A coaxial cannula for extracting samples from tissue having a biopsy needle unit provided with a space for sample extraction and a sample separation unit that coaxially encompasses the biopsy needle on an external wall and that is longitudinally movable. The coaxial cannula includes a sealing element that 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. The sealing element opens an air outlet when the biopsy needle unit is inserted, and prevents air intake when the biopsy needle unit is positioned and vacuum is produced in the internal chamber of the biopsy needle.