Are extracellular osmolality and sodium concentration determined by Donnan effects of intracellular protein charges and of pumped sodium?
Although we are used to attribute almost identical extracellular fluid (ECF) sodium concentrations in birds, amphibians, reptiles, and mammals to the composition of the primordial oceans in which, presumably, all life originated, this interpretation is not supported by geological data suggesting that the ocean salinity was never much lower than the present-day values, still four times higher than our plasma sodium. Here presented interpretation is that the similar ECF salt concentrations are dictated by the opposed Donnan effects on the cell membrane. The only way for the cell to reach the osmotic equilibrium is to alter cell volume, until concentration of nondiffusible intracellular ions (mainly charges on intracellular proteins) is equal to the ECF restricted ions (mainly Na+ ions, restricted by pumping out of cells). The achievement of electroneutrality requires that the sum of all anions equals concentration of positive ions in the cell (mainly K+). Negative charges on cytoplasmic
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