The pump (10) comprises a drive portion (11) and a pump portion (12) which have such a small diameter that they can be pushed through a blood vessel (31). The pump portion (12) has a flexible cannula (18) connected thereto. In order to reduce the flow resistance of the cannula (18), the cannula (18) can be dilated to a diameter that is larger than that of the drive portion (11) and the pump portion (12), respectively. To be able to introduce the pump into the body by puncturing the blood vessel (31) according to the Seldinger technique, the cannula (18) is set into the constricted state in which it has a small diameter. In the blood vessel (31), it is dilated so that it offers a small flow resistance to the blood to be pumped there.