The present invention relates to a device and method for conveying fluids into the treatment unit of a medical treatment apparatus. The device and method according to the present invention are based on the fact that the fluid with which the treatment unit is supplied, circulates in a fluid circuit including the treatment unit. To balance fresh and used fluid fed to the treatment unit or conveyed from the treatment unit, a balancing unit with a balancing chamber is used, which can be incorporated into the fluid circuit including the treatment unit. It is thus possible to supply the fluid circuit continuously with fresh fluid or to carry away used fluid continuously from the fluid circuit. The supply and discharge of fresh and used fluid can take place at a different flow rate from the flow rate at which the fluid circulates in the fluid circuit via the treatment unit.