Lecture 20: The Tubule

Introduction to Kidney Physiology
  • Human kidneys perform essential functions including filtration, reabsorption, and secretion.
  • Understanding nephron structure and tubular function is critical for grasping kidney physiology.
Case of the Day
  • Patient: 15-year-old boy
  • Findings:
    • Routine urinalysis: normal blood and protein, glucose detected
    • Urgent blood glucose: normal (4.5 mmol/L)
    • Repeated urinalyses: consistent glucose presence
  • Question: Is he diabetic?
Overview of Tubule Function
  • Tubule function centers around transport:
    1. General comments about kidney tubular physiology
    2. Water handling in tubules
    3. Sodium handling in tubules
    4. Focus on glucose reabsorption
Kidney Anatomy and Function
  • Glomerular filtration rate (GFR): Normal range >10 mL/min or >150 L/day
  • Total body water distribution:
    • Total body water: 40 L (60% body weight)
    • Extracellular fluid: 15 L (20% body weight)
  • Kidneys filter ~150-170 L of fluid daily based on Starling forces.
Physiology of Tubules
  • Reabsorption of Solutes and Water
    • Kidneys reabsorb >99% of filtered water and sodium every day.
    • Important for maintaining normal body composition and fluid volumes.
Glomerulus and Tubular Segmentation
  • Nephrons consist of various segments:
    • Proximal tubule: Bulk reabsorption of sodium, chloride, water
    • Loop of Henle: Adjusts concentration of urine
    • Distal convoluted tubule: Further sodium and water reabsorption
    • Collecting duct: Fine-tuning of sodium and water; influenced by hormones.
Sodium Reabsorption Mechanisms
  • Epithelial cells:
    • Polarized; distinct proteins in apical and basolateral membranes
    • Primary mechanism involves Na-K-ATPase to maintain sodium gradients.
  • Transport Proteins:
    • Sodium-Hydrogen Exchanger (NHE3): Primarily in proximal tubule
    • Sodium-Potassium-2 Chloride Co-transporter (NKCC2) in thick ascending loop
    • Sodium-Chloride Co-transporter (NCC) in distal convoluted tubule
    • Epithelial Sodium Channel (ENaC) in collecting duct
Glucose Reabsorption
  • Mechanism:
    • Glucose reabsorption is driven by sodium-dependent transporters, specifically SGLT2 in the proximal tubule.
    • Filtration and reabsorption of glucose proportional to plasma concentration until the transport maximum (Tm) is reached.
    • Renal threshold: glucose excretion occurs when glucose exceeds Tm.
  • Clinical Context:
    • In the case presented, renal glycosuria may indicate a benign inherited mutation leading to glucose recovery issues despite normal blood glucose levels.
Summary of Tubular Function
  • Tubules’ primary roles:
    • Reabsorption and secretion are linked with sodium transport.
    • Proximal tubule: major site for solute and water reabsorption.
    • Collecting ducts: regulate electrolyte and water balance under hormonal control.
Conclusion
  • Understanding the mechanisms of kidney function, especially tubular reabsorption, aids in recognizing conditions like diabetes or renal glycosuria.
  • Implications for treatment: SGLT2 inhibitors being explored for diabetes management.
Next Case: Cholera in Haiti
  • Patient: 16-year-old girl with severe diarrhea and dehydration symptoms, needing urgent fluid management.