The General Adaptation Syndrome (G.A.S.), developed by physician Hans Selye in the 1950s, explains the body’s consistent physiological reaction to any demand placed upon it. G.A.S. is a three-stage response pattern—alarm, resistance, and exhaustion—that is non-specific, meaning the physical changes are largely the same regardless of the nature of the stressor. The initial phase, the Alarm Phase, represents the body’s immediate mobilization in response to a perceived threat. This reaction, often called the “fight-or-flight” response, is a temporary state of shock and readiness designed for immediate survival.
Immediate Neurological Recognition and Activation
The Alarm Phase begins with an instantaneous neurological process when the brain registers a stressor. A distress signal is rapidly transmitted to the hypothalamus, a small region deep within the brain that acts as the command center for the body’s automatic functions. This immediate recognition triggers the rapid activation of the autonomic nervous system, specifically its sympathetic branch. This neurological activation ensures that the body’s defense mechanisms are deployed within moments of perceiving danger.
The hypothalamus initiates the “fight-or-flight” response through direct neural pathways, bypassing slower hormonal cascades for the most immediate reaction. Simultaneously, this brain region begins hormonal signaling by releasing corticotropin-releasing hormone (CRH). CRH travels to the pituitary gland, initiating the slower, yet more sustained, Hypothalamic-Pituitary-Adrenal (HPA) axis. The activation of these two systems—the fast neural sympathetic response and the slower HPA hormonal axis—prepares the body for the chemical changes that follow.
The Release of Stress Hormones
The rapid neurological signaling culminates in the immediate release of catecholamines, primarily epinephrine (adrenaline) and norepinephrine, from the adrenal medulla. These hormones are responsible for the body’s immediate surge of energy and heightened awareness. Epinephrine acts quickly to increase the heart rate and force of contraction. Norepinephrine contributes to the widespread constriction of blood vessels, which dramatically raises blood pressure. This hormonal flood readies the cardiovascular system for intense physical exertion.
Catecholamines also cause a rapid increase in the respiratory rate and a dilation of the bronchial tubes in the lungs, maximizing oxygen intake. This process ensures that the blood is highly oxygenated to fuel the active muscles and brain. While the catecholamine release is the fastest hormonal response, the initial surge of cortisol, released through the HPA axis, also occurs in the Alarm Phase. Cortisol’s primary role is to prepare the body for sustained action by mobilizing energy stores over a longer period.
Resource Mobilization and System Prioritization
The hormonal surge from the adrenal glands facilitates a massive mobilization of the body’s stored energy resources. Epinephrine and cortisol work together to accelerate the breakdown of complex energy molecules like glycogen into glucose, which is released into the bloodstream as immediate fuel. Fatty acids are also mobilized from fat cells, providing a secondary, longer-lasting source of energy for the muscles and brain. This metabolic shift ensures a high concentration of usable fuel is available for the survival response.
A major feature of the Alarm Phase is the strategic redirection of blood flow, a process known as vasoconstriction and vasodilation. Blood vessels leading to the skin, digestive tract, and immune organs constrict, shunting blood away from these non-essential systems. Conversely, blood flow increases significantly to the skeletal muscles, heart, and brain, prioritizing oxygen and nutrient delivery for immediate action. The body temporarily suppresses functions like digestion and immune surveillance to boost immediate physical and cognitive performance.
This prioritization results in noticeable physical manifestations, such as the dilation of the pupils to improve vision and a significant increase in muscle tension. Functions not required for immediate survival, including saliva production and digestive motility, are inhibited. These collective physiological changes mark the body’s commitment to the “fight-or-flight” state, providing a short-term, robust defense against a perceived threat.