Resuscitation bag system useable for resuscitating a person in cardiac arrest, comprising: - a hollow flexible bag comprising an inlet and an outlet, - a first conduct element fluidly connected to the inlet of the hollow flexible bag for providing gas to said flexible bag, - a main conduct fluidly connected to the outlet of the hollow flexible bag for conveying gas provided by said flexible bag, and - a valve element arranged in the main conduct and fluidly communicating with said main conduct, said valve element comprising a venting port, characterized in that it further comprises: - a recirculation unit comprising:. a sensing chamber fluidly communicating with the venting port of valve element for recovering gas vented by said venting port, and. a CO 2 removal line comprising an upstream end fluidly connected to said sensing chamber and a downstream end fluidly connected to the first conduct element, - at least one sensing element sensitive to the presence of oxygen, arranged in said sensing chamber, - a gas control unit comprising a first valve fluidly connected to a first and to a second conduct, said first and second conducts (922, 923) being arranged in parallel and further fluidly connected to the first conduct element, the first conduct comprising first flow restriction means configured for limiting the gas flow to a first flow rate, and the second conduct comprising second flow restriction means configured for limiting the gas flow to a second flow rate, said second flow rate being less than the first flow rate, and - a monitoring module comprising processing means (701, 702) and being configured for: i) cooperating with said at least one sensing element of the sensing chamber for determining the concentration of O 2 in the gas contained into the sensing chamber, and ii) controlling the first valve of the gas control unit for directing the gas into the first conduct or the second conduct, on the basis of said determined O 2 concentration. - a hollow flexible bag comprising an inlet and an outlet, - a first conduct element fluidly connected to the inlet of the hollow flexible bag for providing gas to said flexible bag, - a main conduct fluidly connected to the outlet of the hollow flexible bag for conveying gas provided by said flexible bag, and - a valve element arranged in the main conduct and fluidly communicating with said main conduct, said valve element comprising a venting port, characterized in that it further comprises: - a recirculation unit comprising:. a sensing chamber fluidly communicating with the venting port of valve element for recovering gas vented by said venting port, and. a CO 2 removal line comprising an upstream end fluidly connected to said sensing chamber and a downstream end fluidly connected to the first conduct element,. a sensing chamber fluidly communicating with the venting port of valve element for recovering gas vented by said venting port, and. a CO 2 removal line comprising an upstream end fluidly connected to said sensing chamber and a downstream end fluidly connected to the first conduct element, - at least one sensing element sensitive to the presence of oxygen, arranged in said sensing chamber, - a gas control unit comprising a first valve fluidly connected to a first and to a second conduct, said first and second conducts (922, 923) being arranged in parallel and further fluidly connected to the first conduct element, the first conduct comprising first flow restriction means configured for limiting the gas flow to a first flow rate, and the second conduct comprising second flow restriction means configured for limiting the gas flow to a second flow rate, said second flow rate being less than the first flow rate, and - a monitoring module comprising processing means (701, 702) and being configured for: i) cooperating with said at least one sensing element of the sensing chamber for determining the concentration of O 2 in the gas contained into the sensing chamber, and ii) controlling the first valve of the gas control unit for directing the gas into the first conduct or the second conduct, on the basis of said determined O 2 concentration. i) cooperating with said at least one sensing element of the sensing chamber for determining the concentration of O 2 in the gas contained into the sensing chamber, and ii) controlling the first valve of the gas control unit for directing the gas into the first conduct or the second conduct, on the basis of said determined O 2 concentration.