Coughing out blood is a pretty alarming condition. However, the seriousness of the condition is contingent upon the source of blood. The blood could be issued from the mouth or somewhere down the respiratory system.
Hemoptysis is the medical term for blood coughing. It points out some issues in the blood vessels or pneumonia which should be checked and treated immediately.
Coughing out so much blood simply means that the condition is at a critical stage that not only calls for concern but might need an ambulance call. It is an occurrence that should never be looked over no matter what you think the reason is. In sum coughing blood should make you quickly get to the nearest hospital or medical practice.