Are the original Donnan assumptions applicable to serum?

Area of Science:

• Biochemistry
• Physical Chemistry
• Physiology

Background:

• The Donnan equilibrium model is widely used to predict ion distribution across semipermeable membranes in biological systems.
• Previous analyses of serum electrolytes (sodium, potassium, chloride) have shown deviations from predicted Donnan values.
• Discrepancies suggest limitations in the current understanding of ion behavior in protein-rich solutions.

Purpose of the Study:

• To investigate the discrepancies between experimental serum electrolyte levels and the predictions of the Donnan model.
• To propose and validate an alternative model that accurately reflects experimental findings.
• To quantify bound sodium, bound water, and albumin molecule characteristics.

Main Methods:

• Analysis of diffusible serum fractions for sodium, potassium, and chloride.
• Measurement of sodium activity using a sodium glass electrode in protein solutions and filtrate.
• Comparison of experimental data with Donnan model predictions.

Main Results:

• Observed ratios of protein solution to filtrate levels did not align with Donnan values.
• Sodium activity measurements contradicted the Donnan model's prediction of higher activity in the protein solution.
• An alternative model was developed, providing reasonable estimations for bound sodium, bound water, and albumin radius.

Conclusions:

• The Donnan equilibrium model is insufficient for accurately describing ion distribution in serum.
• An alternative model offers a better explanation for experimental observations regarding electrolyte and water binding.
• Findings necessitate a re-evaluation of assumptions within the original Donnan theory.