A sheath for producing a fully sealed access to the interior of a vessel of an animal or human body comprises a base sheath having a tubular body defining a pass-through channel. The base sheath is adapted to be inserted into the vessel through a vessel aperture. The sheath further comprises an expansion device which is adapted to cooperate with the base sheath such that the outer diameter of the sheath increases in the region of the vessel aperture with the sheath in a stationary position in the vessel and upon actuation of the expansion device.