Learning Objective: By the end of this lesson, the learner will be able to interpret changes in intracellular (ICF) and extracellular (ECF) volumes using Darrow-Yannet diagrams, predict the effects of hypertonic, hypotonic, and isotonic fluid shifts on cell and ECF volumes, and relate these changes to common clinical scenarios such as dehydration, hemorrhage, fluid infusion, SIADH, and adrenal insufficiency.
Graphical Representation of Body Compartments (Darrow-Yannet Diagrams)
Body Compartments
- ICF ≈ 2/3 of total body water
- ECF ≈ 1/3 of total body water
Steady-State Osmolality
- ICF osmolality = ECF osmolality
- Water moves freely across the cell membrane to balance osmolality
Volume Changes
- ECF volume changes with net fluid gain or loss
- ICF volume changes only if ECF osmolality changes
General Principles
- ECF hypertonicity → cells shrink
- ECF hypotonicity → cells swell
Activity
Activity
Clinical Darrow-Yannet Patterns
| Scenario | ECF Volume | ECF Osmolality | ICF Volume | Example Causes |
|---|---|---|---|---|
| Loss of isotonic fluid | ↓ | no change | no change | Hemorrhage, diarrhea, vomiting |
| Loss of hypotonic fluid | ↓ | ↑ | ↓ | Dehydration, diabetes insipidus, and alcoholism |
| Gain of isotonic fluid | ↑ | no change | no change | Isotonic saline, aldosterone excess |
| Gain of hypotonic fluid | ↑ | ↓ | ↑ | Water intoxication, SIADH, hypotonic IV |
| Gain of hypertonic fluid | ↑ | ↑ | ↓ | Hypertonic saline, mannitol infusion |
| Loss of hypertonic fluid | ↓ | ↓ | ↑ | Adrenal insufficiency (Na⁺ loss) |
