Saliva vs. Blood Plasma: Osmotic Pressure and Tonicity

Saliva is hypotonic to blood plasma.

Explanation

• Osmotic pressure depends on solute concentration.
• Plasma: high protein (albumin, globulins), electrolytes, glucose → ~285–295 mOsm/kg.
• Saliva: low protein, variable electrolytes (hypotonic at rest, isotonic during stimulation) → ~50–100 mOsm/kg at rest.

Physiological Implication

• Water moves from plasma → saliva across salivary gland epithelium (osmotic gradient).
• Saliva remains hypotonic even after stimulation because active reabsorption of Na⁺/Cl⁻ in ducts exceeds water permeability.

Clinical Relevance

• Salivary flow reflects hydration/autonomic status.
• Hypotonic saliva protects teeth (less mineral precipitation) and oral mucosa.