Physiology of Stress

It is difficult to imagine any mental reactions in the human body that do not lead to physical changes, and vice versa. That is why a complete understanding of stress is only possible by studying both its psychological and physiological manifestations.

Before diving into the details, let’s use an analogy to clarify why it’s important to understand the mechanisms and manifestations of stress. Many people drive cars, but only a few know how the different parts work and what happens in various situations. However, professionals know exactly how everything functions, which allows them to control the car much better.

To understand what stress is, we’ll look at what it consists of, its stages, which of these stages are relevant to us, how the body systems responsible for stress are structured, what happens to these systems during stress, and the consequences.

Definition of Stress

Stress is a protective response of the body to changes in the conditions in which a living organism exists, aimed at maintaining the internal environment (homeostasis) within limits suitable for survival and reproduction.

To sustain life, the body needs a certain stable environment, both external and internal. External examples include a specific ambient temperature, certain nutrients, and, for humans as social beings, communication in a particular context. Internal examples include body temperature and the stability of the quantity and proportion of various substances in the body.

Since the body’s influence on the external environment is strictly limited, over the course of evolution (or perhaps by divine will), living organisms have developed ways to make internal changes to maintain the stability of their internal environment.

Stressors and Types of Stress

All external stimuli that trigger a stress response are called “stressors.” There are physiological and psychological stressors. Physiological stressors directly affect body tissues, such as pain, cold, high temperatures, excessive physical exertion, and more. Psychological stressors are stimuli that indicate the biological or social significance of an event. It’s important to note that a stimulus becomes a stressor due to cognitive interpretation—the meaning a person assigns to it. However, stress can also occur without conscious awareness of the stressor (a technique used in NLP and Ericksonian hypnosis in profiling).

Not every stressor can trigger a stress response. Some people have what is called psychosomatic immunity, which can be due to physiological, social, or genetic factors. (This immunity can be weakened by the techniques mentioned above.)

By Duration

• Short-term stress – involves intense use of the body’s surface-level adaptation reserves, sometimes affecting deeper reserves.
• Long-term stress – involves slow, gradual depletion of both surface and deep adaptation energy.

By Stressor Type

• Psychological stress
• Physiological stress

In profiling, we are interested in short-term stress caused by psychological stressors, as we need to observe the immediate reaction without causing self-destruction. Therefore, we will focus only on this type.

Types of Psychological Stress

• Informational stress – arises when a person is unable to make the right decision within a short, fixed period under high responsibility and information overload. Stress occurs when a person is invested in doing the job correctly but is unable to do so.
• Emotional stress – occurs under negative emotional influences such as threats, insults, or conflicts where it is impossible to satisfy primary biological or social needs.

Stages of Stress

Stress develops in three stages:

• Alarm stage – characterized by low resistance and sharp reactions to any stimuli.
• Resistance stage – during which a person is most adapted and adjusted to the conditions.
• Exhaustion stage – a drop in adaptability, inevitably leading to death if the stress continues.

Physiological Mechanisms of Stress

To understand the physiology of stress, let’s look at the body’s structures and systems involved in these changes. Stress affects almost all functional systems, but the most noticeable effects are on the autonomic nervous system, which is less adapted to negative influences and more easily thrown out of balance.

The Autonomic Nervous System

The autonomic nervous system controls processes in the body that occur independently or are only indirectly influenced by conscious activity. Its divisions are activated during activity and various stress states, increasing metabolic rates in organs and tissues (the Orbeli-Penetzinsky phenomenon). The substances produced by the sympathetic nervous system are called catecholamines: adrenaline and noradrenaline, also produced by the adrenal glands. Body cells have receptors that respond to catecholamines, causing metabolic changes: adrenergic receptors—alpha 1, alpha 2, beta 1, beta 2.

The parasympathetic nervous system supports homeostatic reactions at rest and restorative responses, such as sneezing, coughing, and pupil constriction. It also triggers emptying of hollow organs (digestive tract, gallbladder, bladder), relaxation of sphincters, and contraction of smooth muscles. The substance produced by the parasympathetic system is acetylcholine, which acts on muscarinic and nicotinic cholinergic receptors in cells.

The metasympathetic nervous system consists of microganglionic formations in organ walls, including sensory, motor, and interneurons. It forms local reflex responses and allows internal organs to function without central nervous system involvement, regulating organ blood flow.

The sympathetic and parasympathetic systems are antagonists, causing opposite reactions. Normally, they are balanced, maintaining the body’s viability. This balance shifts when the body receives a stimulus—in our case, a stressor (provocation).

What Happens in the Body During Stress?

Let’s examine the process of psychological stress. The cerebral cortex receives a signal from the environment (provocation). After neocortical-limbic interpretation and assessment as threatening, the signal is transmitted via nerve pathways either to the posterior hypothalamus (for sympathetic activation) or the anterior hypothalamus (for parasympathetic activation).

The nerve impulse then travels via sympathetic pathways through the thoracic and lumbar spinal cord, through a chain of sympathetic ganglia to the relevant internal organs, where noradrenaline is released and a somatic response occurs. In addition to noradrenaline released by nerve endings, stimulation of the adrenal glands by sympathetic nerves causes the release of adrenaline, which has the same effect as noradrenaline.

Via parasympathetic pathways, the impulse travels through the cranial and sacral spinal cord to target organs, where acetylcholine is released and a somatic response occurs.

Sympathetic activation represents general arousal of internal organs, mobilizing the body’s resources and preparing muscles for action—allowing the individual to fight or flee from a threat. Parasympathetic activation causes inhibition, slowing, or normalization (after excessive arousal) of organ function—a “trophotropic” response.

In humans, sympathetic arousal during stress is more common, but with strong emotional and autonomic responses, it is replaced by parasympathetic inhibition, returning internal organs to normal function.

We are particularly interested in the reaction during the alarm stage (fear). Care must be taken not to push the subject into the second stage (though this is unlikely).

Alarm Stage Details

The alarm stage is marked by mobilization of general adaptation resources and a sharp drop in resistance to stress. It has three distinct periods:

1. First period – sharp activation of the body’s defense mechanisms, accompanied by stimulating emotions and increased performance. This period is short, lasting minutes or hours.
2. Second period – formation of a new “functional” level that allows the body to exist in changed conditions. There is a general decline in condition and performance, which may be offset by high motivation. However, chronic diseases may flare up under severe strain. Both periods together last about 11 days, regardless of stress intensity or type.
3. Third period – unstable adaptation, lasting about 20–60 days, after which the resistance stage begins, during which the body can feel relatively comfortable for a long time.

Observable Physiological Changes During Stress

These changes occur in all organs, but we are interested in those that are noticeable—namely, in the brain, skin, cardiovascular system, digestive system, and muscles. Here is a general overview of what happens, including external signs:

Digestive System

When a stimulus causes a clear emotional reaction, significant changes occur in the gastrointestinal tract, though responses vary among individuals. Emotions like fear, jealousy, disappointment, irritation, pain, and distress weaken the muscle tone of the stomach and duodenum, while surprise increases it. Joy, delight, and amazement do not cause any measurable changes.

• Inhibition of the GI tract: dry mouth (frequent swallowing, licking or touching lips, increased thirst).
• Activation of the GI tract: excessive salivation, audible stomach rumbling (frequent and loud swallowing, frequent touching of the abdomen).

Cardiovascular System

Changes in internal organ activity also depend on the emotional state. Blood pressure rises when there is readiness for action (tension, anger); if a person feels helpless and does nothing despite a harmful factor, blood pressure may drop. Fear and anger cause different changes: anger increases diastolic pressure and may slow the heart, while fear increases both systolic and diastolic pressure and heart rate. It is believed that anger is associated with noradrenaline release, and fear with adrenaline, explaining the difference.

• Inhibition: decreased blood pressure and pulse rate.
• Activation: increased blood pressure and pulse rate (visible carotid artery pulsation in the neck, pale skin, reddish-bluish facial tint).

Respiratory System

Breathing rate and depth, as well as the ratio of inhalation to exhalation, change. Strong emotional arousal increases breathing rate and depth. Sudden stimuli may cause brief breath-holding, followed by irregular breathing. Stressful situations increase inhalation duration. These changes can be observed by watching the chest, supraclavicular areas, and listening to breathing sounds.

Skeletal Muscles

Emotional tension increases muscle tone, especially in the neck, nose, and mouth. Constant tension in these muscles, when emotions cannot be directly expressed, can cause chronic headaches (migraines) often associated with stress. These changes are visible, as is trembling, since muscle tone increases unconsciously while the person tries to relax consciously. Voice changes also occur due to laryngeal muscle tension.

Skin

The skin reacts with changes in blood supply and galvanic response. It becomes pale and sweat production increases.

Other Changes

Other non-localized changes from prolonged stress include changes in body temperature, pupil dilation, and reduced quantity and altered composition of saliva, urine, and sweat.