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Overview of Renal Reabsorption and Secretion

Apr 9, 2025

Lecture Notes on Renal Physiology: Reabsorption and Secretion

Key Concepts

Importance of Passive Reabsorption

  • Electrochemical Gradients: Created by symporters and antiporters.
  • Ions Involved: Chloride, potassium, calcium, magnesium, and urea.
  • Movement: Ions move transcellularly or paracellularly into interstitial fluid and peritubular capillary.
  • Osmosis: Water follows salt due to the permeability of the proximal convoluted tubule (PCT).

Proximal Convoluted Tubule (PCT)

  • High Water Permeability: Major site of water reabsorption due to aquaporin channels.

Loop of Henle

  • Function: Regulates volume and osmolarity of body fluids.
  • Descending Limb: Allows movement of water and solutes; reclaims ~15% of water.
  • Ascending Limb: Impermeable to water; sodium and other ions move across to the vasa recta.
    • Transporters: Thin porters for sodium, potassium, and chloride.
  • Energy Usage: Sodium pumped out using ATP, chloride moves passively, potassium is recycled.

Distal Convoluted Tubule (DCT)

  • Reabsorption: Sodium, chloride, and calcium, influenced by parathyroid hormone (PTH).
  • Parathyroid Hormone (PTH):
    • Activates osteoclasts.
    • Increases calcium reabsorption in urine.
    • Activates calcitriol for dietary calcium absorption.

Collecting Duct

  • Final Adjustments: Concentration of urine and hormonal regulation.
  • Hormones Involved:
    • Antidiuretic Hormone (ADH): Concentrates urine, increases blood volume and pressure.
    • Aldosterone: Increases sodium and water reabsorption; synthesizes new pumps and channels.
    • Atrial Natriuretic Peptide (ANP): Decreases blood volume and pressure.

Principal and Intercalated Cells

  • Principal Cells: Reabsorb sodium; use sodium-potassium pump with ATP.
  • Intercalated Cells:
    • Function: Reabsorb potassium and bicarbonate.
    • Proton Pumps: Use ATP to move hydrogen ions against the gradient.
    • Buffering: Bicarbonate helps decrease blood acidity; urine is buffered by phosphate and ammonia to prevent damage.

Other Important Terms

  • Obligatory vs. Facultative Water Reabsorption: Difference in water reabsorption processes.
  • Transport Maximum: Maximal rate of reabsorption.
  • Active Transport: Primary and secondary, and their role in ion movement.
  • Symporters and Antiporters: Mechanisms involved in ion reabsorption.
  • Glucosauria: Condition related to glucose reabsorption.

Goals and Summary

  • Objectives:
    • Define and understand tubular secretion and reabsorption.
    • Recognize where major reabsorption occurs.
    • Differentiate between transcellular and paracellular reabsorption.
    • Comprehend how salt and water reabsorption are connected.
  • Understanding the Chart:
    • Main reabsorbed and secreted substances in different nephron regions: PCT, Loop of Henle, DCT, and collecting duct.

Take time to review the chart mentioned in the lecture for a clear understanding of secretion and reabsorption processes.

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