Overview of the Renin-Angiotensin-Aldosterone System (RAAS)

Introduction

• The body has a complex system for controlling blood pressure known as the Renin-Angiotensin-Aldosterone System (RAAS).
• RAAS involves cells and hormones that work together to regulate blood pressure.

Key Components of RAAS

Cells

• Juxtaglomerular (JG) Cells
  • Location: Blood vessels in the kidney.
  • Type: Specialized smooth muscle cells.
  • Function: Release the hormone renin.

Hormones

• Renin: Released by JG cells to help raise blood pressure when it is detected to be low.
• Angiotensinogen: Produced by liver cells, inactive until it meets renin.

Triggers for Renin Release

1. Low Blood Pressure: Detected by JG cells.
2. Sympathetic Nerve Stimulation: Triggered by major stress (e.g., escaping danger, injury).
3. Low Salt in Distal Convoluted Tubule
   • Detected by macula densa cells in the kidney.
   • Macula densa cells send prostaglandins to JG cells to trigger renin release.

Hormonal Pathway

1. Renin meets Angiotensinogen
   • Renin converts angiotensinogen to angiotensin I.
2. Angiotensin I
   • Travels to endothelial cells, often in the lungs, and converts to angiotensin II.

Effects of Angiotensin II

• Angiotensin II is an active hormone affecting multiple systems:
  1. Vasoconstriction: Acts on smooth muscle cells in blood vessels to increase resistance.
  2. Increased Kidney Water Retention: Raises blood volume and stroke volume.
  3. Pituitary Gland Stimulation
     • Releases Antidiuretic Hormone (ADH) to increase resistance and water retention.
  4. Adrenal Gland Stimulation
     • Releases aldosterone to increase kidney water retention.

Summary

• The RAAS begins and ends with the kidney, playing a crucial role in blood pressure control.
• The system involves several hormones (renin, angiotensinogen, angiotensin I and II, ADH, aldosterone) and affects various target cells and organs.
• Understanding RAAS is essential for comprehending how blood pressure is regulated through hormonal interactions and physiological responses.

Key Takeaways

• RAAS is a critical regulatory system for blood pressure, involving a cascade of hormonal activities.
• The kidney is a central organ in both the initiation and culmination of blood pressure control processes within RAAS.
• Multiple organs and cell types are influenced by the RAAS hormones, highlighting the complexity of this system.