The invention relates to a control unit (30) for determining a pressure in a blood vessel (A, V, F), especially in an arteriovenous fistula (F) that is in fluid communication with at least one section (28, 41) of a blood-conveying system (39), especially of an extracorporeal blood circuit (II), said blood-conveying system (39) being assigned at least one pressure-generating means (3, 8, 19, 24, 26) apt to act on the section (28, 41), and said control unit (30) being configured to perform the following steps: a) ensuring that none of the at least one pressure-generating means (3, 8, 19, 24, 26) acts on the section (28, 41), b) interrupting the fluid communication of the section (28, 41) with the blood vessel (A, V, F) by actuating an interruptor (11, 29), c) adjusting the pressure in the section (28, 41) to a predetermined setpoint value, especially to the ambient pressure, by means of a pressure sensor (13, 14) in the section (28, 41), d) re-establishing the fluid communication of the section (28, 41) with the blood vessel (A, V, F) by actuating the interruptor (11, 29), and e) measuring a pressure which is established in the section (28, 41) by means of the pressure sensor (13, 14).
