Physiology Pearls – Gibbs-Donnan Equilibrium

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The Gibbs-Donnan Equilibrium or Effect describes the behavior of charged particles near a semipermeable membrane. It explains the balance between electrostatic and osmotic forces.

The effect is named after the American physicist Josiah Willard Gibbs and the British chemist Frederick G. Donnan. The Gibbs-Donnan Factor for monovalent cations is 0.95, for example this would mean the sodium concentration in the interstitial fluid is 0.95 that of the concentration in plasma. For monovalent anions, its 1.05. Divalent cations like calcium are partially protein bound, and the Gibbs – Donnan effect only applies to the ionized forms. For them, the factor is 0.90 (and conversely 1.10 for the anions).

See the diagrams below for a more simple visual explanation (pictures from derangedphysiology.com.)

The last illustration is imporant it that in shows how a patient with increase capillary pressure (ie heart failure) can overcome oncotic pressure and cause edema.

Source: DerangedPhysiology