The invention relates to a coaxial cannula (1) for extracting samples from tissue comprising a biopsy needle unit (9) provided with a space for sample extraction and a sample separation unit (21) which coaxially encompasses the biopsy needle (9) on an external wall and is longitudinally movable. The inventive coaxial cannula (1) comprises a sealing element (3) which is arranged on the proximal end thereof and closes the space between the internal wall of the coaxial cannula and the external wall of the sample separation unit. Said coaxial cannula (1) is characterised in that the sealing element (3) opens an air outlet when the biopsy needle unit (9) is inserted, and prevents air intake when the biopsy needle unit (9) is positioned and vacuum is produced in the internal chamber of the biopsy needle.