The Renin-Angiotensin System (RAS) regulates blood pressure, fluid balance, and electrolyte homeostasis, with critical roles in kidney function and cardiovascular health.
Components of the RAS
1.Renin:
Released by the juxtaglomerular cells in response to low blood pressure, low blood volume, or low sodium.
2.Angiotensinogen:
A protein produced by the liver that is converted to angiotensin I by renin.
3.Angiotensin I:
Inactive, but converted into angiotensin II by angiotensin-converting enzyme (ACE).
4.Angiotensin II:
A potent vasoconstrictor that also stimulates aldosterone secretion from the adrenal cortex.
Role of RAS in Kidney Functions
1. Regulation of Blood Pressure and Volume
Angiotensin II causes vasoconstriction, raising blood pressure, and stimulates aldosterone secretion to increase sodium and water reabsorption, raising blood volume.
2. Sodium and Water Reabsorption
Aldosterone promotes sodium reabsorption in the distal tubules and collecting ducts, and water follows sodium, increasing blood volume.
3. Direct Action on the Kidneys
Angiotensin II enhances sodium reabsorption in the proximal tubule and reduces renal blood flow by constricting arterioles.
4. Feedback Regulation
High levels of angiotensin II inhibit further renin release. Natriuresis (excretion of sodium) helps balance sodium reabsorption and water retention, maintaining blood pressure.
Pathophysiological Implications
Overactivation of RAS can lead to hypertension and chronic kidney disease, creating a cycle that worsens both kidney function and blood pressure regulation.