Normal saline is defined by which concentration?

Normal saline is defined by which concentration?

• A. 0.9% NaCl
• B. 3% NaCl
• C. 0.45% NaCl
• D. 5% NaCl

Answer: (A)

Normal saline is defined to be isotonic with blood plasma, meaning its osmolarity is about the same as plasma so it expands the extracellular fluid without making cells shrink or swell. That balance comes from a NaCl concentration of 0.9% w/v. In practical terms, this equals roughly 154 mEq/L of sodium and 154 mEq/L of chloride, giving about 308 mOsm/L—close to plasma’s osmolarity. Because it’s isotonic, it stays in the extracellular space when infused, which is why it’s commonly used for fluid resuscitation and maintenance. The other concentrations are not isotonic with plasma: 3% and 5% NaCl are hypertonic and pull water out of cells or are used for specific, controlled situations; 0.45% NaCl is hypotonic and can cause cells to swell by drawing water into them.

Normal saline is defined to be isotonic with blood plasma, meaning its osmolarity is about the same as plasma so it expands the extracellular fluid without making cells shrink or swell. That balance comes from a NaCl concentration of 0.9% w/v. In practical terms, this equals roughly 154 mEq/L of sodium and 154 mEq/L of chloride, giving about 308 mOsm/L—close to plasma’s osmolarity. Because it’s isotonic, it stays in the extracellular space when infused, which is why it’s commonly used for fluid resuscitation and maintenance.

The other concentrations are not isotonic with plasma: 3% and 5% NaCl are hypertonic and pull water out of cells or are used for specific, controlled situations; 0.45% NaCl is hypotonic and can cause cells to swell by drawing water into them.