When a person is unconscious due to head injury, but the autonomous nervous system of the body keeps the body function normal by ensuring the flow of information between the brain and the rest of the body part.
The autonomous nervous system is the part of the nervous system of the body that is not controlled by our will but it works in the background in order to make sure that the body functions normally during an emergency.
When somebody loses consciousness because of head injury, the automatic nervous system regulates the flow of the blood vessels, heartbeat, and all of the useful functions of the chest area.
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