Renin Angiotensin Aldosterone System (RAAS)

Renin-Angiotensin-Aldosterone System

Overview

• The Renin-Angiotensin-Aldosterone System (RAAS) is a critical hormonal cascade that regulates fluid retention and vasoconstriction in the body.
• It involves a series of hormones, enzymes, and proteins working together to maintain blood pressure and fluid balance.

Initiation of RAAS

• The kidneys initiate the RAAS in response to:
  • Decreases in blood pressure
  • Decreases in blood volume
• These decreases can be caused by:
  • Dehydration
  • Bleeding
  • Sodium level issues
• The kidneys release renin to start the cascade.

Renin

• Renin is an enzyme released into the bloodstream by juxtaglomerular cells in the kidneys.
• Its primary function is to convert angiotensinogen into angiotensin I.
• Angiotensinogen is a protein produced in the liver.
• Renin cleaves angiotensinogen, resulting in the release of angiotensin I.

Angiotensinogen

• Angiotensinogen is a glycoprotein synthesized by the liver.
• It is excreted into the bloodstream and serves as the substrate for the enzyme renin.

Angiotensin Converting Enzyme (ACE)

• Angiotensin Converting Enzyme (ACE) is a protein that converts the inactive peptide angiotensin I into the active hormone angiotensin II.
• ACE is primarily found in the:
  • Lungs
  • Liver
  • Endothelial lining of blood vessels

Angiotensin II

• Angiotensin II is a peptide hormone with several key functions:
  • Causes vasoconstriction, leading to increased blood pressure.
  • Stimulates the release of aldosterone from the adrenal cortex.
  • Stimulates the release of antidiuretic hormone (ADH) from the pituitary gland.
  • Increases thirst, promoting fluid intake.
  • Increases sodium craving, encouraging sodium consumption.
  • Causes renal tubules to retain sodium and water, reducing fluid loss.

Aldosterone

• Aldosterone is a steroid hormone produced by the adrenal cortex.
• Its main functions include:
  • Promoting sodium reabsorption in the kidneys, leading to increased water retention.
  • Increasing potassium excretion in the kidneys.
  • Regulating blood pressure by controlling sodium and water balance.
  • Assisting in acid/base balance by promoting hydrogen excretion in the kidneys.

Antidiuretic Hormone (ADH)

• Angiotensin II stimulates the release of ADH from the pituitary gland.
• ADH promotes increased water retention by the kidneys.
• This leads to:
  • Increased blood volume
  • Increased blood pressure

Clinical Significance

• The RAAS is a crucial mediator of:
  • Cardiac physiology
  • Vascular physiology
  • Renal physiology
• It regulates vascular tone and salt and water homeostasis.
• The RAAS plays a significant role in the pathophysiology of:
  • Hypertension
  • Heart failure
  • Other cardiovascular diseases
  • Renal diseases
• Blockade of the overactivation of RAAS by various medications has been shown to improve outcomes in various cardiovascular and renal diseases.