Does 3% Saline Exert Oncotic Pressure?
No, 3% saline does not exert oncotic pressure—it exerts osmotic pressure through its high sodium concentration (513 mmol/L), which creates a hyperosmolar gradient that draws water from the intracellular to the intravascular space. 1
Understanding the Mechanism of Action
3% hypertonic saline works through osmotic, not oncotic, mechanisms:
- The osmolarity of 3% saline is 1026 mOsmol/L, making it significantly hyperosmolar compared to plasma 1
- This creates an osmotic gradient that shifts fluid from intracellular compartments into the intravascular space 1
- The primary therapeutic effect is achieved through this osmotic fluid redistribution, not through protein-mediated oncotic pressure 1
Key Distinction: Osmotic vs Oncotic Pressure
Oncotic pressure is specifically generated by proteins (primarily albumin), not by crystalloid solutions like saline:
- Oncotic pressure (colloid osmotic pressure) is measured in studies using albumin-containing solutions, not crystalloid saline 2
- When saline is used as a replacement fluid, it actually causes a decrease in oncotic pressure by diluting plasma proteins 2
- Studies measuring colloid osmotic pressure show that saline replacement reduces COP, while albumin administration maintains or increases it 2
Clinical Implications
The therapeutic effects of 3% saline in critical care are entirely osmotic in nature:
- For raised intracranial pressure: 3% saline reduces ICP through osmotic dehydration of brain tissue, with maximum effect occurring 10-15 minutes after administration 3
- For hyponatremia: 3% saline corrects serum sodium through direct sodium delivery and osmotic water redistribution 4, 5
- For hypotensive states: any volume expansion effect is through osmotic fluid shifts into the vascular space, not oncotic retention 6
Common Pitfall to Avoid
Do not confuse the intravascular volume expansion caused by 3% saline with oncotic pressure: