Blood, Electrolytes, and Intravenous Infusions
2-8
2-8. DIFFUSION AND OSMOSIS
Even though the predominate movement of hypotonic solutions and hypertonic solutions is in one direction, there is always a weak movement in the opposite direction. This weak movement is caused by diffusion. This process is in contrast to osmosis, which is a directional movement.
a. Diffusion. When a solute is added to a fluid, the molecules will begin immediately to spread throughout the fluid. This process is diffusion. The movement is random. It is the nature of molecules to move constantly in a fluid. Even though no special effort (like stirring) is made to mix the solution, the solute will be evenly
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distributed within a period. An illustration of diffusion is to add ice cubes to a warm liquid. Without stirring, the cold molecules will distribute themselves among the warmer ones. This will create a liquid of uniform temperature. The diffusion process is not limited by the presence of a semi-permeable membrane as long as the molecules are small enough to travel through the membrane. Cell walls are semi-permeable membranes.
b. Osmosis. The process of osmosis is unidirectional across a semi-permeable membrane. When the molecules of a solute are too large to travel through the membrane wall, they will remain on one side. When a solvent is added to the other side of the membrane, the molecules of the solvent will travel through the membrane to the side with the greater concentration of the solute. This is the principle by which the hypotonic and hypertonic solutions move fluid across the cell membranes.
