Osmosis, osmolarity and tonicity !

Concept
Exam priority: high

Water crosses a membrane towards the side with more dissolved particles; that is how cells swell or shrink.

Updated 2026-11-01

Also called: Osmotic pressure, Effective osmolality

What it is

Osmosis is the net movement of water across a semipermeable membrane towards the side with more dissolved particles. Osmolarity counts every particle; tonicity counts only those that cannot cross the cell membrane (effective osmoles), so it predicts whether a cell swells or shrinks. This difference explains how IV fluids and hyponatraemia behave.

How it works, step by step

A solute that cannot cross the membrane is more concentrated on one side.
On that side the water concentration is lower.
Water diffuses down its own gradient, through the bilayer and aquaporins.
That side's volume rises until concentrations match or a pressure stops it.
The pressure needed to stop this water movement is the solution's osmotic pressure.

Equations

Osmotic pressure (van 't Hoff)π = i × C × R × T

i: particles per molecule (NaCl ≈2)
C: molar concentration
R: gas constant
T: absolute temperature

At 37 °C each non-permeant mOsm/L generates ≈19.3 mmHg.

Calculated plasma osmolalityOsm_calc ≈ 2 × Na⁺ + glucose/18 + BUN/2.8

Na⁺: mEq/L
glucose and BUN: mg/dL

Effective osmolality (tonicity) leaves out BUN. Osmolar gap = measured − calculated; normal < 10.

Reference values

Parameter	Value	Note
Plasma osmolality	≈285–295 mOsm/kg
Serum Na⁺	≈135–145 mEq/L
Osmolar gap	< 10 mOsm/kg	higher with methanol, ethylene glycol, ethanol or mannitol
Pressure per 1 mOsm/L	≈19.3 mmHg	at 37 °C

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

IV solutions and their effect on cells

Solution	Osmolarity (mOsm/L)	Tonicity in the body	Effect on the cell
0.9% NaCl	≈308	Isotonic	No change; expands extracellular fluid
0.45% NaCl	≈154	Hypotonic	Gains water
3% NaCl	≈1026	Hypertonic	Loses water
5% dextrose	≈278	Hypotonic once metabolised	Gains water; spreads as free water

Clinical notes

Severe hyperglycaemia: glucose pulls water out of cells and dilutes Na⁺ (translocational hyponatraemia) although osmolality is high.
Uraemia: urea raises measured osmolality but not tonicity; on its own it does not dehydrate cells.
High osmolar gap with acidosis: suggests methanol or ethylene glycol poisoning, common with adulterated liquor.

Test yourself

Think of the answer, then tap the question to see it.

Why does a urea solution that is iso-osmotic with plasma make red cells burst?

Urea crosses the membrane into the cell; the solution effectively becomes water, which follows it in until haemolysis. Iso-osmotic is not isotonic.

A patient has very high glucose and Na⁺ of 128 mEq/L. Is this hypotonic hyponatraemia?

Not necessarily: glucose is an effective osmole that draws water out of cells and dilutes Na⁺. Tonicity may be normal or high.

What does a high osmolar gap with a high anion gap metabolic acidosis suggest?

An unmeasured low-weight solute such as methanol or ethylene glycol, whose acid metabolites cause the acidosis.

Why is 5% dextrose a poor plasma expander?

The glucose is metabolised, leaving free water that spreads through all body water: only ≈1/12 stays in the plasma.

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In Biochemistry

Serum osmolality and osmolar gap

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 26.2 Water Balance (OpenStax (Rice University), 2022); 3.1 The Cell Membrane (OpenStax (Rice University), 2022); Low blood sodium: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 25). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.