When erythrocytes are placed in an isomotic solution of a penetrating but otherwise harmless substance such as glycerol, the entrance of the solute followed by that of water results in their swelling and eventual hemolysis. The time of hemolysis depends upon the rate of entrance both of the solute and of the solvent, but chiefly upon the former, which of the two processes is usually by far the slower. The osmotic nature of this type of hemolysis is shown by the possibility of preventing it, or of stopping it at any point, by the addition to the external solution of a suitable concentration (e.g., 0.16 M) of a non-penetrating solute such as NaC1.
If the concentration of NaCl added, to the glycerol solution be below the critical hemolytic concentration, hemolysis is not prevented, but is in general slowed to an extent that varies with the concentration of the electrolyte. If, however, this concentration be sufficiently low, e.g., 0.01 M, the unexpected result is obtained that the rate of hemolysis may be increased rather than decreased, in spite of the fact that the total osmolar concentration of the external solution is still greater by a significant amount than that of the glycerol solution alone.