The pressure of fluid being conveyed through a fluid flow path during a biological fluid procedure is detected during the procedure. The fluid is conveyed through the fluid flow path 1) at a first pre-determined rate if pressure is zero to a first pressure threshold； 2) at the first pre-determined rate if pressure is greater than the first pressure threshold but less than or equal to a second pressure threshold and if an immediately preceding rate is equal to the first pre-determined rate； 3) at a second pre-determined rate if pressure is greater than the first pressure threshold but less than or equal to the second pressure threshold and if an immediately preceding rate is equal to the second pre-determined rate； and 4) at the second pre-determined rate if pressure value is greater than the second pressure threshold but less than or equal to a third pressure threshold.
