A patient breathing system includes a ventilator that provides a driving gas flow to generate patient inspiration. The ventilator further includes a gas inlet for driving gas, the patient breathing system being connectable to a circle which includes an inspiratory hose and an expiratory hose connected to patient, through which circle expired gases can be circulated back to the patient. The circle further includes a fresh gas inlet, an arrangement for enabling the gas flow in a desired direction, a unit for removal of the exhaled carbon dioxide, and a ventilator port for the ventilator connection. The patient breathing system further includes an arrangement by which driving gas of the ventilator has been separated from the patient gases flowing in the circle.