Renin-Angiotensin-Aldosterone System (RAAS)

What is the renin-angiotensin-aldosterone system?

The renin-angiotensin-aldosterone system (RAAS) is a system of hormones, proteins, enzymes and reactions that regulate your blood pressure and blood volume long-term. It regulates your blood pressure by increasing:

• Sodium (salt) reabsorption in your kidneys
• Water reabsorption (retention)
• Vascular tone (the degree to which your blood vessels narrow)

The RAAS consists of three major substances:

• Renin: Your kidneys mostly make the enzyme renin. It helps control your blood pressure. It also helps balance your sodium and potassium levels.
• Angiotensin II: This is a hormone. It helps increase blood volume, blood pressure and sodium levels.
• Aldosterone: Your adrenal glands make the hormone aldosterone. It helps manage sodium and potassium levels in your blood.

What does the renin-angiotensin-aldosterone system do?

The RAAS is key to controlling your blood volume and the amount of force or pressure your blood vessels put on your blood (systemic vascular resistance). These affect how much blood your heart can pump and your arteries’ average pressure (mean arterial pressure).

What will trigger it into action?

The RAAS involves several steps, including:

1. When your blood pressure falls, your kidneys release renin into your bloodstream.
2. Renin splits angiotensinogen into pieces. Angiotensinogen is a protein your liver makes and releases. One piece is the hormone angiotensin I.
3. Angiotensin I is inactive (doesn’t cause any effects). It flows through your bloodstream. Angiotensin-converting enzyme (ACE) in your lungs and kidneys split the angiotensin I into pieces. One of those pieces is Angiotensin II. Angiotensin II is an active hormone.
4. Angiotensin II causes the muscular walls of small arteries to narrow. This increases blood pressure. Angiotensin II triggers your adrenal glands to release aldosterone. It also triggers your pituitary gland to release antidiuretic hormone (ADH or vasopressin).
5. Together, aldosterone and ADH cause your kidneys to hold onto (retain) sodium. Aldosterone also causes your kidneys to release potassium through your pee.
6. The increase in sodium in your bloodstream causes your body to hold onto water. More water increases blood volume and blood pressure. This completes the renin-angiotensin-aldosterone system.

Other hormones also activate the RAAS, including:

• Corticosteroids
• Estrogen
• Thyroid hormone

What organs and body systems are involved in the RAAS?

The RAAS involves several organs, glands and tissues. These include your:

• Kidneys
• Liver
• Blood vessels
• Lungs
• Adrenal glands
• Pituitary gland
• Hypothalamus

How is the RAAS related to heart failure?

The renin-angiotensin-aldosterone system is key to the process of heart failure. At first, changes in the RAAS happen because your heart can’t meet the blood flow demands of important organ systems.

Then, the RAAS releases more angiotensin II to make up for the lack of blood flow. This increase in angiotensin II makes heart failure worse. Experts believe that too much angiotensin II can make your heart enlarge (dilate). Because of this, healthcare providers use different medications that directly affect the RAAS to treat heart failure. These medications include:

• Beta-blockers
• ACE inhibitors
• Angiotensin receptor blockers (ARBs)
• Angiotensin receptor/neprilysin inhibitor (ARNI)
• Mineralocorticoid receptor antagonists (MRA)
• Sodium-glucose cotransporter 2 (SGLT2) inhibitors

What is the difference between the renin-angiotensin-aldosterone system and the baroreceptor reflex?

The RAAS and baroreceptor reflex (baroreflex) both play important roles in regulating your blood pressure. The RAAS regulates blood volume and vascular resistance in the long term. The baroreflex is a short-term regulator of your blood pressure. Arterial baroreceptors tell your autonomic nervous system if there are small or rapid beat-to-beat changes in your blood pressure.