Osmotic Pressure

"Cells are about 75 percent water and blood plasma is about 95 percent water. Why then, does the water not flow from blood plasma to cells? The force of water also known as hydrostatic pressure maintains the volumes of water between fluid compartments against the force of all dissolved substances. The concentration is the amount of particles in a set volume of water. (Recall that individual solutes can differ in concentration between the intracellular and extracellular fluids, but the total concentration of all dissolved substances is equal.) The force driving the water movement through the selectively permeable membrane is the higher solute concentration on the one side. Solutes at different concentrations on either side of a selectively permeable membrane exert a force, called osmotic pressure. The higher concentration of solutes on one side compared to the other of the U-tube exerts osmotic pressure, pulling the water to a higher volume on the side of the U-tube containing more dissolved particles. When the osmotic pressure is equal to the pressure of the water on the selectively permeable membrane, net water movement stops (though it still diffuses back and forth at an equal rate). One equation exemplifying equal concentrations but different volumes is the following: 5 grams of glucose in 1 liter = 10 grams of glucose in 2 liters (5g/L = 5g/L)."