- To and Fro: This system was used in the past but has largely been abandoned. It's advantages are conservation of respiratory moisture and heat. The conservation of heat can be a problem, for example, the patient may become too hot. The major disadvantages are a bulky absorber close to the working space around the patient, the potential for inhalation of the caustic soda lime dust especially with positive pressure ventilation, and an increase in equipment dead space with time. The circle system doesn't have any of these problems.
- Circle System: The most common breathing system used in people and animals. There are 2 subsystems of the circle system defined by gas flow- closed and semi-closed.