Structure and Function of the RAAS
The system includes the following key components:
- Renin — a proteolytic enzyme (protease) secreted by the juxtaglomerular cells of the kidneys. It cleaves a peptide from angiotensinogen to form angiotensin I.
- Angiotensin II — a potent vasoconstrictor with a pronounced blood vessel-constricting effect. It is also a dipsogenic substance, meaning it stimulates water intake (thirst). It interacts with specific membrane receptors on target cells (AT1 and AT2), increases sympathetic nervous system tone, and stimulates aldosterone secretion.
- Aldosterone — a mineralocorticoid produced by the adrenal cortex. It stimulates the reabsorption of sodium ions ($Na^+$) and the excretion of potassium ions ($K^+$) in the renal tubules.
How the System Regulates Blood Volume and Pressure
The RAAS plays a central role in regulating blood pressure and fluid-electrolyte homeostasis.
compensatory mechanism during hypovolemia and decreased blood pressure involves the following steps:
- A decrease in circulating blood volume activates antidiuretic hormone (ADH); constriction of interlobular and afferent arterioles reduces the glomerular filtration rate (GFR) and leads to ischemia of the juxtaglomerular apparatus (JGA) cells.
- JGA ischemia triggers the secretion of renin, leading to the production of angiotensin II.
- Angiotensin II increases arteriolar tone, stimulates catecholamine release, and activates aldosterone secretion.
- Aldosterone stimulates tubular reabsorption of $Na^+$.
- Elevated blood $Na^+$ levels cause plasma hyperosmolality, stimulating vascular osmoreceptors and prompting additional ADH secretion by the hypothalamus.
- Result: fluid retention within the vasculature is enhanced, and plasma volume is restored.
RAAS Changes During Pregnancy
During normal pregnancy, the components of the RAAS undergo significant changes:
- Angiotensin II levels increase 2- to 3-fold.
- Plasma renin activity increases 2- to 4-fold.
- Plasma and urinary aldosterone levels rise 3-fold in the first trimester and up to 10-fold by the third trimester.
Role of the RAAS in Pathology
The RAAS is implicated in several pathological conditions:
- Hemorrhage: during compensatory fluid shifts following blood loss, JGA ischemia triggers renin secretion and angiotensin II formation; aldosterone stimulates $Na^+$ reabsorption, assisting in the restoration of plasma volume.
- Hypertension: the RAAS acts as a pressor mechanism (Renin → Angiotensin II → Aldosterone), leading to $Na^+$ retention and increased circulating blood volume.
- Renal Pathology: activation of the RAAS in the setting of suppressed prostaglandin and kallikrein-kinin systems is associated with sodium and water retention, hypervolemia, hypernatremia, and elevated total peripheral resistance.
- Chronic Heart Failure: reduced left ventricular systolic function is accompanied by the activation of pressor systems, including the RAAS. The hemodynamic effects of these neurohumoral factors include peripheral vasoconstriction and sodium/fluid retention, which increase left ventricular preload and afterload. Furthermore, they stimulate myocardial fibrosis and cardiomyocyte apoptosis.
- Pharmacological Effects: drug-induced reduction in blood pressure triggers compensatory mechanisms involving sodium and water retention. Lowered blood pressure reduces renal blood flow, increases renin secretion, activates angiotensin II formation, raises aldosterone levels, and ultimately leads to sodium and water retention.