An in-line sampling syringe for selectively introducing and aspirating a fluid from a downstream fluid conduit connected to a catheterized patient. The syringe includes a housing with a distal component defining a distal chamber having a proximal opening and a distal outlet port to be coupled to a downstream fluid conduit； and a pair of proximal wall components assembled together in an airtight manner defining a proximal chamber having a proximal opening for coupling to an external source of fluid and a distal opening that is coupled to the proximal opening of the distal component. The syringe also includes an elongated volume regulator positioned inside the housing. A curved element is positioned at and in alignment with the distal outlet and retention elements are defined in the distal component at the distal outlet port to mount and retain the curved element at the distal outlet port while enabling flow around the curved element relative to the distal outlet port. A conduit extends from the proximal opening of the proximal chamber through the elongated volume regulator to the curved element at the distal outlet port.
