The present invention refers to a ventilation system used in hospital respiratory devices for the administration of anesthesia to newly born, pediatric and adult patients. More specifically, the present invention refers to a ventilation system that promotes a respiratory circuit with re-inhalation for the administration of anesthesia and that overcomes all inconveniences and deficiencies existing in respiratory devices of the state of the art. The ventilation system in respiratory devices comprises a bellows assembled within a reservoir, which is provided with a manifold with various gas exhaling valves, more specifically release valves to take out the excess of gases, and an exhaling valve for control gases. It is additionally constituted of a valve for the free flow of oxygen to quickly renew or replace the gases inside the respiratory circuit.
