SN2 SHORT NOTE 5M

Write a short note on the Gibbs–Donnan equilibrium.

Physiology · 1st Professional — Pre-Clinical

Definition

Gibbs–Donnan equilibrium = the unequal distribution of diffusible ions that results when a non-diffusible charged particle (e.g. Protein) is present on one side of a semipermeable membrane.

Conditions Required

1. A semipermeable membrane

2. A non-diffusible ion on one side — usually protein, which is negatively charged at body pH

3. Freely diffusible ions on both sides

Rules at Equilibrium

1. Electroneutrality — each compartment stays electrically neutral

2. Product rule — the product of diffusible ion concentrations is equal on both sides:[Na+]1 × [Cl−]1 = [Na+]2 × [Cl−]2

3. The compartment with protein has more diffusible cations and fewer diffusible anions

4. Total osmotically active particles are greater on the protein side

Worked Illustration

	Compartment 1 (with protein)	Compartment 2
Na+	12	8
Cl−	4	8
Protein−	8	0
Product Na × Cl	48	64
Total particles	24	16

At true equilibrium the products equalise; the protein side always ends with more total osmotically active particles

Consequences in the Body

Effect	Explanation
↑ Na+ in plasma	Plasma protein is anionic → attracts cations
↓ Cl− in plasma	Anions repelled
↑ osmotic pressure of plasma	Adds to colloid osmotic pressure
Donnan effect on oncotic pressure	True oncotic pressure 25 mmHg vs 19 mmHg from protein alone — the extra 6 mmHg is the Donnan effect

Applied Aspects

Explains why plasma oncotic pressure is higher than protein concentration alone would predict → important in capillary fluid exchange (Starling forces)
Basis of the chloride shift in RBCs
Relevant in haemodialysis and in interpreting CSF electrolytes

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