## Osmolarity and Tonicity

The osmotic concentration of body fluids has such a great effect on cellular function that it is important to understand the units in which it is measured. One osmole is 1 mole of dissolved particles. If a solute does not ionize in water, then 1 mole of the solute yields 1 osmole (osm) of dissolved particles. A solution of 1 molar (1 M) glucose, for example, is also 1 osm/L. If a solute does ionize, it yields two or more dissolved particles in solution. A 1 M solution of NaCl, for example, contains 1 mole of sodium ions and 1 mole of chloride ions per liter. Both ions affect osmosis and must be separately counted in a measure of osmotic concentration. Thus, 1 M NaCl = 2 osm/L. Calcium chloride (CaCl2) would yield three ions if it dissociated completely (one Ca2+ and two Cl_), so 1 M CaCl2 = 3 osm/L.

Osmolality is the number of osmoles of solute per kilogram of water, and osmolarity is the number of osmoles per liter of solution. Most clinical calculations are based on osmolarity, since it is easier to measure the volume of a solution than the weight of water it contains. At the concentrations of human body fluids, there is less than 1% difference between osmolality and osmolarity, and the two terms are nearly interchangeable. All body fluids and many clinical solutions are mixtures of many chemicals. The osmolarity of such a solution is the total osmotic concentration of all of its dissolved particles.

A concentration of 1 osm/L is substantially higher than we find in most body fluids, so physiological concentrations are usually expressed in terms of milliosmoles per liter (mOsm/L) (1 mOsm/L = 10~3 osm/L). Blood plasma, tissue fluid, and intracellular fluid measure about 300 mOsm/L.

Tonicity is the ability of a solution to affect the fluid volume and pressure in a cell. If a solute cannot pass through a plasma membrane, but remains more concentrated on one side of the membrane than on the other, it causes osmosis. A hypotonic15 solution has a lower concentration of nonpermeating solutes than the intracellular fluid (ICF). Cells in a hypotonic solution absorb water, swell, and may burst (lyse) (fig. 3.16a). Distilled water is the extreme example; given to a person intravenously, it would lyse the blood cells. A hypertonic16 solution is one with a higher concentration of nonpermeating solutes than the ICF. It causes cells to lose water and shrivel (cre-nate) (fig. 3.16c). Such cells may die of torn membranes and cytoplasmic loss. In isotonic17 solutions, the total concentration of nonpermeating solutes is the same as in the ICF—hence, isotonic solutions cause no change in cell volume or shape (fig. 3.16fc).

It is essential for cells to be in a state of osmotic equilibrium with the fluid around them, and this requires that the extracellular fluid (ECF) have the same concentration of nonpermeating solutes as the ICF. Intravenous fluids given to patients are usually isotonic solutions, but hypertonic or hypotonic fluids are given for special purposes. A 0.9% solution of NaCl, called normal saline, is isotonic to human blood cells.

It is important to note that osmolarity and tonicity are not the same. Urea, for example, is a small organic molecule hypo = less + ton = tension

Saladin: Anatomy & I 3. Cellular Form and I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

Chapter 3 Cellular Form and Function 109

Chapter 3 Cellular Form and Function 109

Figure 3.16 Effects of Tonicity on Red Blood Cells (RBCs). (a) In a hypotonic medium such as distilled water, RBCs absorb water, swell, and may burst. (b) In an isotonic medium such as 0.9% NaCl, RBCs gain and lose water at equal rates and maintain their normal, concave disc shape. (c) In a hypertonic medium such as 2% NaCl, RBCs lose more water than they gain and become shrunken and spiky (crenated).

Figure 3.16 Effects of Tonicity on Red Blood Cells (RBCs). (a) In a hypotonic medium such as distilled water, RBCs absorb water, swell, and may burst. (b) In an isotonic medium such as 0.9% NaCl, RBCs gain and lose water at equal rates and maintain their normal, concave disc shape. (c) In a hypertonic medium such as 2% NaCl, RBCs lose more water than they gain and become shrunken and spiky (crenated).

that easily penetrates plasma membranes. If cells are placed in 300 mOsm/L urea, urea diffuses into them (down its concentration gradient), water follows by osmosis, and the cells swell and burst. Thus, 300 mOsm/L urea is not isotonic to the cells. Sodium chloride, by contrast, penetrates plasma membranes poorly. In 300 mOsm/L NaCl, there is little change in cell volume; this solution is isotonic to cells.

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### Responses

• faruz
What effect does physiological saline and urea have on red cells?
3 years ago
• Agatino
Is 0.9% urea hypotonic?
3 years ago
• thomas
Does isosmotic solutions cause change in cellular volume?
3 years ago
• aziza ali
Why is tonicity more important than osmolarity in a clinical enviornment?
2 years ago
• rowan
Why is it important to know the difference between osmolarity and tonicity?
1 year ago
• Daniel
Why is understanding the concept of tonicity important in the study of physiology?
1 year ago
• Vigo Boffin
How does osmolarity affect the weight of a cell?
1 year ago
• Roxy
Why is tonicity in the cell important?
2 months ago
• Simret
Why is it important to know the osmolarity of a tissue?
2 months ago
• diana
How does tonicity affect osmosis?
2 months ago
• MERICO
How does osmolarity affect cells?
23 days ago
• RAYYAN
Is urea isotonic to human cells?
2 days ago