Stress is a biological response to a perceived threat. The threat may be physical, as in a near-miss car accident, or it may be emotional or mental, such as job or relationship stress. No matter what kind of stress is involved, however, the body's reaction is similar. This stress response is also known at the “fight or flight” response.

In situations of physical danger, the person’s sensory receptors (e.g., eyes and ears) detect the change and send messages to the brain via somatosensory fibers. This message basically says, “Something’s out there; what do you think of it?” The limbic system, the part of the brain associated with emotion, may send out a “red alert” message if it perceives a threat.

The part of the brain that responds to the emergency call is the hypothalamus, which is responsible for a wide variety of functions from sleeping and waking to sexual response. The hypothalamus sends out the warning to the rest of the body by doing four things very quickly. First, it sends a message directly to the adrenal glands, which are located on top of the kidneys, to release two hormones, epinephrine (adrenaline) and norepinephrine (noradrenaline). These two hormones race through the bloodstream to speed up the body’s organs in preparing for fight or flight. For example, the rate and force of the heartbeat will increase significantly.

Second, the hypothalamus releases a chemical called corticotrophin releasing factor (CRF). The CRF carries the warning message to the nearby pituitary gland, which responds by releasing adrenocorticotropic hormone (ACTH). The ACTH travels quickly to the adrenal glands, which receive the message and release a variety of chemicals in response, especially cortisol and aldosterone. Cortisol is responsible for supplying the body with necessary energy for the situation, which it does in a variety of ways; for example, it stimulates the liver’s release of cholesterol, fats, protein and glucose, and it takes energy from the immune system, thus temporarily shutting the immune system down. Aldosterone is responsible for increasing blood pressure for maximum readiness, which it does by helping the body retain sodium (and, therefore, water) in the blood.

Third, the hypothalamus stimulates the pituitary gland to release another two hormones: oxytocin and vasopressin (antidiuretic hormone; ADH). These hormones are responsible for keeping the blood pressure elevated, so that the heart, muscles and brain can get the oxygen needed for optimal functioning in the face of danger. Saliva may dry up in the mouth as fluids are transferred to more essential areas, such as the brain and muscles. The blood’s clotting ability is also increased, so that the body will lose less blood/fluids in the event of an injury.

Finally, the hypothalamus releases thyrotropic releasing hormone (TRH), which stimulates the pituitary gland to release thyrotropic stimulating hormone (TSH). As this hormone travels through the bloodstream, it stimulates the thyroid gland (located in the neck) to produce two chemicals: thyroxine and triiodothyronine. These two chemicals are responsible for speeding up the body’s metabolism, resulting in the acceleration of the following processes: blood pressure, breathing, heart rate, thinking processes and perspiration. The liver produces sugar from its stores of glycogen (composed of excess sugars, proteins and fat) and releases it into the bloodstream to provide extra energy for the body.

In situations of lasting emotional or mental stress, parts of this system may be activated on a more or less constant basis (chronic stress). The semi-permanent presence of stress chemicals in the body raises the risk of heart disease and other medical conditions. If this stress response is activated by a single event, it is known as acute stress. Many studies have shown that acute stress is linked to heart attack.