A breathing circuit comprising first and second conduits, wherein at least one of the conduits is a non-conventional conduit. In an embodiment, a multilumen unilimb breathing circuit has first and second conduits, wherein when the proximal ends of said first and second conduits are each connected to an inlet and outlet fitting, respectively, axial extension or contraction of the second conduit causes a corresponding axial extension or contraction of the first conduit. In an embodiment, at least one of said conduits is coiled. In another embodiment, a coiled conduit is contained within an outer flexible conduit that is axially extendable and compressible, forming a unilimb, multilumen respiratory circuit. The outer flexible conduit may be pleated to provide for non-rebounding axial extension and contraction. The multilumen respiratory circuit can provide a variable rebreathing volume. In an embodiment, at least one tube in a multilumen respiratory conduit is radially collapsible and radially expandable to a maximum radius for carrying respiratory gases to and from a patient. In an embodiment, first and second conduits are formed of accordion-like pleated tubing, wherein both conduits can be connected to a proximal fitting or proximal terminal at their proximal end; in an embodiment, the first and second accordion-like pleated tubes can also be connected to a common distal fitting. The methods and systems can be used to administer anesthesia and for other purposes.