The Fight Or Flight Response

Norepinephrine and acetylcholine neurotransmitters produce a fight or flight response (see chart). In a fight response, eyes dilate so you can see better and lungs inspire more oxygen while increasing your heart rate. Blood vessels constrict increasing blood pressure. Smooth muscles along the bladder and the GI tract relax so that energy is not expended on digestion. Salivary glands reduce the secretion of saliva giving the person the dry mouth feeling in an emergency.

The flight response is really a misnomer because it doesn't help you run away. Instead, the flight response is really the opposite of fight and allows the individual to relax and function normally. In the flight response or the non-fight mode, pupils constrict, the heart rate slows, the GI tract reduces function, and breathing slows down.

Body Tissue/Organ

Sympathetic (Fight) Response

Parasympathetic (Flight) Response


Dilates pupil

Constricts pupil


Dilates bronchioles

Constricts bronchioles and increases secretions


Increases heart rate

Decreases heart rate

Blood vessels




Relaxes smooth muscles

Increases peristalsis


Relaxes bladder muscle

Constricts bladder


Relaxes uterine muscle

Salivary gland

Increases salivation

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