Renin-Angiotensin-Aldosterone System (RAAS)

What is the renin-angiotensin-aldosterone system (RAAS)?

The renin-angiotensin-aldosterone system (RAAS) is the system of hormones, proteins, enzymes and reactions that regulate your blood pressure and blood volume on a long-term basis.

It regulates your blood pressure by increasing sodium (salt) reabsorption, water reabsorption (retention) and vascular tone (the degree to which your blood vessels constrict, or narrow). The RAAS consists of three major substances, including:

• Renin (an enzyme).
• Angiotensin II (a hormone).
• Aldosterone (a hormone).

Enzymes are proteins that help trigger chemical reactions in your body. They build some substances and break others down.

Hormones are chemicals that coordinate different functions in your body by carrying messages through your blood to your organs, muscles and other tissues. These signals tell your body what to do and when to do it.

What is blood pressure?

Your blood pressure is a measurement of the pressure or force inside your arteries with each heartbeat. Normal pressure is important for the proper flow of blood from your heart to your body’s organs and tissues.

Blood pressure depends on many things, including how much blood your heart is pumping and the diameter (the width) of the arteries your blood is moving through.

Many factors can affect your blood pressure, such as certain medications, high cholesterol and smoking. The renin-angiotensin-aldosterone system (RAAS) is responsible for the basic regulation of blood pressure. If you have high blood pressure (hypertension) or low blood pressure (hypotension), it doesn’t necessarily mean there’s an issue with your body’s RAAS.

What’s the difference between the renin-angiotensin-aldosterone system (RAAS) and the baroreceptor reflex?

Both the RAAS and the baroreceptor reflex, or baroreflex, play important roles in regulating blood pressure.

The renin–angiotensin–aldosterone system (RAAS) is a critical regulator of blood volume and systemic vascular resistance on a long-term basis. The baroreceptor reflex, on the other hand, responds in a short-term manner to decreased blood pressure. Arterial baroreceptors inform your autonomic nervous system of minor or rapid beat-to-beat changes in blood pressure.

What organs and body systems are involved in the RAAS?

Several organs, glands and tissues are involved in the RAAS, including your:

• Kidneys.
• Liver.
• Blood vessels.
• Lungs.
• Adrenal glands (small, triangle-shaped endocrine glands that are located on top of each of your two kidneys).
• Pituitary gland (a small, pea-sized endocrine gland located at the base of your brain below your hypothalamus).
• Hypothalamus (a region of your brain that coordinates your autonomic nervous system and the activity of your pituitary gland).

What are the renin-angiotensin-aldosterone system (RAAS) steps?

The renin-angiotensin-aldosterone system (RAAS) involves several steps, including:

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

The renin-angiotensin-aldosterone system is also activated by other hormones, including corticosteroids, estrogen and thyroid hormones.

How is the RAAS related to heart failure?

The renin-angiotensin-aldosterone system (RAAS) is a central feature in the process of heart failure. Initially, adaptations in the RAAS occur in response to the heart’s inability to meet the blood flow demands of vital organ systems.

In particular, the RAAS releases more of the hormone angiotensin II to try to compensate for the lack of blood flow. This excess of angiotensin II subsequently makes heart failure worse. Scientists believe excess angiotensin II contributes to growth in the size of your heart.

Because of this, healthcare providers use different medications that directly affect the RAAS to treat heart failure. These medications include:

• Beta-blockers.
• Angiotensin-converting enzyme (ACE) inhibitors.
• Angiotensin receptor blockers.
• Mineralocorticoid receptor antagonists (MRA).

A note from Cleveland Clinic

The renin-angiotensin-aldosterone system (RAAS) is crucial for blood pressure regulation. While several factors outside of the RAAS can affect your blood pressure levels, the RAAS can sometimes be responsible. If you’re experiencing symptoms of high or low blood pressure or you’re having a difficult time managing your blood pressure, talk to your healthcare provider. They’re available to help.