An anesthetic circuit for treating a patient, comprising: a flow passage; a molecular anesthetic agent inlet in fluid communication with the flow passage for introducing an external anesthetic agent comprising a volatile molecular anesthetic agent into the flow passage; an exit outlet in fluid communication with the flow passage for providing at least the external anesthetic agent comprising a volatile molecular anesthetic agent to the patient; an entry inlet for receiving an exhaled fluid mixture from the patient, the exhaled fluid mixture comprising an exhaled oxygen, an exhaled carbon dioxide and an exhaled volatile molecular anesthetic agent, the flow passage being in fluid communication with the entry inlet for receiving the exhaled fluid mixture from the entry inlet; a membrane comprising at least one polymeric material, in fluid communication with the flow passage, located downstream from the entry inlet, and at least partially impervious to the exhaled volatile molecular anesthetic agent to at least partially retain the exhaled volatile molecular anesthetic agent in the flow passage after the exhaled fluid mixture contacts the membrane, wherein the membrane comprises a dense membrane and has a non-porous consistency, the membrane is pervious to the exhaled oxygen such that the membrane has an exhaled oxygen-to-exhaled volatile molecular anesthetic agent selectivity of greater than 1, the membrane is pervious to the exhaled carbon dioxide such that the membrane has an exhaled carbon dioxide-to-exhaled volatile molecular anesthetic agent selectivity of greater than 1, the membrane is capable, upon contact with the exhaled fluid mixture, of leaving a modified fluid mixture in the flow passage having a lower amount of the exhaled carbon dioxide than the exhaled fluid mixture, and the exit outlet is located downstream from the membrane and provides at least the modified fluid mixture to the patient; and the membrane is inert with respect to the exhaled volatile molecular anesthetic agent and inert with respect to the exhaled carbon dioxide; and a fluid inlet for introducing an external fluid into the flow passage to be added to the modified fluid mixture provided to the patient. a flow passage; a molecular anesthetic agent inlet in fluid communication with the flow passage for introducing an external anesthetic agent comprising a volatile molecular anesthetic agent into the flow passage; an exit outlet in fluid communication with the flow passage for providing at least the external anesthetic agent comprising a volatile molecular anesthetic agent to the patient; an entry inlet for receiving an exhaled fluid mixture from the patient, the exhaled fluid mixture comprising an exhaled oxygen, an exhaled carbon dioxide and an exhaled volatile molecular anesthetic agent, the flow passage being in fluid communication with the entry inlet for receiving the exhaled fluid mixture from the entry inlet; a membrane comprising at least one polymeric material, in fluid communication with the flow passage, located downstream from the entry inlet, and at least partially impervious to the exhaled volatile molecular anesthetic agent to at least partially retain the exhaled volatile molecular anesthetic agent in the flow passage after the exhaled fluid mixture contacts the membrane, wherein the membrane comprises a dense membrane and has a non-porous consistency, the membrane is pervious to the exhaled oxygen such that the membrane has an exhaled oxygen-to-exhaled volatile molecular anesthetic agent selectivity of greater than 1, the membrane is pervious to the exhaled carbon dioxide such that the membrane has an exhaled carbon dioxide-to-exhaled volatile molecular anesthetic agent selectivity of greater than 1, the membrane is capable, upon contact with the exhaled fluid mixture, of leaving a modified fluid mixture in the flow passage having a lower amount of the exhaled carbon dioxide than the exhaled fluid mixture, and the exit outlet is located downstream from the membrane and provides at least the modified fluid mixture to the patient; and the membrane is inert with respect to the exhaled volatile molecular anesthetic agent and inert with respect to the exhaled carbon dioxide; and a fluid inlet for introducing an external fluid into the flow passage to be added to the modified fluid mixture provided to the patient. the membrane is pervious to the exhaled oxygen such that the membrane has an exhaled oxygen-to-exhaled volatile molecular anesthetic agent selectivity of greater than 1, the membrane is pervious to the exhaled carbon dioxide such that the membrane has an exhaled carbon dioxide-to-exhaled volatile molecular anesthetic agent selectivity of greater than 1, the membrane is capable, upon contact with the exhaled fluid mixture, of leaving a modified fluid mixture in the flow passage having a lower amount of the exhaled carbon dioxide than the exhaled fluid mixture, and the exit outlet is located downstream from the membrane and provides at least the modified fluid mixture to the patient; and the membrane is inert with respect to the exhaled volatile molecular anesthetic agent and inert with respect to the exhaled carbon dioxide; and a fluid inlet for introducing an external fluid into the flow passage to be added to the modified fluid mixture provided to the patient.