Learning Objective: By the end of this lesson, the learner will be able to explain how negative feedback regulates RAAS and ADH secretion in response to changes in extracellular volume and osmolality, and predict hormonal responses in various fluid balance scenarios.
RAAS (Renin–Angiotensin–Aldosterone System)
- Stimulus: ↓ renal perfusion / ↓ extracellular volume
- Effect: ↑ renin → ↑ Ang II → ↑ aldosterone → ↑ Na⁺ reabsorption → ↑ extracellular volume
- Feedback: Restoration of blood pressure and ECF volume inhibits further RAAS activation
ADH (Vasopressin)
- Stimuli:
- ↑ plasma osmolality → ↑ ADH
- ↓ blood volume/pressure → ↑ ADH
- Effect: ↑ water reabsorption → restores plasma osmolality and volume
- Feedback: Normalization of osmolality and blood pressure inhibits ADH
Activity
Application to Darrow-Yannet Scenarios
| Figure | ECF Volume / Osmolality Change | RAAS Response | ADH Response | Notes |
|---|---|---|---|---|
| I-1-5 | ↓ ECF volume, no osmolality change | ↑ RAAS | ↑ ADH | Loss of isotonic fluid → extracellular volume loss stimulates both |
| I-1-6 | ↓ ECF volume, ↑ osmolality | ↑ RAAS | ↑↑ ADH | Loss of hypotonic fluid → strong ADH stimulus due to hypertonicity |
| I-1-7 | ↑ ECF volume, ↑ osmolality | ↓ RAAS | Variable ADH | Hypertonic fluid gain → RAAS inhibited; ADH depends on the magnitude of osmolality vs volume change |
| I-1-8 | ↑ ECF volume, ↓ osmolality | ↓ RAAS | ↓ ADH | Hypotonic fluid gain → both RAAS and ADH inhibited |
| I-1-9 | ↑ ECF volume, no osmolality change | ↓ RAAS | ↓ ADH | Isotonic fluid gain → RAAS and ADH inhibited |
| I-1-10 | ↓ ECF volume, ↓ osmolality | ↑ RAAS | Variable ADH | Hypertonic fluid loss (e.g., adrenal insufficiency) → RAAS stimulated; ADH depends on the balance of osmolality vs volume |
