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Stress physiology is a general term used to describe the physiological responses that various organisms experience when placed in stress-inducing situations. When placed in situations of stress, the body tends to experience a substantial chemical and physical response aimed at redirecting the body's resources in order to handle the source of stress. While stress physiology tends not to be particularly effective in situations that cause stress in many modern humans, it is an effective survival mechanism for organisms that need to deal with predators and other physical threats. One of the most important parts of stress physiology is known as the "fight or flight response" and involves a set of physiological changes intended to allow an organism to fight or flee from a given threat.
The fight or flight response is among the most commonly studied and researched aspects of stress physiology because of its prevalence in the stress responses of many organisms in a variety of different situations. In the fight or flight response, blood flow is increased to the muscles and to the respiratory system to allow for heightened physical activity. Metabolism increases in order to provide a high level of usable energy to the body. Overall, the body is optimized for the rapid and strong muscular and cardiovascular activity necessary to escape or fight potential threats.
Many parts of the body are largely unnecessary for fight or flight activities, so stress physiology includes a reduction of activity in some areas. Blood supply, for instance, is reduced to some parts of the body including the digestive system and some parts of the brain. Digestion of food and brain functions such as those allowing speech are not generally necessary during fight or flight situations.
There are, in some cases, different stages of stress physiology. Upon perceiving a potential threat, an animal may enter a state of heightened sensory awareness prior to the full fight-or-flight response. This allows the animal to determine accurately whether or not a threat exists. Upon identifying a threat with certainty, the full fight or flight response begins, allowing for the necessary rapid muscular action.
One of the major elements of stress physiology that facilitates the fight or flight response is the release of hormones, such as adrenaline and noradrenaline. Such hormones are chemical signals that, when released into the blood stream, initiate the various changes involved in stress physiology. Adrenaline, produced in the adrenal gland, is one of the most important chemical signals involved in human stress response.