The sympathetic nervous system becomes activated.
The sympathetic nervous system is one of the sub-divisions of the autonomic nervous system. The sympathetic nervous system gets activated in situations that trigger a fight-or-flight response. In this case, when you duck your head down when a flying object is coming towards you are reacting (ducking) from the source of danger (flying object) to protect yourself from harm or injury.
The sympathetic nervous system also produces the following reactions physiologically:
Increased heart rate
Faster shallow breaths
Dilated pupils
Inhibition of digestive and excretory functions
