In 1936 Hans Selye discovered that test rats displayed the same physiological response to any and
all stressors (cold, heat, noise, chemicals, etc.). Regardless of the nature of the stressor, the
animal’s response involved virtually every major body system. (1, 2) Selye called this adaptive
response to stress the General Adaptation Syndrome (GAS) and defined it thus: “The sum of all
non?specific systemic reactions of the body which ensue upon long continued exposure to systemic stress.”
About the GAS, Selye made the following points: “The reaction is general. It is elicited by those
stresses that are generalised, that affect large portions of the body, and evoke a generalised, systemic,
defence phenomenon. The reaction is adaptive. It helps the acquisition and maintenance of a state of
inurement. The reaction is a syndrome. Its individual manifestations are co?ordinated and interdependent.”
(14) Simply put, the GAS is the bodies in built mechanism of resistance and through it the body is
able to resist stress. Selye discovered the GAS occurred in three distinct and sequential phases,
the Alarm Reaction, the State of Resistance, and the State of Exhaustion. (1, 2)
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Selye defined the Alarm Reaction as “all non?specific phenomena which occur after exposure to a
stressor.” For the Alarm Reaction (AR) to be initiated, Selye stipulated “The stressor must affect
large portions of the body and the organism must be qualitatively and quantitatively not adapted to the
stressor.” Selye also observed that the AR immediately followed exposure to a stressor. (3)
The AR exhibited the same pattern each time. In order for the body to rapidly deploy energy,
sympathetic nervous activities increased and the hypothalamus/ pituitary/ adrenal gland axis
was stimulated. Selye found that catecholamine levels rose while adrenal gland weight
diminished. He demonstrated that the loss of adrenal weight was due to the usage of stored
ascorbic acid and cholesterol, which are used in the production of adrenaline and corticosterols.
Additionally, Selye found that the weight of the thymus, spleen, lymphatic glands, and livers also
decreased. He also observed acute damage to the digestive tract and the development of ulcers in
this phase of the GAS. Body temperatures lowered and fat tissues began to show signs of
catabolism – a hallmark of the AR. (1, 2)
Selye observed that the AR, in turn, could be divided into two distinct phases: Phase of Shock
and Phase of Counter?shock. (3)
The Phase of Shock lasts between minutes and 24 hours and is followed by the Phase of Countershock.
Selye observed that the same list of physiological changes were always seen in the Phase
of Shock: hypothermia, hypotension, depression of the nervous system, decrease in muscular
tone, hemo?concentration, deranged capillary and cell membrane permeability, generalised tissue
break down (catabolic impulse), hypochloremia, hyperkalaemia, transitory hyperglycaemia
followed by hypoglycaemia, leucopoenia followed by leucocytosis, eosinopenia, and acute gastric
ulceration. (3).
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The Phase of Counter?shock is characterised by a reversal of the changes seen in the Phase of
Shock including: an increase in blood pressure, hyperthermia, hyperchloremia, hyperglycaemia,
increased blood volume, and increased diuresis. Additionally, there is an enlargement of adrenal
cortices with increased signs of adrenal activity (loss of lipids, ascorbic acid, cholesterol, etc.) and
acute involution of the thymicolymphatic tissues. Selye observed that the reversals were largely
due to the release of corticoids into the blood stream. In addition, he noted that Counter?shock
represents a transition to the State of Resistance. (3)
If exposure to the harmful influence continued for several days, the State of Resistance (SR)
began. (1, 2) Selye defined the State of Resistance as “The sum of all non?specific systemic reactions
elicited by prolonged exposure to stress stimuli to which the organism has acquired resistance.” In other
words, he observed that the SR was characterised by an increased resistance to the stressor agent.
(4)
The hallmark of this stage is normalisation of physiological function; the morphological and
biochemical changes that appeared in the Alarm Reaction disappear during the SR. The
catabolism of the AR is replaced with anabolism, the adrenal loss of lipids is replaced with lipid
deposition, hyperglycaemia gives way to normal blood sugar levels, and hypertension gives way
to normal blood pressure levels. The animal adapts to the stress despite its continued presence
and body functions return to those of a normal resting state. (4).
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Biological Activities
General Adaptation Syndrome (GAS)
Written by Mehrzad | 
