High-yield map
Key points before the detail
- 01
Use 60% of body weight only as an approximation for a lean adult man, not a universal constant.
- 02
Roughly two-thirds of total body water is intracellular and one-third extracellular.
- 03
Osmolality counts dissolved particles; tonicity describes the effective osmotic force that changes cell volume.
- 04
After glucose is metabolised, 5% glucose behaves largely as electrolyte-free water and is not a resuscitation fluid.
The major compartments
| Compartment | Approximate 70 kg lean-male example | Dominant cation |
|---|---|---|
| Total body water | About 42 L | — |
| Intracellular fluid | About 28 L | Potassium |
| Extracellular fluid | About 14 L | Sodium |
| Interstitial fluid | About 10.5 L | Sodium |
| Plasma | About 3.5 L | Sodium |
What separates the spaces?
Cell membranes are highly permeable to water but maintain strong concentration gradients for many ions through transporters and pumps. The capillary endothelium separates plasma from interstitial fluid and is much more permeable to small electrolytes than to large plasma proteins. These different barriers explain why sodium-rich crystalloids mainly expand the extracellular space while water can ultimately distribute across total body water.
Osmolality versus tonicity
Osmolality is the concentration of osmotically active particles per kilogram of water. Tonicity is narrower: it describes the effect of non-freely permeable solutes on cell volume. A solute can contribute to measured osmolality without producing a sustained osmotic gradient if it crosses cell membranes readily.
- Hypertonic extracellular fluid draws water out of cells.
- Hypotonic extracellular fluid allows water to enter cells.
- Rapid changes in effective osmolality are especially important in the brain because the rigid skull limits expansion.
Why common fluids behave differently
| Fluid concept | Where it distributes | Practical implication |
|---|---|---|
| Isotonic crystalloid | Extracellular space | Only a minority remains in plasma after equilibration; useful when extracellular volume needs expansion. |
| 5% glucose after metabolism | Water distributes across total body water | Very little remains intravascular; unsuitable for resuscitation. |
| Hypertonic solution | Raises extracellular effective osmolality | Draws water from intracellular to extracellular space; specialist indications require careful monitoring. |
Surgical links
- Vomiting, stomas, fistulae and drains change both volume and electrolyte balance.
- Inflammation and sepsis alter vascular permeability and can redistribute fluid away from the effective circulation.
- Oedema does not automatically mean the circulation is adequately filled.
- Renal and hormonal systems continually adjust water and sodium handling, so fluid balance is dynamic rather than a static set of boxes.
Active recall
Close the notes and answer these
Try each question from memory before revealing the answer. These public prompts are a small preview of the integrated retrieval system inside SurgAtlas.
01What fraction of total body water is intracellular in the simple adult model?
About two-thirds.
02What is the difference between osmolality and tonicity?
Osmolality counts dissolved particles; tonicity reflects the effective osmotic gradient created by solutes that do not freely cross cell membranes and therefore change cell volume.
03Why is 5% glucose not a resuscitation fluid?
Once glucose is metabolised, the remaining water distributes across total body water, leaving very little sustained intravascular expansion.
Sources & editorial basis
References & editorial basis
- SurgAtlas production chapter — Applied Physiology — Fluids, Electrolytes & Renal Physiology. Primary source for this public lesson. Stable physiological principles have been condensed from the corresponding SurgAtlas production teaching.
- Guyton and Hall Textbook of Medical Physiology. Reference for stable body-fluid and osmoregulatory physiology.
This lesson is derived from the corresponding SurgAtlas production teaching material. Where the source makes current management or guideline claims, the public lesson uses the cited contemporary guidance. It is written for education and examination preparation, not as patient-specific clinical advice.
Editorial details
Medical Doctor (MD) · MRCS Part A · Physician · Surgical Educator
SurgAtlas is an educational resource. For patient care, verify current national guidance, local antimicrobial and transfusion policies, specialty pathways and individual patient factors.
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