Why Normal Saline Is Called "Normal"
Normal saline (0.9% sodium chloride solution) is called "normal" for historical reasons rather than physiological accuracy, as it is neither truly normal to human physiology nor isotonic to human plasma. 1
Historical Origin and Naming
The term "normal saline" originated from historical misconceptions rather than scientific accuracy:
- The development of this "balanced salt solution" was intended to maintain iso-osmolarity based on anticipated physiological requirements in terms of concentration gradient and charge 1
- Despite its name, 0.9% saline is actually slightly hyperosmolar compared to human plasma, with an osmolarity of 308 mOsmol/L 1
- The term "normal" likely came from Hartog Jakob Hamburger's in vitro studies of red cell lysis in the late 19th century, not from its physiological compatibility with the human body 2
Actual Composition and Physiological Implications
When examining the composition of normal saline compared to human plasma:
- Normal saline contains equal concentrations of sodium and chloride (154 mmol/L each) 1, 3
- This is physiologically imbalanced, as human plasma typically has higher sodium than chloride concentrations
- The sodium and chloride concentration of "normal saline" is not isosmotic to human plasma, but slightly hyperosmolar 1
Clinical Considerations
The non-physiological nature of normal saline has important clinical implications:
- The increased chloride load in normal saline has been associated with hyperchloremic metabolic acidosis 4
- In urological procedures, normal saline is strongly recommended as the standard irrigation solution because it is isotonic and isoosmolar, reducing the risk of hemolysis, hyponatremia, and heart failure if absorbed 1
- For fluid resuscitation, balanced crystalloids may be preferable to normal saline in many situations 4, 5
Modern Perspective
Current medical understanding recognizes the limitations of normal saline:
- Normal saline is neither "normal" nor "physiological" in relation to human body fluids 6
- Despite these limitations, it remains one of the most widely used solutions in clinical practice 7
- Balanced solutions such as Ringer's Lactate and Plasma-Lyte A are increasingly preferred in many clinical contexts 4
Common Pitfalls
When using normal saline, be aware of these important considerations:
- Large volume infusion of normal saline can cause metabolic acidosis due to its high chloride content 4
- Normal saline should be used with caution in patients with renal impairment or those at risk for fluid overload
- The term "normal" may create a false sense of physiological compatibility that isn't supported by its actual composition
Normal saline has persisted in clinical practice largely due to historical precedent rather than physiological appropriateness, and its name reflects historical terminology rather than its actual relationship to human physiology.