Stress in biological terms is an organism's bodily response to some kind of challenge. This can be

environmental, like from the ground shifting or shaking underneath it, or in response to the

presence of another organism. Or it can be internal, some kind of subconscious misfire that makes

this organism think that there's something going on environmentally when in fact

there is not. So when an organism's environment changes in a way that causes stress, parts of its

biology light up. In humans and other creatures with relatively sophisticated biologies,

our autonomic nervous system, the part of us that

regulates heart rate, pupil size, sexual arousal, and other subconscious things of that

nature, will trigger as will our hypothalamic pituitary adrenal, or HPA, axis, which is

made up of glands that modulate the hormones that are active in

our bodies. One such hormone is called cortisol, and it can trigger changes in our metabolic,

psychological, and immunological bodily functions, changing things like our blood flow,

our thinking and perception, and our body's defense mechanisms.

The consequences of these quick-twitch physical responses can be immense. They can alter our

internal reward systems, changing what we feel like doing and feel good doing. They can alter

our memories, causing us to recall

things happening differently than they actually did, and they can even make us more susceptible

to disease and injury.

And running parallel to all of this is what is called the sympathetic nervous system,

which can be triggered by these other changes, and in turn

can activate our so-called fight-or-flight response, which further deviates our bodies from

their normal functions in order to essentially optimize us, to mess someone up, to fight something,

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