A bag for medical use having three compartments, and having a compartment containing a solvent, a compartment containing a solute and a compartment containing a solvent, the compartment containing a solute being separated from the other two compartments by pincers, the opening of which allows liquid to flow by gravity, and, during opening of one of the pincers and under the effect of a pressure, the liquid contained in one compartment is propelled towards another compartment. A method for using a bag for medical use having three compartments is also provided.
