Learning Objective: By the end of this lesson, the learner will be able to explain osmosis, differentiate osmolarity and osmolality, and identify effective osmoles that determine fluid shifts across cell and capillary membranes.

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Osmosis

Osmosis is the diffusion of water across a semipermeable membrane from a region of higher water concentration to a region of lower water concentration.

Water concentration is inversely related to solute concentration:

• ↑ Solute → ↓ Water concentration
• ↓ Solute → ↑ Water concentration

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Key Concepts

1. Osmolarity

• mOsm/L
• Number of solute particles per liter of solution

2. Osmolality

• mOsm/kg
• Number of solute particles per kilogram of solvent (water)
• More accurate in physiology because water volume may vary, but mass does not.

Clinically: The body regulates osmolality, not osmolarity.

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Mechanism of Osmosis

Two fluid compartments are separated by a membrane permeable to water but not solute.

• Side B has more solute → lower water concentration
• Water diffuses from A → B
• Fluid rises on side B and falls on side A

This is the basis for osmotic gradients in cells and capillaries.

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Effective Osmoles

A solute is an effective osmole if it cannot freely cross a membrane, forcing water to shift. Effective osmoles by membrane type

Capillary Membrane

• Plasma proteins (e.g., albumin)
  • Do not cross capillaries
  • Maintain plasma oncotic pressure

Cell Membrane

• Na+
  • Does not cross cell membranes easily
  • Major determinant of ECF osmolality and volume

Not effective osmoles across cell membranes:

• Urea (freely crosses the cell membrane → no sustained water shift)

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