Abstract: Provided is a breathing system comprising a flexible reservoir bag (6) having bag walls and provided with an exhaled gas inlet and a gas outlet (7). The bag defines a passageway for flow of gas in a first direction. The gas outlet of the reservoir bag is in communication with a collecting tube (8) which defines a passageway for flow of gas in a second direction and in use is connectable to a gas collector. At least part of the walls of the bag extend beyond the sides of the collecting tube, and the first and second directions of gas flow are substantially parallel to, and laterally offset from, one another. Additionally, the collecting tube is attached to an outer surface of the reservoir bag.