The Fight or Flight Response

The fight or flight response is an automatic survival mechanism inherent to human biology. its primary function is to prepare the body to take immediate physical action when faced with life-threatening danger.
The response prepares the body for two specific modes of action: running away from the threat (flight) or, if escape is not possible, engaging in physical combat (fight).
Physiological sensations produced during this state occur for highly specific reasons related to survival. However, these sensations may be perceived as uncomfortable or distressing if the person experiencing them is unaware of their biological purpose.

Racing Thoughts: Cognition speeds up significantly during the response. This increased pace of thinking is a functional adaptation to help the individual evaluate danger quickly and arrive at rapid decisions.
Concentration Shifts: When the fight or flight response is active, the brain prioritizes survival above all else. Consequently, it becomes extremely difficult for an individual to concentrate on anything except the immediate danger or potential escape routes.
Acuteness of Vision: Vision can become remarkably acute so that more attention can be directed toward the source of danger.
Visual Phenomena: The individual may experience "tunnel vision," a narrowing of the visual field, or a sense that their vision has become "sharper" and more focused.

Increased Breathing Rate: Respiration becomes noticeably quicker and shallower. The purpose of this change is to maximize the intake of oxygen, which serves to power the muscles for the exertion required to fight or escape.
Dizziness and Lightheadedness: If the body takes in extra oxygen through rapid breathing but the individual does not perform the physical exercise (e.g., running or fighting) to use that oxygen, they may quickly begin to feel lightheaded or dizzy.
Accelerated Heartbeat: The heart rate increases to pump more blood to the muscles. This enhanced blood flow significantly improves the body's physical ability to perform at a high level during a crisis.

Dry Mouth: The mouth is an integral component of the digestive system. During a dangerous situation, digestion is temporarily shut down so that the body's energy can be diverted toward the muscles. This cessation of digestive function causes the mouth to feel dry.
Nausea and 'Butterflies': These sensations in the stomach are a direct result of blood being diverted away from the digestive system and toward more critical systems needed for immediate physical action.
Bladder Urgency: In circumstances of extreme stress, the muscles in the bladder may sometimes relax, which creates a sudden sense of urgency to urinate.

Muscle Tension: Muscles across the entire body tense up as a preparatory measure to get the individual ready to run or fight.
Shaking and Trembling: If an individual remains still while their muscles are prepared for action, those muscles may shake or tremble. This is a physiological method of maintaining a state of "ready for action."
Cold Hands and Vascular Constriction: The blood vessels located in the skin contract. This process forces the blood away from the extremities and toward the major muscle groups that are most critical for survival maneuvers.
Sweaty Palms and Cooling: The body begins to sweat to maintain a cool internal temperature. The guide notes that "a cool machine is an efficient machine," and sweating increases the likelihood of survival during a dangerous event by keeping the body's systems running efficiently.

Release of Adrenaline: The adrenal glands are responsible for the rapid secretion of adrenaline into the body.
Rapid Communication: Once released, adrenaline acts as a chemical messenger that quickly signals all other parts of the body to reach a state of readiness to respond to the perceived threat.