RAAS

• Renin-Angiotensin-Aldosterone System (RAAS)

  • Slow, hormonal mechanism 
  • Used in long-term blood pressure regulation by adjustment of blood volume.
• RAAS is

  • Slow response compared to baroreceptors
• Trigger: A decrease in renal perfusion (lower blood volume = reduced blood pressure = less fluid passing through kidneys).
• Sensing Mechanisms:

  • Afferent arteriole stretch receptors in the kidneys sense low perfusion.
• • Low sodium levels detected by the Macula Densa in the distal convoluted tubule of the kidney.
• Juxtaglomerular apparatus: Specialized smooth muscle cells in the kidney walls that respond to reduced blood pressure and perfusion.
• Renin Release:

  • A decrease in renal perfusion triggers the release of renin from the juxtaglomerular cells.
• Renin Cascade:

  • Renin converts angiotensinogen (an inactive form) into angiotensin I (still inactive).
• Angiotensinogen is produced by the liver.
• Angiotensin I is converted to Angiotensin II (active form) by the enzyme ACE (Angiotensin Converting Enzyme).
• ACE is found in the lungs and kidneys.
• Effects of Angiotensin II:

  • Potent vasoconstrictor: Constricts arteries, increasing vascular resistance and thus raising blood pressure.
• Actions of Angiotensin II:

  1. In the adrenal cortex: Stimulates the production of aldosterone, which promotes sodium reabsorption and water retention. Since water follows sodium, this helps restore blood volume and pressure.
• Primary trigger: Decreased blood pressure → decreased blood volume → aldosterone release → more sodium retained → more water retained → regulation of blood volume and blood pressure.
• 2. In the posterior pituitary gland: Stimulates the release of antidiuretic hormone (ADH), which promotes water reabsorption in the kidneys, returning more water to the system, thus increasing blood pressure.
• Increases sympathetic activity: Angiotensin II also boosts sympathetic nervous system activity, contributing to the restoration of homeostasis (blood pressure regulation).
• Negative Feedback:

  • Once blood pressure returns to homeostasis, the system stops releasing renin, ceasing the cascade.