During the process of relaxed homeostasis, the autonomic nervous system (ANS) controls the basic functions of all of the physiological functions such as breathing, the heart rate, digestion, blood sugar, etc.
by influencing the endocrine system. The two components of the ANS, the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS), physiologically stimulate and inhibit the endocrine glands, which excrete hormones in a regular rate, thus providing normal cell function. When stress occurs, their functionality instantly changes in order to provide an adequate reaction. Acute stress situation, either psychological or environmental, activates the sympathetic nervous system.
This system is responsible for what is referred to as the “fight or flight” response (Alderman, 2004). The stressor stimulates the hypothalamus, which acts as a control center and instantly stimulates the SNS while at the same time inhibits the PNS. The SNS directly influences the adrenal medulla, which releases adrenaline (epinephrine) into the bloodstream. Adrenaline acts as a circulating hormone, increasing the breathing rate, dilating blood vessels in the heart and large muscles, and elevating blood pressure and heart rate. Adrenaline also stimulates the liver to metabolize and produce more blood sugar. This response provides more oxygen and energy (glucose) to the brain, muscles, and heart, making one act more instinctively, faster, and stronger (Aguilera, 2012). If the stressor persists, the hypothalamus releases a corticotropin-releasing factor; this stimulates the pituitary gland to release adrenocorticotropic hormone (ACTH) into circulation. ACTH stimulates the cortex of the adrenal glands to produce adrenocortical steroids, such as cortisol (Ziegler, 2004).
Cortisol further increases blood sugar and stimulates the repair of cells. The release of cortisol also inhibits the inflammatory and immune reactions, thus increasing the stress endurance for a longer period of time. Once a stressor ceases to exist, the hypothalamus inhibits the SNS and stimulates PNS, metabolically diminishing the emergency effect and repairing and replenishing the depleted bodily reserves. This phase is called “rest and digest”, which sole function is to achieve the homeostasis that once was, as soon as possible (Goldstein, 2010).
