Overview
This lecture discusses the differences between osmolarity and osmolality, their definitions, accuracy, and practical use in clinical settings, and reviews related calculations and concepts from previous lessons.
Review of Previous Questions
- If solute is higher in column A than B, water moves from B to A until equilibrium.
- Osmolality calculations involve ionizable compounds splitting into individual particles (osmoles).
- Example: 1 mole NaCl in 1 L = 2 osmoles; 1 mole glucose in 0.5 L = 2 osmoles per liter; 1 mole CaCl2 gives 3 osmoles; 6 moles NaCl in 0.5 L gives 12×2=24, check calculation context.
Key Concepts: Osmosis & Diffusion
- Osmosis is the passive movement of water from high water concentration to low (high solute) concentration.
- Simple diffusion is the movement of molecules from high to low concentration without energy.
Osmotic Pressure
- Osmotic pressure is the pressure needed to stop osmosis.
Osmolarity vs. Osmolality
- Osmolality: Number of osmoles per kilogram of solvent (mass-based).
- Osmolarity: Number of osmoles per liter of solvent (volume-based).
- Mnemonic: "Osmolality by the kilo, osmolarity by the liter."
- Osmolality is more accurate because mass is constant even if temperature changes; volume may expand with heat.
- Osmolarity is more practical in clinical settings, as volume is easier to measure than mass.
- Clinicians often use the terms interchangeably due to practical convenience.
Clinical Application Example
- Giving 100% water instead of saline to a patient causes red blood cells to swell and burst (hemolysis).
- Normal saline is a salt (NaCl) solution used to maintain electrolyte balance.
Key Terms & Definitions
- Osmosis — Diffusion of water across a semipermeable membrane from low to high solute concentration.
- Osmotic Pressure — The force required to prevent osmosis.
- Osmolarity — Number of osmoles of solute per liter of solution.
- Osmolality — Number of osmoles of solute per kilogram of solvent.
- Simple Diffusion — Movement of molecules from high to low concentration without energy use.
Action Items / Next Steps
- Review concepts of osmosis, osmolarity, and osmolality.
- Practice calculating osmolarity/osmolality for different solutions.
- Prepare for next lesson on edema and hydrostatic pressure.