Understanding the Renin-Angiotensin-Aldosterone System

Sep 30, 2024

Lecture Notes: Renin-Angiotensin-Aldosterone System (RAAS)

Introduction

  • The RAAS system manages blood pressure, especially when it drops.
  • Goal: Increase blood pressure by activating angiotensin II.

Overview of RAAS

  • Angiotensin II:
    • Causes major vasoconstriction to increase blood pressure.
    • Increases blood volume, further raising pressure.
  • Sequence of activation: Renin → Angiotensin I → Angiotensin II → Aldosterone.

Step-by-Step Process

  1. Initiation:
    • Blood pressure drops.
    • Sympathetic nervous system is stimulated (fight or flight response).
    • Juxtaglomerular (JG) cells in kidneys release renin.
  2. Renin in Circulation:
    • Activates angiotensinogen (from liver) into angiotensin I.
  3. Conversion to Angiotensin II:
    • ACE (Angiotensin-Converting Enzyme) converts angiotensin I to angiotensin II.
    • ACE is located in the endothelium of lungs and kidneys.

Actions of Angiotensin II

  • Vasoconstriction:
    • Acts on smooth muscle vessels.
    • Increases systemic vascular resistance and blood pressure.
    • Analogy: Narrowing a water hose increases the pressure of water.
  • Increase Blood Volume:
    • Stimulates kidneys to conserve sodium and water.
    • Stimulates adrenal cortex to produce aldosterone:
      • Kidneys retain sodium and water; excrete potassium.
    • Stimulates pituitary gland to release ADH (Antidiuretic Hormone):
      • Increases water retention in kidneys.

End Result

  • Increased blood pressure through vasoconstriction and increased blood volume.

ACE Inhibitors

  • Prevent ACE from converting angiotensin I to angiotensin II.
  • Used to lower blood pressure.

Recap

  • Drop in blood pressure stimulates sympathetic nervous system.
  • JG cells release renin, activating angiotensinogen to angiotensin I.
  • ACE converts angiotensin I to angiotensin II.
  • Angiotensin II causes:
    • Vasoconstriction.
    • Sodium and water retention (via aldosterone and ADH).
  • Overall result: Increased blood pressure.

Conclusion

  • Understanding RAAS is crucial for grasping blood pressure management.
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