Because of the diffusion force water is "pushed" into a solution which contains less water (in other words it is more concentrated). This is called osmotic pressure.
Osmotic pressure is directly related to the concentration of solutes in the solution.
So if red blood cells (erythrocytes) are in a solution (blood serum) that has the same water concentration as the water concentration inside the red blood cells, the erythrocytes are said to be in isotonic solution.
If the water concentration outside of erythrocytes is less than inside (red blood cells are floating in a denser, less water containing medium) we say they are in hypertonic medium.
On the other hand, if the water concentration inside of erythrocytes is greater than outside (red blood cells are floating in a more aqueous medium) we say they are in a hypotonic medium.
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