Measurement of Osmolality

Plasma and other biological fluids contain many organic molecules and electrolytes. The osmolality of such complex solutions only can be estimated by calculations. Fortunately, however, there is a relatively simple method for measuring osmolality. This method is based on the fact that the freezing point of a solution, like its osmotic pressure, is affected by the total concentration of the solution and not by the chemical nature of the solute.

One mole of solute per liter depresses the freezing point of water by -1.86° C. Accordingly, a 1.0 m glucose solution freezes at a temperature of -1.86° C, and a 1.0 m NaCl solution freezes at a temperature of 2 x-1.86 = -3.72° C because of ion-ization. Thus, the freezing-point depression is a measure of the osmolality. Since plasma freezes at about -0.56° C, its osmolality is equal to 0.56 + 1.86 = 0.3 Osm, which is more commonly indicated as 300 milliosmolal (or 300 mOsm).

Clinical Investigation Clues

Remember that Jessica's plasma has a higher than normal osmolality.

What is the normal osmolality of plasma?

What is the relationship between the glucose in Jessica's urine, her frequent urination, and her high plasma osmolality?

Interactions Between Cells and the Extracellular Environment

Volume = X Volume = Z

o o

CO CO

• •

C» cm cm cm

cm cm

• . .

• H,0-1 •

—cm

. cm

1.0 m glucose

'/. ' ' H2O-/ •

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Volume = 2/33C Volume = 4/3X

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1.5 mgli • / •

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■ Figure 6.10 The effect of ionization on the osmotic pressure.

(a) If a selectively permeable membrane (permeable to water but not to glucose, Na+ or Cl-) separates a 1.0 m glucose solution from a 1.0 m NaCl solution, water will move by osmosis into the NaCl solution. Osmosis occurs because NaCl can ionize to yield one-molal Na+ plus one-molal Cl-. (b) After osmosis, the total concentration, or osmolality, of the two solutions is equal.

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