Osmotic behaviour of human red blood cells: an interpretation in terms of negative intracellular fluid pressure

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Abstract
The observation that human red blood cells do not shrink in hypertonic media as much as expected for ieal osmometers was previously explained in terms of a marked increase in the osmotic coefficient of the cell contents or an increase in the Cl- content of the cells. Changes in suspension pH and hematocrit were observed when the concentration of the unbuffered NaCl medium was doubled. The small increases in external pH and the size of the volume decreases are inconsistent with variations in the Cl- content as a significant factor in the non-ideal osmotic responses. Membrane potentials of red cells in buffered media were followed using the fluorescent dye, diS-C3-(5). On shrinking at pH 7.4, the cells hyperpolarized approximately 5 mV as predicted if changes in the osmotic coefficeint rather than in Cl- content explained the osmotic behavior. Regarding Hb in concentrated solution as a solute with high osmotic coefficient is formally correct but is little help in understanding the properties of the solution. It was useful to separately consider Hb and the rest of the contents of the cell. The Hb then supports part of the total hydrostatic pressure on the cell leaving the crystalloid solution to experinece a reduced fluid pressure. In greatly shrunken cells the contents act like a gel with the matrix of Hb under compression and the fluid which fills the spaces within the matrix under tension.