Water diffuses across most cell membranes fairly easily, but many dissolved substances do not. Water is moving in and out rapidly and constantly. The NET movement of water is from an area of lower dissolved material to an area of higher dissolved materials. Or, if you phrase it as you have, from an area of high WATER concentration to an area of low WATER concentration. The terms isotonic, hypotonic, and hypertonic refer to the concentration of dissolved material. "Iso-" means equal; "hypo-" means low; and "hyper-" means high--all in reference to dissolved material (solutes). So, the net movement of water is from LOW solute concentration to HIGH solute concentration. If a solution surrounding a cell has a solute concentration equal to the solutes in the cell, then there will be no net movement. The following chart might help:
Concentration of solutes Concentration of water Direction of water Term applied to solution
compared to the cell compared to the cell movement surrounding the cell
HIGH LOW OUT OF CELL HYPERTONIC
LOW HIGH INTO CELL HYPOTONIC
EQUAL EQUAL NO NET MOVEMENT ISOTONIC
As an example, a solution of about 0.85 g/100 milliliters of sodium chloride or solution of about 5 grams/100 milliliters of glucose is isotonic and can be infused into the veins without problems. Various other solutions are made for intravenous (within the veins) infusion. Not all are isotonic, but for various reasons are not damaging. Whatever the case, these principles must be kept in mind for IV (intravenous) solutions.