Every living organism tends to remain in a state of homeostasis. In homeostasis, all physical and psychological systems function smoothly. When a stressor disrupts homeostasis, the body adjusts with an adaptive response, or an attempt to restore homeostasis. This adaptive response to stress varies in intensity and physical manifestation from person to person and from stressor to stressor. The three-stage response to stress is called the general adaptation syndrome. The phases of the general adaptation syndrome are alarm, resistance, and exhaustion.
During the alarm phase, a stressor disturbs homeostasis. The brain subconsciously perceives the stressor and prepares the body either to fight or to run away, a response sometimes called the fight or flight response. When the mind perceives a stressor, the cerebral cortex, is called to attention. If the cerebral cortex consciously or unconsciously perceives a threat, it triggers an autonomic nervous system response that prepares the body for action. The autonomic nervous system is the portion of the central nervous system that regulates bodily functions that we do not normally consciously control. When we are stressed, the rate of all these bodily functions increases dramatically to give us the physical strength to protect ourselves against an attack, or to mobilize internal forces. In addition to this, the hypothalamus, a section of the brain, functions as the control center and determines the overall reaction to stressors. When the hypothalamus perceives that extra energy is needed to fight a stressor, it stimulates the adrenal glands to release the hormone epinephrine, also called adrenaline. Epinephrine causes more blood to be pumped with each beat of the heart, dilates the air sacs in the lungs to increase oxygen intake, increases the breathing rate, stimulates the liver to release more glucose, and dilates the pupils to improve visual sensitivity. The body is then poised to act immediately.
The resistance phase of the general adaptation syndrome begins almost immediately after the alarm phase starts. In this phase, the body has reacted to the stressor and adjusted in a way that begins to allow the body to return to homeostasis. As the hypothalamus is working to energize the body, other processes are occurring to keep energy levels under control and return the body to a normal level of functioning.
In the exhaustion phase of the general adaptation syndrome, the physical and psychological energy used to fight a stressor has been depleted. Short-term stress would probably not deplete all of a person's energy reserves, but chronic stressors, such as the struggle to get straight As, financial worries, or fights with family and friends may create continuous states of alarm and resistance. When a person no longer has the adaptation energy stores for fighting a distressor, serious illness may result.