Reactions to a specific stressor (an agent that causes stress) vary among individuals. For instance, some people experience frustration and anxiety when confronted with a deadline whereas others do not. The chemical processes that take place in the body when it perceives stress, however, are fairly consistent among all people.The body’s response to a perceived threat (stress) is also known as 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. These messages basically instruct the brain to assess the situation and determine if it is threatening. 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, cortisol 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 maintain a high sodium level (and therefore a high level of water) in the blood.

Third, the hypothalamus stimulates the pituitary gland to release two other hormones: oxytocin and vasopressin (antidiuretic hormone). 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 hormone releasing factor (TRF), which stimulates the pituitary gland to release thyrotropic hormone (TTH). 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.