An apparatus and method for blood treatment for the single-needle mode, the apparatus having an extra-corporeal blood circuit with a blood infeed line running to the inlet and a blood return line leading away from the outlet, of a blood treating unit, includes in the blood return line a device, e.g., container, for amassing blood, which is in flow-permitting connection for gas transfer with a device, e.g., container, for storing gas. A pressure is calculated and set in the device for storing gas before the arterial and venous phases are started, with two constraints. First, the pressure should be sufficiently low to ensure both no infusion of air into the patient and also regulated flow of blood into the patient even at opposing pressures lower than ambient pressure. Second, the pressure should be sufficiently high to expel blood from the device for amassing blood into the device for storing gas.
