeable to water without antidiuretic hormone (ADH).
- ADH triggers aquaporin channels, allowing water reabsorption and controlling urine concentration.
Medullary Collecting Duct
- Final processing site for urine; can reabsorb water (under ADH), secrete H+, and transport urea into the medullary interstitium.
- Urea recycling helps maintain medullary hypertonicity for urine concentration.
Key Terms & Definitions
- Distal Convoluted Tubule (DCT) — The nephron segment after the loop of Henle involved in ion and water regulation.
- Principal Cells — Cells in late DCT/collecting duct regulating sodium and potassium.
- Intercalated Cells — Cells regulating acid-base balance (Type A: secrete H+; Type B: secrete HCO3–).
- Aldosterone — Hormone increasing Na+ reabsorption and K+/H+ secretion.
- Antidiuretic Hormone (ADH) — Hormone that increases water permeability via aquaporins.
- Gitelman Syndrome — Genetic disorder affecting the Na+-Cl– transporter in early DCT.
Action Items / Next Steps
- Review nephron structure and roles of tubular segments.
- Study mechanisms of action for thiazide diuretics, aldosterone, and ADH.
- Prepare for questions on acid-base regulation by intercalated cells.