Water is a small polar molecule that can easily diffuse across plasma membranes through small intermolecular spaces. Osmosis is the net movement of water through a semipermeable membrane down its own concentration gradient from an area of high water concentration to an area of low water concentration. In other words, water moves toward an area of higher solute concentration. The solute particles may be thought of as "drawing" the water toward them. Therefore, the osmotic pressure of a solution is the pressure or force by which water is drawn into the solution through a semipermeable membrane. The magnitude of this pressure depends on the number of solute particles present. An increase in the number of particles in the solution results in an increase in the osmotic pressure and, therefore, an increase in the movement of water toward it.

The plasma membrane is semipermeable because it is not permeable to all solute particles present. As a result, it maintains a concentration difference for many ions and molecules across itself, although water crosses the membrane freely in either direction. The movement of water in and out of the cell will occur whenever there is a difference in osmotic pressure between the intracelluar fluid and the extracellular fluid. For example, an increase in the osmotic pressure of the extracellular fluid (more solute, lower water concentration) will cause water to leave the cell by osmosis. On the other hand, a decrease in the osmotic pressure in the extracellular fluid (less solute, higher water concentration) will cause water to enter the cells.

Pharmacy application: intravenous solutions

Intravenous (i.v.) solutions are commonly administered to patients in hospitals, long-term care facilities, and ambulances. They are used primarily to replace body fluids and to serve as a vehicle for injecting drugs into the body. The advantages of this pharmaceutical dosage form include the rapid onset of action, the ability to treat patients unable to take medication orally and the ability to administer a medication unavailable in any other dosage form.

Intravenous solutions must be isosmotic (same osmotic pressure) with red blood cells. If red blood cells were to be exposed to an i.v. solution that was hypoosmotic (lower osmotic pressure), water would move into the cells causing them to swell and possibly lyse. If red blood cells were to be exposed to a hyperosmotic i.v. solution (higher osmotic pressure), water would move out of the cells causing them to dehydrate and shrink. Both of these conditions would damage the red blood cells and disrupt function.

Patient discomfort is another important consideration. The stinging caused by a hypoosmotic or hyperosmotic i.v. solution is not experienced with one that is isosmotic. Intravenous injections are often prepared with 0.9% sodium chloride or 5% dextrose, both of which are approximately isosmotic with red blood cells.

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