Renin-Angiotensin-Aldosterone System (RAAS)

Overview

• The RAAS is a homeostatic negative feedback loop system.
• Regulates blood pressure, blood volume, and sodium levels.
• Involves kidneys, hormones, and enzymes.
• Key Concept: When blood pressure drops, the system works to restore it.

Components of RAAS

1. Stimulus

   • Decrease in blood volume/pressure due to:
     • Dehydration
     • Blood loss (e.g., hemorrhaging)
   • Sodium deficiency

2. Sensors

   • Baroreceptors: Detect changes in blood pressure.
     • Located in juxtaglomerular cells (afferent arterioles of nephron).
   • Chemoreceptors: Detect sodium levels.
     • Located in macula densa (distal convoluted tubule).

3. Integration

   • Occurs in the bloodstream.
   • Involves three organs: Liver, Kidney, Lungs.
     • Liver: Produces angiotensinogen (inactive hormone).
     • Kidney: Juxtaglomerular cells release renin (enzyme) when low blood pressure/sodium detected.
     • Lungs: Produce Angiotensin Converting Enzyme (ACE).
   • Enzyme interactions:
     • Renin converts angiotensinogen to Angiotensin I (inactive).
     • ACE converts Angiotensin I to Angiotensin II (active hormone).

4. Effectors

   • Adrenal Gland: Releases aldosterone.
     • Increases sodium reabsorption in distal convoluted tubule.
   • Angiotensin II Direct Effects:
     • On Nephron: Increases sodium reabsorption in proximal convoluted tubule.
     • On Brain: Stimulates posterior pituitary to release ADH (antidiuretic hormone).
       • Increases water reabsorption in distal convoluted tubule and collecting duct.
     • Causes vasoconstriction of arterioles, increasing blood pressure.
     • Activates sympathetic nervous system (fight/flight response), increasing blood pressure.

Additional Effects

• Angiotensin II:
  • Increases glomerular filtration rate, ensuring waste filtration despite low blood pressure.

Study Tips

• Review and explain each step of the RAAS process.
• Practice with a blank diagram.
• Attempt to draw the entire process from memory.

Conclusion

• The RAAS is critical for maintaining homeostasis in blood pressure, volume, and sodium levels.
• Understanding this system is essential for comprehending how the body responds to changes in blood pressure and volume.