Renin Angotensinogen Aldosterone System (RAAS)

The RAAS maintains blood volume (and therefore blood pressure) by diluting or concentrating blood serum (and inversely, concentrating or diluting urine), among other mechanisms.

1. Renin is released from juxtaglomerular cells in the afferent arteriole of the nephron when...

   • the juxtaglomerular cells detect reduced blood volume (i.e. hypotension).
   • Macula Densa cells in the DCT detect a drop is serum sodium.
   • increased SNS stimulation triggers the release directly.

2. The liver continuously stores and releases angiotensinogen into the blood stream.

3. Renin converts angiotensinogen into angiotensin I.
4. AT1 is converted into angiotensin II by angiotensin converting enzyme produced in the lungs.

5. AT2 has multiple functions. AT2...

   • triggers the release of aldosterone from the adrenal cortex.
   • constricts the smooth muscle of the efferent arteriole, increasing glomerular filtration of blood, increasing amount of sodium in the DCT.
   • promotes generalized vasoconstriction, increasing blood pressure and therefor GFR.
   • triggers the release of antidiuretic hormone from the posterior pituitary gland, which decreases amount of H\(_2\)O in the DCT.

6. Aldosterone causes the DCT to reabsorb sodium into the ECF (and excrete potassium). Sodium draws water into the ECF via osmosis, increasing ECF volume.