THERMAL ECOLOGY AND STRESS 333 



organism, including increasing the levels of circulating blood glucose, 

 dilating the pupils of the eye, stimulating the frequency and force of 

 heart contractions, etc. These kinds of responses are clearly effective 

 in preparing an individual to respond defensively or offensively to 

 the threat of external attack or provocation. It is appropriate that 

 the alarm stage is sometimes called the fright— flight— fight response. 

 The second phase of the GAS is the resistance stage. Very soon 

 £ifter the alarm stage has run its course, the activity of the autonomic 

 nervous system is diminished, and the release of epinephrine from 

 the adrenal medulla also slows down. If the stressor persists in time, 

 however, there is a substantial rise in the level of circulating 

 corticosteroids produced by cells in the adrenal cortex (Fig. 2). The 

 cortical cells are stimulated by adrenocorticotrophic hormone 

 (ACTH), which, in turn, is produced by the pituitary gland. The 

 stimuli for discharge of ACTH are releasing factors produced in the 

 hypothalamus; presumably higher centers in the central nervous 

 system are responsive to external and internal stimuli that promote 

 the liberation of the so-called releasing factors from the hypothala- 

 mus. The function of corticosteroids is to mitigate cellular damage. 



STRESSOR 



RECEPTOR 



PITUITARY GLAND 



ACTH 



ADRENAL GLAND 



ADRENOCORTICOSTEROIDS 



PHYSIOLOGICAL, MORPHOLOGICAL, 

 AND BIOCHEMICAL CHANGES 



DEPRESSED CARBOHYDRATE METABOLISM 



PROMOTION OF G LUCONEOGENESIS FROM 

 TISSUE PROTEIN 



PROMOTION OF GLYCOGENESIS IN LIVER 



SUPPRESSION OF INFLAMMATORY RESPONSE 



Fig. 2 Resistance stage of the general adaptation syndrome. 



