A double-lumen breathing circuit assembly with separate breathing circuit conduit lengths can be joined together by differently arranged connectors or adapters. The connectors may each have different shapes and associated accessories that serve different functions. The breathing circuit conduit lengths may have an inner septum that extends both diametrically and axially for segregating inspiratory and expiratory lumens. Each septum may terminate at opposite distal ends of the conduit lengths and form a pair of semi-circular lumen openings. One of the bi-lumen breathing circuits may be connected to a patient-side connector, and the other bi-lumen breathing circuit may be connected to, or integrally moulded with, a gas collection plug of an anaesthetic gas machine. The latter bi-lumen breathing circuit may also be further connected to, or integrally moulded with, a carbon dioxide sampling connector, a temperature probe, and/or a pressure sensor probe.
