Osmolarity and Tonicity of NaCl Solutions in Human Blood

Chapter 9: Solutions

Osmolarity and Tonicity of NaCl Solutions

Understanding the osmolarity and tonicity of solutions is essential in clinical chemistry, especially when administering intravenous fluids. This section explains why a 0.90% (m/v) NaCl solution is isotonic to human blood, provides a sample osmolarity calculation, and describes the effects of hypotonic and hypertonic solutions on patients.

Key Terms and Definitions

• Osmolarity (Osm): The total concentration of all solute particles in a solution, expressed as osmoles per liter (Osm/L).

• Tonicity: The ability of a solution to cause water movement across a cell membrane due to differences in solute concentration.

• Isotonic Solution: A solution with the same osmotic pressure as body fluids (e.g., blood plasma).

• Hypotonic Solution: A solution with lower osmotic pressure than body fluids.

• Hypertonic Solution: A solution with higher osmotic pressure than body fluids.

Why 0.90% (m/v) NaCl is Isotonic to Human Blood

• Human blood plasma has an osmolarity of approximately 0.28 Osm/L.

• A 0.90% (m/v) NaCl solution contains 0.90 g NaCl per 100 mL of solution.

• NaCl dissociates completely in water into Na+ and Cl− ions.

Osmolarity Calculation for 0.90% (m/v) NaCl Solution

1. Calculate molarity (M):

   • Molar mass of NaCl = 58.44 g/mol

   • 0.90 g NaCl in 100 mL = 9.0 g in 1 L

   • Molarity: $M = \frac{9.0\ \text{g}}{58.44\ \text{g/mol}} = 0.154\ \text{mol/L}$

2. Calculate osmolarity (since NaCl dissociates into 2 particles): $\text{Osmolarity} = 0.154\ \text{mol/L} \times 2 = 0.308\ \text{Osm/L}$

3. This value is very close to the osmolarity of blood plasma (0.28–0.30 Osm/L), making 0.90% NaCl isotonic.

Effects of Hypotonic and Hypertonic Solutions

• Hypotonic Solution: Lower osmolarity than blood plasma. Water enters red blood cells, causing them to swell and possibly burst (hemolysis).

• Hypertonic Solution: Higher osmolarity than blood plasma. Water leaves red blood cells, causing them to shrink (crenation).

• Isotonic Solution: No net movement of water; red blood cells retain their normal shape.

Summary Table: Effects of Different Solutions on Red Blood Cells

Solution Type	Osmolarity Relative to Blood	Effect on Red Blood Cells
Isotonic (0.90% NaCl)	Same	No change
Hypotonic (<0.90% NaCl)	Lower	Cells swell and may burst (hemolysis)
Hypertonic (>0.90% NaCl)	Higher	Cells shrink (crenation)

Example Application

• If a patient receives a hypotonic solution, their red blood cells may undergo hemolysis, which can be dangerous.

• If a patient receives a hypertonic solution, their red blood cells may shrink, impairing oxygen transport.

• Therefore, isotonic solutions like 0.90% NaCl are used in intravenous therapy to maintain cell integrity.