The present invention relates to a connection assembly for guiding a medical liquid, the assembly comprising: a connection piece (11) through which the medical liquid can be pumped； An attachment part (2) attachable to the connection piece (11) and connectable to a pump device (5) for pumping the medical liquid through the connection piece； An opening 210 to which the connecting piece 11 is coupled when the mounting portion 2 is attached to the connecting piece 11； And a sealing element (4) for sealing the passage between the connecting pieces (11, 12) and the attachment part (2). The sealing element 4 can be inserted into the opening 210 of the attachment part 2 in the insertion direction E so that when the attachment part 2 is attached to the connecting piece 11, (2) and the connecting piece (11). The sealing element 4 comprises a sealing head 40 for closing the opening 210 of the attachment part 2 with respect to the liquid passage when the attachment part 2 is attached to the connecting piece 11, And a body 42 that can be disposed in the opening 210 at a radial distance to the attachment 2. The interior of the seal element 4 does not have a tip body to assist the opening process of the sealing head 40. The sealing element 4 protrudes beyond the sealing head 40 in the radial direction in the insertion direction E at the transition between the sealing head 40 and the body 42 and extends along the insertion direction E A shoulder 410 that can be in contact with a pair of abutment surfaces 212 within an opening 210 of the body 210； Is disposed away from the shoulder 410 along the insertion direction E and projects radially outwardly with respect to the body 42 of the sealing element 4 and the lower surface 422 of the flange 420 is connected to the connecting piece 11 Between the attachment part 2 and the connecting piece 11 when the attachment part 2 is attached to the connection piece 11 such that the upper surface 421 of the flange is at least partly exposed, Gt；
