Outward pressure within erythrocytes.

The osmotic pressure of human erythrocyte contents was measured and found to be higher by 35 mosmol/liter, or 0.79 atm, than that of the surrounding plasma. The pressure difference also exists under hypotonic conditions, although it is smaller than under isotonic conditions. Binding capability of the concentrated intracellular protein for additional ions was shown. It is suggested that the osmotic pressure difference is balanced by the inward pressure exerted by the membrane together with the osmotic pressure of the surrounding plasma.