Lecture Notes on Kidney Physiology

Importance of Passive Reabsorption

• Location: Second half of the proximal convoluted tubule (PCT)
• Mechanism: Electrochemical gradients produced by symporters and antiporters
  • Allows ions (Cl-, K+, Ca2+, Mg2+, urea) to diffuse into the interstitial fluid and peritubular capillary
• Osmosis: Water follows the reabsorbed solutes ("where salt goes, water follows")
• Water Permeability: PCT has aquaporin channels, facilitating water reabsorption

Loop of Henle

• Function: Regulates volume and osmolarity of body fluids
• Descending Loop:
  • Permeable to water and solutes
  • Reclaims ~15% of water
• Ascending Loop:
  • Impermeable to water; only ions move across
  • Na+ pumped out using ATP; Cl- passively moves, K+ recycled

Distal Convoluted Tubule (DCT)

• Reabsorption:
  • Sodium and chloride using symporters
  • Calcium reabsorption influenced by parathyroid hormone (PTH)
• Parathyroid Hormone (PTH) Actions:
  • Activates osteoclasts
  • Increases calcium reabsorption from urine
  • Activates calcitriol for dietary calcium absorption
• End Result: ~95% of solutes and water reabsorbed

Collecting Duct

• Final Adjustments: Regulates urine concentration
• Hormonal Influence:
  • Antidiuretic Hormone (ADH): Concentrates urine, increases blood volume/pressure
  • Aldosterone: Enhances Na+ and water reabsorption, increases blood volume/pressure
  • Atrial Natriuretic Peptide (ANP): Decreases blood volume/pressure

Regulating Blood pH

• Intercalated Cells:
  • Reabsorb K+ and bicarbonate
  • Use proton pumps (ATP) for hydrogen ion secretion
  • Maintain blood pH by secreting H+ and adding bicarbonate
  • Urine buffered with phosphate and ammonia to prevent damage

Key Concepts

• Tubular Secretion and Reabsorption: Importance and locations
• Reabsorption Sites: PCT, Loop of Henle, DCT, Collecting Duct
• Transport Processes:
  • Transcellular vs. Paracellular Reabsorption
  • Obligatory vs. Facultative Water Reabsorption
  • Transport Maximum and Types of Transport (Primary, Secondary, Symporters, Antiporters)
• Substances Secreted/Reabsorbed: Varies by nephron region

Functions of Principal and Intercalated Cells

• Principal Cells: Na+ reabsorption and K+ secretion via sodium-potassium pump
• Intercalated Cells: Regulate pH through H+ secretion and bicarbonate reabsorption

Summary

• Understand filtration, reabsorption, and secretion in nephron regions
• Focus on electrolyte and water balance as well as hormonal regulation

Study Tip: Review charts summarizing main reabsorbed and secreted substances in nephron regions.