The invention relates to a control unit (30) for determining the pressure in a blood vessel (A, V, F), in particular in an arteriovenous fistula (F), which is in fluid connection with at least one section (28, 41) of a blood line system (39), in particular an extracorporeal blood circulation (II), at least one pressure-generating device (3, 8, 19, 24, 26) being assigned to the blood line system (39), this pressure-generating device being suitable for acting on the section (28, 41), and the control unit (30) being configured to perform the following steps: a) being sure that none of the at least one pressure-generating device (3, 8, 19, 24, 26) acts on the section (28, 41), b) interrupting the fluid connection of the section (28, 41) with the blood vessel (A, V, F) by triggering an interrupt means (11, 29), c) setting the pressure in the section (28, 41) at a predetermined ideal value, in particular at the ambient pressure, with the help of a pressure sensor (13, 14) in the section (28, 41), d) restoring the fluid connection of the section (28, 41) with the blood vessel (A, V, F) by triggering the interrupt means (11, 29), and e) measuring a resulting pressure in the section (28, 41) with the help of the pressure sensor (13, 14).
