Renin-Angiotensin-Aldosterone (RAA) System
The renin-angiotensin-aldosterone (RAA) system is one of the body's mechanisms for detecting falling blood pressure and bringing the blood pressure back to normal. The kidneys depend on a constant, normal blood pressure to filter the blood and remove waste products.
If the blood pressure drops too low (for example, if you are in a car accident and lose lots of blood), renal failure can result. The loss of blood volume causes a decrease in blood pressure.
A drop in blood pressure , a decrease in sodium ion (Na+) levels, or an increase in potassium ion (K+) levels can set the following cascade of events in motion:
1. The juxtaglomerular cells in the kidneys detect the drop in blood pressure.
2. The juxtaglomerular cells respond by secreting the enzyme renin.
3. Renin helps convert angiotensinogen into angiotensin I.
4. Angiotensin-converting enzyme (ACE) then helps convert angiotensin I into angiotensin II.
5. Angiotensin II then increases blood pressure in 2 ways:
a. It stimulates vasoconstriction of the blood vessels
b. It stimulates the adrenal cortex to secrete aldosterone. Aldosterone increases blood volume by stimulating increased reabsorption of sodium ions. (Recall the general rule, water follows the salt," so when sodium ions are reabsorbed, water is reabsorbed. An increase in water volume results in an increase in blood volume and ultimately blood pressure.)