The ionic composition of the interstitial fluid and blood plasma vary due to the Gibbs-Donnan effect.

Collagen fibres exert the compressive force, whereas the Gibbs-Donnan effect & cartilage proteoglycans create osmotic pressure which tends to draw water in.

The Gibbs-Donnan effect (also known as the Donnan effect, Donnan law, Donnan equilibrium, or Gibbs-Donnan equilibrium) is a name for the behavior of charged particles near a semi-permeable membrane that sometimes fail to distribute evenly across the two sides of the membrane.