Abstract: Arrangement in connection with an anaesthesia/ventilation system for a patient comprising means for flowing inspiratory gas to the patient and means for flowing expiratory gas from the patient to a gas separation means and further through the gas separation means back to the inspiratory flow. The gas separation means comprises a chamber unit having a high pressure side and a low pressure side, the high pressure side and the low pressure side being separated by a membrane where carriers are fixed on polymer backbone. The expiratory gas from the patient containing retentates such as anaesthetic agents, N2O, O2, air, and permeates such as CO2, flows to the high pressure side of the chamber unit to have contact with the membrane surface whereby the membrane reacts with the permeates so that the permeates flow through the membrane to the low pressure side of the chamber unit and the retentates flow through the high pressure side of the chamber unit without permeating the membrane.