A catheter system includes a flexible, elongated base body adapted to be applied to a vein central-venously and having a base distal end a fiber-optic probe having a probe distal part a fiber-optic lumen receiving the fiber-optic probe and an attachment element configured to avoid a longitudinal displacement of the fiber-optic probe relative to the fiber-optic lumen and configured to detach so as to allow removal of the fiber-optic probe through the longitudinal displacement of the fiber-optic probe relative to the fiber-optic lumen. The attachment element has a connector piece firmly connected to the fiber-optic probe and a counter-piece firmly connected to the base body, wherein the connector piece is connectable to the counter-piece, the connector piece having a shaft piece adjustable lengthwise and disposed on the connector piece so as to vary a length of the connector piece.
