Table 2 Empirical equilibrium concentrations of main small ions (in mOsm/L H2O) in human blood plasma (assumed protein equivalent: − 12.2 mEq/L) from Guyton and Hall17 and the associated equilibrium concentrations of ions in a protein-free fluid along with the theoretical values of the Gibbs–Donnan factor calculated using the presented method (eq. (16)).

From: Calculation of the Gibbs–Donnan factors for multi-ion solutions with non-permeating charge on both sides of a permselective membrane

Ion

Empirical equilibrium concentrations: plasma

Estimated equilibrium concentrations: protein-free fluid

Estimated Gibbs–Donnan factor

Na+

142.0

136.3

0.96

K+

4.2

4.0

0.96

Ca2+

1.3

1.2

0.92

Mg2+

0.8

0.7

0.92

Cl

108.0

112.5

1.04

HCO3

24.0

25.0

1.04

Lactate

1.2

1.3

1.04

SO42−

0.5

0.5

1.09

HPO42−

2.0

2.2

1.09