Body Fluid Compartments and Tonicity: A Study Guide

A compartment tells you where water is; tonicity helps you predict whether cells gain or lose it. This free guide for MBBS revision and postgraduate foundational review separates those questions.

Updated 26 October 2026.

Learning objectives

  • Distinguish intracellular fluid, interstitial fluid and plasma.
  • Explain why osmolarity and tonicity answer different questions.
  • Predict cell-volume changes in an idealised non-penetrating-solute experiment.

1. Start with the barriers

Intracellular fluid (ICF) is water inside cells. Extracellular fluid (ECF) is outside cells and includes interstitial fluid and plasma. Cell membranes separate ICF from ECF; capillary walls separate plasma from the surrounding interstitial fluid. Red-cell water belongs to ICF, so blood volume and plasma volume are different quantities.

Sodium is the principal extracellular cation; potassium is the principal intracellular cation. Plasma contains more protein than interstitial fluid. A difference in composition does not imply a sustained difference in overall osmolality across a water-permeable cell membrane.

2. Count particles, then ask whether they cross

Osmolarity counts dissolved particles per litre of solution; osmolality counts particles per kilogram of solvent. Tonicity describes the effect of a solution on cell volume and depends on solutes that remain effectively separated by the relevant membrane. State the membrane and timescale before predicting a response.

A penetrating solute can contribute to measured osmolality yet fail to sustain the same water shift as a non-penetrating solute. This is why “same osmolarity” does not automatically establish “same tonicity”.

3. Compare cell responses

Idealised cells exposed to solutions classified relative to their interior
External solution Net water movement initially Cell-volume response
Hypotonic Into cells Swelling
Isotonic No sustained net movement No sustained change
Hypertonic Out of cells Shrinkage

4. Worked example: predict before calculating

Imagine a cell initially containing 300 mOsm/L of trapped solute, placed in a large bath containing 200 mOsm/L of non-penetrating particles. Assume water crosses freely, intracellular solute amount stays fixed, the bath concentration stays constant, and there are no mechanical or regulatory limits.

The external solution is hypotonic. Water enters. At osmotic equilibrium, intracellular concentration approaches 200 mOsm/L. Since concentration equals amount divided by volume, 300 × initial volume = 200 × final volume. Final volume is therefore 1.5 times initial volume: a 50% increase.

This is a teaching model, not a prescription or an exact prediction for a living cell. Real membranes, cell-volume regulation and membrane rupture can change the outcome.

5. Common misconceptions

  • “ECF means plasma.” Plasma is only one part; interstitial fluid is also ECF.
  • “Isotonic means water stops moving.” Water exchanges in both directions; there is no sustained net shift changing volume.
  • “All counted particles sustain tonicity equally.” Membrane permeability matters.
  • “A water shift requires ATP-driven water pumping.” Osmosis is passive; transporters help establish the solute distributions.

6. Revision questions and answers

  1. Which barrier separates ICF from interstitial fluid?
    Answer: The cell membrane.
  2. Does red-cell water contribute to plasma volume?
    Answer: No. It is intracellular water.
  3. What happens when external effective solute concentration rises?
    Answer: Water tends to leave cells, reducing their volume, assuming the solute remains extracellular.
  4. Why can two equally osmolar solutions affect cells differently?
    Answer: Their solutes may have different membrane permeabilities.
  5. What assumption makes the worked calculation possible?
    Answer: Intracellular solute amount is fixed, with a constant external concentration and freely moving water.

7. Choose a next step

Try the free Homeostasis sample to explore a related foundation. For focused reading, see the Body Fluid Compartments chapter. Compare scope and current details on the listing before purchasing. You can also use our physiology resource-selection hub.

References and further reading

Original educational explanations checked against OpenStax Anatomy and Physiology 2e: 26.1, Body Fluids and Fluid Compartments (compartment definitions and composition), and 3.1, The Cell Membrane (osmosis and cell responses). Use your prescribed physiology text for further depth.