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[Physiology](https://durgical.com/en/physiology/) · [Cell and membrane](https://durgical.com/en/physiology/#s-cell)

# 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

## Key points

- Osmosis depends on the number of particles, not their size or charge: 1 mmol of NaCl gives ≈2 mOsm.
- Effective osmoles (Na⁺, glucose without insulin, mannitol) move water; ineffective ones (urea, ethanol) cross the membrane and do not.
- Calculated osmolality ≈ 2 × Na⁺ + glucose/18 + BUN/2.8 (mg/dL); normal ≈285–295 mOsm/kg.
- Isotonic: the cell does not change. Hypotonic: it swells. Hypertonic: it shrinks.
- 5% dextrose is iso-osmotic in the bag but acts as free water: the glucose is metabolised and leaves water behind.

## 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 osmolality**Osmcalc ≈ 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

| Parameter | Value | Note |
| --- | --- | --- |
| Plasma osmolality | ≈285–295 mOsm/kg | |
| Serum Na⁺ | ≈135–145 mEq/L | |
| Osmolar gap |

| 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.

## Memory aid

Tonicity counts the ones that stay: only solutes that cannot cross the membrane decide where water goes.

## 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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## Related topics

- [IV fluids: tonicity and where they go](https://durgical.com/en/physiology/iv-fluids-tonicity-and-where-they-go/)
- [Cell volume regulation](https://durgical.com/en/physiology/cell-volume-regulation/)
- [ADH, thirst and water balance](https://durgical.com/en/physiology/adh-thirst-and-water-balance/)
- [Body fluid compartments](https://durgical.com/en/physiology/body-fluid-compartments/)
- [Urine dilution, free water and osmotic diuresis](https://durgical.com/en/physiology/urine-dilution-free-water-and-osmotic-diuresis/)

## Related conditions

- [Hyponatraemia](https://durgical.com/en/conditions/hyponatraemia/)
- [Hypernatraemia](https://durgical.com/en/conditions/hypernatraemia/)
- [Syndrome of inappropriate antidiuretic hormone secretion (SIADH)](https://durgical.com/en/conditions/syndrome-of-inappropriate-antidiuretic-hormone-secretion/)
- [Diabetic ketoacidosis](https://durgical.com/en/conditions/diabetic-ketoacidosis/)

## In Biochemistry

- [Serum osmolality and osmolar gap](https://durgical.com/en/biochemistry/topics/serum-osmolality-and-osmolar-gap/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://openstax.org/books/anatomy-and-physiology-2e/pages/26-2-water-balance) (OpenStax (Rice University), 2022); [3.1 The Cell Membrane](https://openstax.org/books/anatomy-and-physiology-2e/pages/3-1-the-cell-membrane) (OpenStax (Rice University), 2022); [Low blood sodium: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000394.htm) (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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