The stress response begins in the brain (credit: Harvard source). When someone confronts an oncoming car or other danger, the eyes or ears (or both) send the information to the amygdala, an area of the brain that contributes to emotional processing. The amygdala interprets the images and sounds. When it perceives danger, it instantly sends a distress signal (an "alarm notification") to the hypothalamus.
The hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions of the hypothalamus is to link the nervous system to the endocrine system via the pituitary gland (hypophysis).
The hypothalamus is responsible for certain metabolic processes and other activities of the autonomic nervous system. The hypothalamus synthesizes and secretes certain hormones and these in turn stimulate or inhibit the secretion of pituitary hormones.
Hormones are chemical substances that are secreted directly into the blood and carried to organs and tissues of the body to exert their functions.
Thought these substances the hypothalamus controls body temperature, hunger, thirst, fatigue, sleep and circadian rhythms. The hypothalamus is a bit like a command center. This area of the brain communicates with the rest of the body through the autonomic nervous system, which controls such involuntary body functions as breathing, blood pressure, heartbeat, and the dilation or constriction of key blood vessels and small airways in the lungs called bronchioles.
The autonomic nervous system has two components, the sympathetic nervous system and the parasympathetic nervous system. The state of the body at any given time represents a balance between the sympathetic and parasympathetic systems.
The sympathetic nervous system functions like a gas pedal in a car. It triggers the fight-or-flight response, providing the body with a burst of energy so that it can respond to perceived dangers. The parasympathetic nervous system acts like a brake. It promotes the "rest and digest" response that calms the body down after the danger has passed.
After the amygdala sends a distress signal, the hypothalamus activates the sympathetic nervous system by sending signals through the autonomic nerves to the adrenal glands.
These glands respond by pumping the hormone epinephrine (i.e. adrenaline) into the bloodstream. As epinephrine circulates through the body, it brings on a number of physiological changes. The heart beats faster than normal, pushing blood to the muscles, heart, and other vital organs. Pulse rate and blood pressure go up. The person undergoing these changes also starts to breathe more rapidly. Small airways in the lungs open wide. This way, the lungs can take in as much oxygen as possible with each breath. Extra oxygen is sent to the brain, increasing alertness. Sight, hearing, and other senses become sharper.
Meanwhile, epinephrine triggers the release of blood sugar (glucose) and fats from the liver for temporary storage sites in the body. These nutrients flood into the bloodstream, supplying energy to all parts of the body.
All of these changes happen so quickly that people aren't aware of them. In fact, the wiring is so efficient that the amygdala and hypothalamus start this cascade even before the brain's visual centers have had a chance to fully process what is happening."
That's why people are able to jump out of the path of an oncoming car even before they think about what they are doing.
As the initial surge of epinephrine subsides, the hypothalamus activates the second component of the stress response
system — known as the HPA axis. This network consists of the hypothalamus, the pituitary gland, and the adrenal glands.
The HPA axis relies on a series of hormonal signals to keep the sympathetic nervous system — the "gas pedal" — pressed down.
If the brain continues to perceive something as dangerous, the hypothalamus releases corticotrophin-releasing hormone (CRH), which travels to the pituitary gland, triggering the release of adrenocorticotropic hormone (ACTH). This substance travels to the adrenal glands, prompting them to release into the bloodstream of another key-substance: cortisol, also known as "the stress hormone".
When the threat passes, cortisol levels fall. The parasympathetic nervous system — the "brake" — then dampens the stress response.
According to the General Adaptation Syndrome theory by Hans Selye developed in 1939, a healthy/positive stress response is when the body learns to cope efficiently with stressors thus stress-hormonal levels return normal in a defined period of time and the body can come back at its normal resting state.
For instance, when we are hungry (stressor agent) our body is pumping stress-hormones (stress response switch on) just long enough to deal with the offending stressor. When we shout down the stressor by eating, the cortisol levels fall (stress response switch off).
This is how a healthy stress response may look like:
After a moment of acute stress, a "recovery phase" is essential for restoring our internal physiological balance.
"Homeostasis" is an essential concept for stress response. An organism's attempt at restoring physiological conditions back to or near homeostasis thanks to the stress response. The stress response is also known as "allostasis" – which comes from the Greek "allo", meaning "variable", and "stasis" meaning "stable". Allostasis refers to the capacity (the response) of an organism to maintain its internal stability producing internal changes like the release of hormones. Homeostasis is in fact the property (or tendency) of an organism to achieve internal stability. Basically some variables (for example temperature, PH of its extracellular fluids or the concentration of minerals in the blood plasma) are actively regulated to remain very nearly constant.
In biology, most biochemical processes strive to maintain this equilibrium, a steady state that exists more as an ideal and less as an achievable condition. In fact environmental factors, internal or external stimuli, continually disrupt homeostasis.