Simple weightless tubing, which connects a few necessary components does this job to be given the name “circuit”. The anesthesia machine supplies the needed drugs in precise quantities, which have to be safely given to the patient without any additions and omissions. The administration of drugs through lungs as inhalational method forms an essential part of the armamentarium of anesthesiologists worldwide. It was widely used in the past and now is only of historical importance. The Waters to and fro system, a classic example of bidirectional flow systems with a canister to absorb carbon dioxide, is valveless and portable. There are bidirectional flow systems with carbon dioxide absorption. With the introduction of low resistance valves, improved soda lime canisters and low dead space connectors, the use of less complicated pediatric circle systems is gaining popularity to anesthetize children. During the function, the arrangement of components is significant only at higher fresh gas flows. The functional analysis is explained in detail. The components of the circle system can be arranged in different ways with adherence to basic rules: (1) Unidirectional valve must be present between the reservoir bag and the patient on both inspiratory and expiratory sides (2) fresh gas must not enter the system between the expiratory unidirectional valve and the patient and (3) the APL valve must not be placed between the patient and the inspiratory unidirectional valve. Such a circle system can be used with flows as low as 250 to 500 mL and clinically can be termed as low-flow systems. When the adjustable pressure limiting (APL) valve is closed and all the exhaled gases without carbon dioxide are returned to the patient, the system becomes a totally closed one. The system contains a component (absorber), which absorbs exhaled carbon dioxide and it is not necessary to give high fresh gas flows as in Mapleson systems. The flow of exhaled gases is unidirectional in the system. When the components are arranged as a circle, it is termed a circle system. A breathing system is defined as an assembly of components, which delivers gases from the anesthesia machine to the patients’ airways.
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