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M04.03.05 Graphical Representation of Body Compartments
Understanding how body osmolality, and intracellular, and extracellular volumes change in clinically relevant scenarios is essential for diagnosing and managing fluid imbalances. This section explains these changes using Darrow-Yannet diagrams.
Overview of Osmolality and Fluid Distribution
- Y-Axis: Represents solute concentration or osmolality.
- X-Axis: Represents fluid volume, divided into:
- Intracellular Fluid (ICF): 2/3 of total body fluid.
- Extracellular Fluid (ECF): 1/3 of total body fluid.
In the control state, represented by a solid line, dashed lines demonstrate changes in osmolality, intracellular, and extracellular volumes. For example, a decrease in osmolality and extracellular volume, along with an increase in intracellular volume, can be visualized.
Darrow-Yannet Diagram Explanation
The diagram illustrates fluid shifts between ICF and ECF. It’s a tool for understanding fluid volume changes in different clinical situations.
| Parameter | Range |
|---|---|
| Na+ | 136–145 mEq/L |
| K+ | 3.5–5.0 mEq/L |
| Cl– | 100–106 mEq/L |
| HCO3– | 22–26 mEq/L |
| BUN | 8–25 mg/dl |
| Cr | 0.8–1.2 mg/dl |
| Glucose | 70–100 mg/dl |
Key Concepts
- Extracellular Volume (ECV)
- ECV enlarges when there’s a net gain of fluid in the body.
- ECV decreases with a net loss of body fluid.
- Osmolality
- Solute concentration correlates to body osmolality.
- At a steady state, intracellular and extracellular osmolalities are equal since water moves freely across cell membranes.
- Intracellular Volume
- Changes with the effective osmolality of the extracellular compartment.
- Fluids and solutes enter/leave via the extracellular compartment first.
- ICF volume only changes if ECF osmolality changes.
- Effects of Osmolality Changes on Cells
- Increased ECF osmolality: Cells lose water and shrink.
- Decreased ECF osmolality: Cells gain water and swell.
Clinical Examples with Darrow-Yannet Diagrams
Below are examples of fluid and osmolality changes in clinical settings:
| Figure | Condition | Description | Loss of intracellular and extracellular volumes increased osmolality. |
|---|---|---|---|
| Figure I-1-5 | Isotonic fluid loss | Loss of extracellular volume with no change in osmolality. | Hemorrhage, diarrhea, vomiting. |
| Figure I-1-6 | Hypotonic water loss | Loss of intracellular and extracellular volumes increased osmolality. | Dehydration, sweating, diabetes insipidus. |
| Figure I-1-7 | Hypertonic solute gain | Gain of extracellular volume, increased osmolality, decreased intracellular volume. | Ingestion of salt, hypertonic infusion. |
| Figure I-1-8 | Hypotonic fluid gain | Increase in extracellular and intracellular volumes, decreased osmolality. | Excess water intake, syndrome of inappropriate ADH (SIADH). |
| Figure I-1-9 | Isotonic fluid gain | Increased extracellular volume with no change in osmolality or intracellular volume. | Isotonic fluid infusion, excess aldosterone. |
| Figure I-1-10 | Hypertonic fluid loss | Decreased extracellular volume and osmolality, increased intracellular volume. | Adrenal insufficiency. |
Points to Remember
- Extracellular volume changes are immediate indicators of fluid imbalances, often due to clinical scenarios such as hemorrhage or water intake.
- Intracellular volume only changes when extracellular osmolality shifts, influencing water movement into or out of cells.
- Normal solute concentration ranges are critical for maintaining osmolality and are provided in clinical exams to avoid memorization but aid in time management.
- Understanding these diagrams helps predict fluid shifts and their clinical causes, such as dehydration or fluid overload.
Bibliography
- Guyton, A. C., & Hall, J. E. (2015). Textbook of Medical Physiology. Elsevier.
- Boron, W. F., & Boulpaep, E. L. (2016). Medical Physiology. Elsevier.
- Marieb, E. N., & Hoehn, K. (2018). Human Anatomy & Physiology. Pearson.