Renin–Angiotensin System (RAS)
Introduction
The Renin–Angiotensin System (RAS) is a hormonal mechanism that plays a major role in regulating blood pressure, blood volume, and electrolyte balance.
It ensures that the kidneys receive an adequate blood supply and that the body maintains proper blood pressure and fluid balance.
Original Notes:
Components of the Renin–Angiotensin System
The RAS involves the following major components:
- Renin – an enzyme secreted by the juxtaglomerular cells of the kidneys.
- Angiotensinogen – a plasma protein produced by the liver.
- Angiotensin I – an inactive peptide formed from angiotensinogen.
- Angiotensin-Converting Enzyme (ACE) – mainly found in the lungs, converts angiotensin I into angiotensin II.
- Angiotensin II – an active hormone that causes vasoconstriction and stimulates aldosterone secretion from the adrenal gland.
Steps of the Renin–Angiotensin Mechanism
- Trigger (Low Blood Pressure or Sodium Levels):
- When blood pressure drops or sodium levels decrease, the juxtaglomerular cells in the kidney detect this change.
- They respond by secreting renin into the bloodstream.
- Formation of Angiotensin I:
- Renin acts on angiotensinogen (from the liver) and converts it into angiotensin I, which is inactive.
- Conversion to Angiotensin II:
- Angiotensin I is carried to the lungs, where Angiotensin-Converting Enzyme (ACE) converts it into angiotensin II.
- Actions of Angiotensin II:
- Vasoconstriction: Narrows blood vessels, increasing blood pressure.
- Aldosterone Secretion: Stimulates the adrenal cortex to release aldosterone, which increases sodium and water reabsorption in the kidneys.
- Increased Thirst: Stimulates the hypothalamus, leading to water intake.
- ADH Release: Promotes secretion of antidiuretic hormone (ADH) from the posterior pituitary, aiding water retention.
Effects of the Renin–Angiotensin System
| Effect | Description | Result |
|---|---|---|
| Vasoconstriction | Constriction of arterioles | Increases blood pressure |
| Aldosterone Secretion | Increases sodium and water reabsorption | Increases blood volume |
| ADH Secretion | Enhances water retention | Prevents dehydration |
| Stimulation of Thirst | Promotes water intake | Restores fluid balance |
Physiological Importance
- Maintains Blood Pressure: Raises blood pressure during hypotension.
- Regulates Blood Volume: By increasing sodium and water reabsorption.
- Supports Kidney Function: Ensures sufficient glomerular filtration rate (GFR).
- Maintains Electrolyte Balance: By controlling sodium and potassium levels.
Clinical Significance
- Overactivation of RAS can cause hypertension (high blood pressure) and heart failure.
- ACE Inhibitors (e.g., captopril, enalapril) and Angiotensin Receptor Blockers (ARBs) (e.g., losartan) are used to block the RAS, lowering blood pressure.
