Our senses are continually providing a whole range of ongoing information to the brain about the external and our internal environment. This includes:

What we see
What we hear
What we smell
What we taste
What we physically feel
What we are emotionally experiencing
What we are thinking

When there is any perceived threat our internal alarm system focuses our attention as our brain evaluates the situation and determines how much threat we are under and our capabilities of dealing with the threat. This alarm-driven focus of attention is often experienced as:

Visual focus on threat, reducing peripheral vision and creating a tunnel-vision experience
Heightened sensory awareness
Experiencing distortion in time
Experiencing distortion in sound
Emotional numbing

Depending on the degree of perceived threat a degree of the fight/flight response is activated. A fight/flight (stress) response can include:

Increased adrenalin and cortisol levels
Increased heart rate and blood pressure
Faster and shallower breathing
Blood is diverted away from surface skin and non-essential functions, such as digestion
Digestion slows down or stops
Evacuation urge (three ways)
Blood is diverted to major muscles
Major muscles tense, trembling as body is “super-charged”
Glucose and fatty acids released from the liver for energy boost
Fine motor skills decrease
The body cools itself by sweating
Pupils dilate to focus on threat

The freeze and flop responses occur when fight and flight are not perceived as possible; either the situation requires us to be stationary to give us the best chance of survival or and the brain is preparing for the potential for death. Dissociation and the freeze response are usually associated with higher levels of PTSD.