D2.3.7—Medical applications
Medical fluids and organ baths must be isotonic so body cells avoid harmful osmotic swelling or shrinkage during treatment and transplantation.
- Syllabus
- First assessment 2025
- Objective
- D2.3.7
- Level
- SL
Exam analysis
Chance of appearing1%of analysed past papers
Latest appearanceMay 2021
Most common paperPaper1
Typical marks1
Common command terms
- Explain
Recent exam appearances
May 2021Paper1 ["SL"] · TZ14[ 1 ]D2.3.7—Medical applications
Practice this objective
Coverage 2021–2021 · Updated 16 Jul 2026
Water Potential Guides Medical Fluid Choices
Medical fluids are made isotonic with body tissues to avoid harmful net water gain or loss by cells.
| Application | Why isotonic conditions matter |
|---|---|
| Intravenous fluid | Prevents red blood cells and other body cells swelling, lysing, shrinking or crenating while fluid/solutes are delivered |
| Organ awaiting transplantation | An isotonic bathing solution limits osmotic damage to the organ's cells before implantation |
An isotonic saline infusion replaces extracellular fluid without causing appreciable net movement of water into or out of red blood cells.
Isotonic means matched effective osmotic concentration relative to the tissue; it does not mean the solution has the same chemical composition as cytoplasm.
Medical applications
Assessment in practice
1–4 marksHow it is assessed
This objective is assessed through structured response, multiple choice, commonly using Explain.
Command terms
Explain
What earns marks
Build the answer around this relationship: Isotonic fluids prevent net water movement into or out of body cells.
Representative question
Question 1
Explain the need for isotonic conditions in human blood plasma and tissue fluid.
Core Osmosis Effects
Water forms hydration shells around ions and polar solutes; hydrogen bonding and charge attraction reduce free water movement. Water moves by osmosis across partially permeable membranes from hypotonic/lower solute solutions toward hypertonic/higher solute solutions. Osmosis direction depends on internal and external solute concentration; isotonic conditions have dynamic water movement but no net osmosis. Plant tissue changes mass or length in sucrose solutions; percentage change graphs estimate isotonic or osmotic concentration. Animal cells can lyse in hypotonic solutions and crenate in hypertonic solutions; freshwater protists use contractile vacuoles to expel excess water. Plant cells become turgid in hypotonic solutions as vacuoles swell; hypertonic solutions cause flaccidity and plasmolysis from water loss. Isotonic saline prevents harmful water gain or loss in body cells; IV fluids and transplant organ baths must match tissue osmotic concentration.
Concept essentials
- Isotonic fluids prevent net water movement into or out of body cells.
- Hypotonic medical fluids can cause cell swelling or lysis.
- Hypertonic fluids can cause cell shrinkage or crenation.
- Transplant organs are bathed in solutions matching tissue osmolarity.