A blood treatment apparatus (1) comprising a degassing unit (15) receiving gas-containing fluid and supplying degassed fluid to a fluid chamber (3) of a blood treatment unit (2). The degassing unit has a gas separator (16), a degassing pump (18) to circulate fluid in a degassing line (12), a flow restrictor (17) for reducing the pressure in the degassing line (12), an absolute pressure sensor (19), and a control unit (20) designed to control the speed of said degassing pump from the absolute pressure signal emitted by the pressure sensor. The apparatus comprises a dialysis machine provided with a device for preparing on-line a dialysis liquid from water and concentrates. The apparatus achieves an accurate and reliable regulation of a desired concentration of gas in the dialysis liquid.
