What is the renin-angiotensin-aldosterone system?
The renin-angiotensin-aldosterone system (RAAS, often improperly called the RAAS system) controls the water and electrolyte balance of our organism and thus has a decisive effect on blood pressure:
Since the functioning of our circulatory system depends on precise regulation of blood volume, mechanisms are required to balance the volume of fluid inside and outside the blood vessels (intra- and extravascular) in the short term. The renin-angiotensin-aldosterone system participates critically in the control of blood volume by regulating fluid and electrolyte balance.
What is the function of the renin-angiotensin-aldosterone system?
When there is a lack of volume in the body (for example, due to severe blood loss), blood flow to the renal arteries is reduced and the pressure prevailing in them drops. In response, certain kidney cells (juxtaglomerular cells) secrete renin as part of the renin-angiotensin-aldosterone system. This protein-cleaving enzyme converts the blood protein (plasma protein) angiotensinogen, which originates from the liver, into the hormone precursor angiotensin I.
Angiotensin II causes the vessels to constrict (vasoconstriction), which increases blood pressure. It stimulates the release of the hormone aldosterone from the adrenal gland. This causes the kidneys to retain more sodium and water in the body (instead of excreting it in the urine). This increases the sodium content and volume of the blood, which also raises blood pressure.
In addition, angiotensin II promotes thirst sensation (fluid intake increases blood volume and thus blood pressure), salt appetite, and the release of ADH (antidiuretic hormone, vasopressin) from the pituitary gland. This hormone inhibits water excretion via the kidneys (diuresis) – blood pressure rises.
A lack of sodium in the body also triggers the release of renin and thus the activation of the renin-angiotensin-aldosterone system (RAAS).
Where is the renin-angiotensin-aldosterone system located?
What problems can the renin-angiotensin-aldosterone system cause?
Drugs can be used to intervene in the renin-angiotensin-aldosterone system and thus influence blood pressure regulation. For example, beta blockers or ACE inhibitors are given to treat high blood pressure. Beta-blockers inhibit renin release, while ACE inhibitors block ACE and thus the formation of angiotensin II. In both cases, this results in a drop in blood pressure.
There are also drugs that inhibit the action of aldosterone (aldosterone antagonists such as spironolactone). They are mainly used as diuretics, for example in heart failure.
In the so-called Conn syndrome (primary hyperaldosteronism), excessive amounts of aldosterone are secreted. The cause is a disease of the adrenal cortex (such as a tumor).
In secondary hyperaldosteronism, the body also secretes too much aldosterone. The reason is usually excessive activation of the renin-angiotensin-aldosterone system, for example due to kidney disease (such as narrowing of the renal arteries = renal artery stenosis).
