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

Introduction

• Purpose: The Renin-Angiotensin-Aldosterone System is activated when blood volume and blood pressure drop.
• Goal: To increase blood volume and blood pressure.

Key Components of RAAS

1. Renin
2. Angiotensin
3. Aldosterone

Functioning of the System

Nephron Structure

• Nephron: Filtration system in kidneys (~1 million nephrons per kidney).
• Location: Most in the cortex; Loop of Henle and collecting ducts in medullary pyramids.
• Filtration Rate: Kidneys filter approximately 120 ml/min, producing about 180 liters/day, with 99% reabsorbed.

Renin Release

• Released from: Juxtaglomerular cells (granular cells) in afferent arteriole.
• Triggers for Renin Release:
  1. Drop in Blood Pressure: Granular cells act as baroreceptors.
  2. Drop in Sodium Concentration: Detected by macula densa cells in distal convoluted tubule communicating with granular cells.
  3. Sympathetic Nervous System: Direct innervation also triggers release.

Renin's Role

• Renin: Enzyme that converts angiotensinogen (inactive) to angiotensin I.

Conversion Process

Angiotensin

• Angiotensinogen (produced by liver) converts to Angiotensin I (by renin).
• Angiotensin I: Slight vasoconstrictor; converted to Angiotensin II by Angiotensin Converting Enzyme (ACE) in the lungs.

Actions of Angiotensin II

1. Generalized Vasoconstriction: Increases blood pressure by constricting arterioles.
2. Constriction of Efferent Arteriole: Increases glomerular filtration rate, fixing low filtration rate.
3. Stimulates Aldosterone Release: From adrenal cortex.
   • Aldosterone's Role: Increases sodium reabsorption in the distal convoluted tubule, boosting blood volume and pressure.
4. Stimulates ADH Release: From hypothalamus; ADH increases water reabsorption in distal convoluted tubule and collecting ducts.

Outcomes of RAAS Activation

• Result: Increased blood volume and blood pressure.
• Stimulus: Initial drop in blood volume and pressure.
• Final Outcome: Restoration of normal blood volume and pressure.

Summary

• Main Goal: Counteract low blood volume/blood pressure by RAAS cascade—Renin release leads to Angiotensin II effects, promoting vasoconstriction, aldosterone, and ADH release, ultimately increasing blood volume and pressure.