Renal Autoregulation

Introduction

• Renal autoregulation: The kidney's ability to modify its blood flow and urine output.
• Intrinsic mechanisms: The kidney regulates itself without external assistance.

Intrinsic Mechanisms

1. Myogenic Mechanism

• Purpose: Respond to changes in blood pressure.
• High Blood Pressure:
  • Increased glomerular hydrostatic pressure, more urine production.
  • Afferent arterioles experience high pressure leading to muscle stretch.
  • Sodium channel activation -> Calcium release -> Muscle contraction -> Vasoconstriction.
  • Reduced blood flow reduces GFR (glomerular filtration rate).
• Low Blood Pressure:
  • Less muscle stretch leads to vasodilation of afferent arterioles.
  • Increased blood flow increases GFR.

2. Tubulo-glomerular Feedback Mechanism

• High Blood Pressure:
  • More sodium chloride reaches distal convoluted tubule.
  • Macula densa cells release adenosine causing vasoconstriction of afferent arterioles.
  • Inhibits renin release.
• Low Blood Pressure:
  • Less sodium chloride reaches distal convoluted tubule.
  • Macula densa cells release PGI2 and nitric oxide causing vasodilation.
  • Stimulates renin release.

Extrinsic Mechanisms

1. Sympathetic Nervous System

• Activated when blood pressure is very low (MAP < 65 mmHg).
• Effects:
  • Increases heart rate and contractility via beta-1 receptors to raise blood pressure.
  • Alpha-1 receptor activation causes systemic and renal vasoconstriction.
  • Reduces kidney perfusion to prioritize other vital organs.

2. Renin-Angiotensin-Aldosterone System (RAAS)

• Triggered by low blood pressure.
• Process:
  • JG cells release renin, converting angiotensinogen to angiotensin I, then to angiotensin II.
  • Angiotensin II increases blood pressure by:
    • Stimulating the release of ADH and aldosterone.
    • Causing vasoconstriction of efferent arterioles.
    • Increasing sodium and water reabsorption.
• High Blood Pressure Response:
  • ANP (Atrial Natriuretic Peptide) opposes all effects of angiotensin II.

Conclusion

• Renal autoregulation is crucial for maintaining consistent kidney function despite changes in blood pressure.
• The kidney employs multiple mechanisms to regulate blood flow and filtration.