The renin-angiotensin-aldosterone system (RAAS) maintains long-term blood pressure primarily through which of the following mechanisms?

Question

Accepted Answer

A. Increased sodium and water reabsorption in the kidney, leading to increased blood volume and pressure. ## RAAS and Long-Term Blood Pressure Regulation

### Overview
**Key Point:** The RAAS is the primary long-term (hours to days) blood pressure regulation system. Unlike the baroreceptor reflex, which operates acutely, the RAAS works through fluid volume expansion and vasoconstriction.

### Mechanism of Action
1. **Trigger:** Decreased renal perfusion pressure or decreased plasma Na^+^ concentration
2. **Renin release:** Juxtaglomerular cells release renin
3. **Angiotensinogen → Angiotensin I:** Renin cleaves angiotensinogen (hepatic origin)
4. **Angiotensin I → Angiotensin II:** ACE (in lung endothelium) converts Ang I to Ang II
5. **Dual effects of Ang II:**
   - **Vasoconstriction:** Direct effect on vascular smooth muscle (immediate, minutes)
   - **Aldosterone secretion:** Stimulates adrenal cortex → ↑ Na^+^ and H~2~O reabsorption in collecting duct
6. **Volume expansion:** Increased blood volume → increased cardiac output → increased blood pressure

### RAAS vs. Baroreceptor Reflex
| Feature | Baroreceptor | RAAS |
|---------|--------------|------|
| **Onset** | Seconds | Hours–days |
| **Primary mechanism** | Neural (sympathetic/parasympathetic) | Hormonal (Ang II, aldosterone) |
| **Main effect** | ↓ HR, ↓ contractility, vasodilation | ↑ Blood volume, vasoconstriction |
| **Adaptation** | Yes (within days) | No (sustained) |
| **Chronic HTN** | Not responsible | Major contributor |

**High-Yield:** The RAAS is responsible for sustained hypertension in many pathological states (renal artery stenosis, primary hyperaldosteronism, chronic kidney disease). ACE inhibitors and ARBs block this system therapeutically.

**Mnemonic:** **RAAS = Volume & Vasoconstriction** — Renin → Angiotensin II → Aldosterone → Sodium/water retention → Blood volume ↑ → BP ↑

Answer

B. Direct vasodilation of arterioles and capillaries to increase tissue perfusion

Answer

C. Rapid sympathetic activation and parasympathetic withdrawal within seconds of pressure drop

Answer

D. Inhibition of antidiuretic hormone (ADH) secretion to promote diuresis

The renin-angiotensin-aldosterone system (RAAS) maintains long-term blood pressure primarily through which of the following mechanisms?

Explanation

RAAS and Long-Term Blood Pressure Regulation

Overview

Mechanism of Action

RAAS vs. Baroreceptor Reflex

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Feature	Baroreceptor	RAAS
Onset	Seconds	Hours–days
Primary mechanism	Neural (sympathetic/parasympathetic)	Hormonal (Ang II, aldosterone)
Main effect	↓ HR, ↓ contractility, vasodilation	↑ Blood volume, vasoconstriction
Adaptation	Yes (within days)	No (sustained)
Chronic HTN	Not responsible	Major contributor