- The renin-angiotensin-aldosterone system (RAAS) is a critical regulatory mechanism that maintains blood pressure and fluid balance in the body.
- It is primarily activated in response to low blood pressure or decreased renal perfusion.
- When the kidneys detect a drop in blood pressure, the juxtaglomerular cells release the enzyme renin.
- Renin then converts angiotensinogen, produced by the liver, into angiotensin I.
- This is subsequently converted to angiotensin II, a potent vasoconstrictor, which helps to increase blood pressure by narrowing blood vessels.
- Additionally, angiotensin II stimulates the release of aldosterone from the adrenal cortex, promoting sodium and water retention by the kidneys, which further increases blood volume and pressure.

In contrast, conditions such as hypernatremia or elevated blood volume typically inhibit the activation of RAAS since the body does not need to conserve more sodium or water. Similarly, increased arterial oxygen tension is unrelated to the primary triggers of RAAS activation.

Therefore, the primary trigger for activating the RAAS is low blood pressure, which ensures the body can restore and maintain adequate perfusion to vital organs.