Lecture Notes: Glomerular Filtration Rate (GFR)

Introduction

• Topic: Glomerular Filtration Rate (GFR)
• Focus: Kidney autoregulation, myogenic response, and tubular feedback

Kidney Autoregulation

• Definition: Kidney's intrinsic ability to regulate blood pressure and filtration independently.
• Goal: Maintain a constant GFR of ~125 ml/min.
• Mechanisms: Myogenic response and tubuloglomerular feedback.

Myogenic Response

• Mechanism: Vascular smooth muscle contracts when stretched, relaxes when relaxed.
• Afferent Arteriole Response:
  • High blood pressure → Vasoconstriction → Reduces blood flow, prevents excessive filtration.
  • Low blood pressure → Vasodilation → Increases blood flow, raises hydrostatic pressure.
• Outcome: Maintains normal GFR by adjusting blood flow automatically.

Tubuloglomerular Feedback

• Backup to Myogenic Response: Acts when autoregulation isn't enough.
• Macula Densa Cells:
  • Detect sodium chloride levels to assess GFR.
  • High NaCl → Release vasoconstrictor → Lower GFR.
  • Low NaCl → Stimulate renin release → Increase GFR.

Range of Effective Autoregulation

• Works best with systemic blood pressure between 80-180 mmHg.
• Outside this range, extrinsic controls are necessary.

Extrinsic Controls

• Physiological Processes: Change in GFR via nervous system and hormonal control.
• Sympathetic Stimulation: During exercise/emergency, decreases GFR to conserve water and maintain blood volume.
• Atrial Natriuretic Peptide (ANP):
  • Released when atria are stretched due to high blood pressure.
  • Relaxes afferent arterioles, increases GFR, and promotes urine formation to decrease blood volume and pressure.

Scenarios

• High Systemic BP (>180 mmHg):
  • Vasoconstriction to prevent excessive urine formation.
• Low Systemic BP (<80 mmHg):
  • Vasodilation to maintain GFR, possibly reducing urine formation to preserve fluid.

Conclusion

• Further discussion on renin-angiotensin mechanism in the next session.
• Transition to the next topic: Reabsorption and secretion of materials in the nephron.