6. 1 Basic Principles of Osmosis and Osmotic Pressure

• What is a frequent problem in treating seriously ill patients?

  Maintaining adequate fluids in compartments
• What determines the relative amounts of extracellular fluid?

  Balance of hydrostatic and colloid osmotic forces
• What mainly determines fluid distribution between intracellular and extracellular compartments?

  Osmotic effect of smaller solutes
• Why are cell membranes highly permeable to water?

  They are relatively impermeable to small ions
• What happens to intracellular fluid in relation to extracellular fluid?

  It remains isotonic with extracellular fluid
• What are the interrelations between intracellular and extracellular fluid volumes?

  • Osmotic factors cause fluid shifts
  • Fluid volumes are regulated by solute concentrations
• What principle was presented in Chapter 4 regarding osmosis?

  Basic principles of osmosis and osmotic pressure
• What occurs when there is a higher concentration of solute on one side of the cell membrane?

  Water diffuses toward the higher solute concentration
• What happens when sodium chloride is added to extracellular fluid?

  Water diffuses from cells into extracellular fluid
• What is the rate of diffusion of water called?

  Rate of osmosis
• How is osmolality defined?

  Osmoles per kilogram of water
• How is osmolarity defined?

  Osmoles per liter of solution
• When can osmolality and osmolarity be used synonymously?

  In dilute solutions like body fluids
• Why are calculations based on osmolarities preferred in clinical settings?

  It's easier to express in liters of fluid
• What does van’t Hoff’s law help calculate?

  Potential osmotic pressure of a solution
• How is the osmotic pressure of a 0.9% sodium chloride solution calculated?

  Using grams per 100 mL of solution
• What is the molarity of a 0.9% sodium chloride solution?

  About $0.154 \text{ mol/L}$
• What is the osmolarity of a 0.9% sodium chloride solution?

  About $308 \text{ mOsm/L}$
• What is the correction factor used for in osmotic pressure calculations?

  To account for interionic attraction
• What is the osmotic coefficient for sodium chloride?

  About 0.93
• What is the actual osmolarity of a 0.9% sodium chloride solution after correction?

  About $286 \text{ mOsm/L}$
• What is the approximate osmolarity of body fluids?

  • About 300 mOsm/L for all compartments
  • Plasma is about 1 mOsm/L greater than interstitial and intracellular fluids
  • 80% of interstitial fluid and plasma osmolarity is due to sodium and chloride ions
• What causes the slight difference in osmolarity between plasma and interstitial fluid?

  Osmotic effects of plasma proteins
• What are corrected osmolar activities of body fluids?

  • Adjustments for interionic attraction
  • Reflect slight decrease in osmotic activity