You've probably heard of the terms stress and the fight-or-flight response. The word stress is, perhaps, a bit overused in our everyday language, but in biology it refers to a specific yet diverse arrangement of physiological behaviors and mechanisms; it is our body's response to what you might call a disequilibrium, or a challenge, to our perceived livelihood. This is why things as different as a car crash, an important test, or a loved one's illness can all cause us to feel stressed; they are threats to our basic instincts for self-preservation, comfort and success.
The fight-or-flight response describes the actual changes in our body as it reacts to these perceived threats; this is why you may feel breathless, restless or sweaty even when the threat bears you no physical threat, such as in the case of the important test. This is indicative of the autonomic nervous system being involved; that is, you do not have conscious control of whether or not the response is activated (although, with exposure or conditioning, you may learn that certain things are not threats and thereby overcome the perception). The fight-or-flight response is aptly named; your body is, in a literal sense, getting ready to attack or run, both of which require the involvement of virtually every other system in the body. The FOF response triggers what is commonly called a "cascade" - a series of reactions wherein one causes another, and another, rippling throughout the various components of the body's systems.
Some of the most commonly cited results of the FOF response include increased heart rate, sweating, rapid breath, and pupil dilation. Some of these effects are triggered directly by the brain, while others are strongly influenced by the hormone adrenaline (also called epinephrine). Adrenaline is produced and secreted by the endocrine system, and regulated by the nervous system. Other effects include inhibition of digestion, so we might say that the digestive system is involved in the response as well, but these are largely secondary.
The primary system involved in the response is the nervous system and sensory organs. Important secondary systems are the circulatory, respiratory, and metabolic systems.
According to one source, six "systems" are involved in perceiving a threat and reacting to it.
1. Autonomic nervous system: This system alerts the body to danger.
2. Adrenal cortex: This mediator of stress responses releases stress hormones.
3. Heart: It begins to beat faster and harder.
4. Lungs: Breathing becomes faster.
5. Thyroid gland: This gland triggers metabolism.
6. Muscles: Large muscles in the body become filled by oxygenated blood.