The brain can survive for 4 minutes without oxygen, then brain cells start dying. So if there is no heartbeat, the heart is not contracting, the blood is not circulating through the body, and there is no oxygen supply. Other tissues and organs can live longer without oxygen. For example an arm or leg can survive up to 6 hours. That is why if there is a deep cut or wound, a tourniquet can be applied to stop the flow of blood to a limb for several hours if needed.