The first stage of the general adaptation stage, the alarm reaction, is the immediate reaction to a stressor. it involves increased activity in the sympathetic adrenal medullary system, SAM and the hypothalamic pituitary adrenocortical axis, HPA. In the initial phase of stress, humans exhibit a "fight or flight" response, which causes one to be ready for physical activity. However, this initial response can also decrease the effectiveness of the immune system, making persons more susceptible to illness during this phase. Seyle said that the alarm reaction develops 6-48 hours after stress.
Stage 2 might also be named the stage of adaptation as well as stage of resistance and involves activity in the HPA. During this phase, if the stress continues, the body adapts to the stressors it is exposed to. Changes at many levels take place in order to reduce the effect of the stressor, the parasympathetic nervous system requires more careful use of the body’s resources in order to cope. The system is being taxed to its limits. This stage is initially marked by an increase in the size of the adrenal glands and a decrease in some pituitary activity. E.g. the production of growth hormone. An example, if the stressor is starvation (possibly due to anorexia), the person might experience a reduced desire for physical activity to conserve energy, and the absorption of nutrients from food might be maximized.
At stage 3, the stage of exhaustion, the stress has continued for some time. The body's resistance to the stress may gradually be reduced, or may collapse quickly. Generally, this means the immune system, and the body's ability to resist disease, may be almost totally eliminated. Patients who experience long-term stress may succumb to heart attacks or severe infection due to their reduced immunity. For example, a person with a stressful job may experience long-term stress that might lead to high blood pressure and an eventual heart attack.
Seyle carried out pioneering research to identify major components of the stress response. His research was very useful in alerting medicine to the importance of stress in disease. Previous researchers had not properly appreciated the damaging effects of prolonged exposure to stress. However there are many limitations to this study. Seyle did not pay much attention to the SAM system and he did not understand fully the relationship between the SAM and the HPA systems. Also he exaggerated when he claimed that stress always produces the same physiological pattern. For example Mason in 1975 found that the various stressors, fear, uncertainty and anger, produced different patterns of adrenaline, noradrenaline and cortisol secretion.
Seyle has been criticised for using non human animals to support his research on human responses to stress. Therefore the results may be different and cannot be used fairly when showing effects of human stressors. This may explain why his model overemphasises physiological factors at the expense of psychological factors such as the role of emotional and cognitive factors in stress. The last criticism arises because Seyle assumed that people respond in a passive way to stressors. However, Mason argued that there is an active process of psychological appraisal when people confront a stressor.
