The invention relates to a connection assembly for conducting a medical liquid comprising a connection piece (11) through which a medical liquid can be pumped an attachment part (2) which can be attached to the connection piece (11) and which can be connected to a pump device (5) for pumping a medical liquid through the connection piece an opening (210) into which the connection piece (11) engages when the attachment part (2) is attached to the connection piece (11) and a seal element (4) for sealing a passage between the connection piece (11 12) and the attachment part (2). The seal element (4) can be inserted into the opening (210) of the attachment part (2) in an insert direction (E) such that the seal element (4) is held between the attachment part (2) and the connection piece (11) when the attachment part (2) is attached to the connection piece (11). The seal element (4) has a sealing head (40) which closes the opening (210) of the attachment part (2) against the passage of liquid when the attachment part (2) is attached to the connection piece (11) and a body (42) which adjoins the sealing head (40) and which can be arranged in the opening (210) at a radial distance to the attachment part (2). The interior of the seal element (4) is free of a tipped body which should aid the opening process of the sealing head (40). The seal element (4) has a shoulder (410) which projects beyond the sealing head (40) radially to the insert direction (E) and which can be brought into contact with a paired contact surface (212) within the opening (210) of the attachment part (2) along the insert direction (E) at the transition between the sealing head (40) and the body (42) a flange (420) which is arranged at a distance from the shoulder (410) along the insert direction (E) projects radially outwards with respect to the body (42) of the seal element (4) and is held between the attachment part (2) and the connection piece (11) when the attachment part (2) is attache
