Lecture on Late Distal Tubules and Collecting Duct

Overview

• Discussion on late distal tubules and introduction to intercalated A and B cells.
• Principal cells and their role in maintaining mineral and water balance.
• Intercalated cells managing acid-base balance.

Key Concepts

Review of Previous Concepts

• Sodium Chloride Reabsorption:
  • Early distal tubules via sodium chloride cotransporters.
• Calcium Reabsorption:
  • Dependent on parathyroid hormone.
• Late Distal Tubules:
  • Aldosterone increases sodium reabsorption and potassium secretion.
  • Antidiuretic hormone (ADH) increases aquaporins for water reabsorption.

Intercalated A and B Cells

• Intercalated A Cells:
  • Respond to acidosis (respiratory/metabolic).
  • CO2 + H2O → Carbonic acid (via carbonic anhydrase) → Protons + Bicarbonate.
  • Bicarbonate is reabsorbed to neutralize acidity.
  • Protons secreted with ATP dependent mechanisms.
  • Ammonia secretion helps buffer acid in urine.
• Intercalated B Cells:
  • Respond to alkalosis (high pH).
  • Process similar to A cells but in reverse.
  • Bicarbonate is secreted, protons reabsorbed to decrease pH.

Collecting Duct

• Principal Cells in Collecting Duct:
  • ADH acts on these cells to reabsorb water and maintain blood volume/pressure.
  • Mechanism involves G-protein signaling cascade activating aquaporin channels.
  • ADH release triggered by high plasma osmolality and angiotensin II.

Additional Functions in Collecting Duct

• Secretion of drugs, toxins, creatinine, ammonia, and protons.

Vasa Recta and Counter-Current Mechanisms

• Vasa Recta:
  • Counter-current exchanger preventing rapid salt loss and providing tissue oxygen.
  • Contributes to medullary interstitial gradient for concentrated urine.
• Urea Recycling:
  • Urea contributes to medullary gradient and concentrated urine formation.

Summary

• Covered mechanisms of acid-base balance in intercalated cells and water balance in principal cells.
• Discussed additional secretory functions of the collecting duct.
• Explained counter-current exchange in vasa recta and role of urea recycling.

Next Steps

• Upcoming video will integrate all reabsorption and secretion processes into a comprehensive overview.

These notes provide a concise overview of the complex processes occurring in the late distal tubules and collecting ducts, focusing on cell types, regulatory mechanisms, and the physiological significance of these processes.