Lecture on Tonicity and Fluid Compartments

Key Topics

• Difference between iso-, hypo-, and hypertonic solutions
• Effects of plasma tonicity changes on intra- and extracellular fluid

Fluid Compartments

• Intracellular Fluid (ICF): Fluid inside cells, separated by the cell membrane
• Extracellular Fluid (ECF): Fluid outside cells, including plasma and interstitial fluid
  • Plasma: Fluid within capillaries
  • Interstitial Fluid: Fluid between cells
• Cell Membrane: Permeable to water but not salts like sodium or chloride
• Capillary Endothelium: Permeable to water and salts but not albumin

Osmolarity Concepts

• Osmolarity: Concentration of solutes in a solution
• Volume: Amount of fluid in a compartment

Examples Using Orange Juice Analogy

• Isotonic Addition: Adding same concentration fluid increases volume but not concentration
• Hypertonic Addition: Adding more concentrated fluid increases volume and concentration
• Hypotonic Addition: Adding less concentrated fluid increases volume but decreases concentration
• Evaporation: Water loss increases concentration while decreasing volume

Effects on Fluid Compartments

• Isotonic Solution (Letter A): Increase in ECF volume without change in osmolarity
• Hypertonic Solution (Letter C): Increase in ECF volume and osmolarity, water moves from ICF to ECF
• Hypotonic Solution (Letter B): Increase in ECF volume, decrease in ECF osmolarity, water moves from ECF to ICF
• Water Loss (Letter D): Decrease in volume, increase in osmolarity, water moves from ICF to ECF

Practical Questions

1. Isotonic Saline Infusion

   • Increase in ECF volume
   • No change in osmolarity
   • Letter A

2. Hypertonic Saline Infusion (3% saline)

   • Increase in ECF volume and osmolarity
   • Water moves from ICF to ECF
   • Letter C

3. Hypotonic Saline Infusion

   • Increase in ECF volume
   • Decrease in ECF osmolarity
   • Water moves from ECF to ICF
   • Letter B

4. Water Loss (e.g., fever, vomiting, diarrhea)

   • Decrease in volume, increase in osmolarity
   • Water moves from ICF to ECF
   • Letter D

Key Takeaways

• Understanding tonicity helps predict fluid shifts between compartments
• Changes in fluid volume and osmolarity have significant physiological effects