Note on Donnan Equilibrium and Osmotic Pressure Relationship between the Cells and the Serum

In a paper on the factors controlling the electrolyte and water distribution in the blood Van Slyke, Wu, and McLean (1) based their mathematical deduction on three fundamental postulates: (1) Approximate neutrality of the blood reaction; sum of base cations equals sum of anions. (2) Validity of Donnan’s law of membrane equilibrium. (3) Equality of the osmotic pressure of the fluid within the cells and that of the serum outside. The first two postulates stood on experimental data familiar in the literature. The third postulate was without experimental basis. That it was substantially correct was shown by the osmolar concentration data presented in Table I of their paper and by the general agreement between the observed effects of changes of 02 and CO, tensions on the water and electrolyte distribution on the one hand with those predicted by the aid of those postulates on the other hand (1, 2). It deserves emphasis, however, that osmotic inequality would be the condition of a Donnan equilibrium in which ions of only one sign were indiflusible, and that the approximate osmotic equality between the cells and the serum is the result of a special condition obtaining in blood; namely, the impermeability of thf cell membrane to the metallic cations as well as to the protein anions. Let us assume, for the sake of simplicity, that the cell contained only sodium bicarbonate and oxyhemoglobin and the serum contained only sodium bicarbonate. The oxyhemoglobin is present partly as salt and partly as free protein. Then at equilibrium, if the cation B+ were diffusible, like the anion A-,