US EPA

7437334 

Journal Article 

The Effect of Exercise on the Hypothalamo—Pituitary—Adrenal Axis 

Wittert, G; , 

2000 

Humana Press 

Totowa, NJ 

Sports Endocrinology 

43-55 

10.1007/978-1-59259-016-2_3 

http://link.springer.com/10.1007/978-1-59259-016-2_3
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In response to any external stimulus that is perceived as a threat to homeostasis (stress), activation of the autonomic nervous system occurs and plasma cortisol levels increase as a result of activation of the hypothalamo—pituitary—adrenal (HPA) axis. The hypothalamic hormones corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) are the major regulators of corticotropin (adrenocorticotrophic hormone; ACTH) secretion from the corticotropes of the anterior pituitary gland (1). ACTH in turn stimulates the production and release of cortisol from the zona fasciculata of the adrenal cortex at a rate of about 12–15 mg/m2/d in nonstressed adults. Approximately 90–93% of circulating cortisol is bound to a protein, cortisol binding globulin (CBG), and only the free fraction can readily diffuse through cell membranes and exert biological effects. Plasma cortisol levels peak at about the time of waking and have their nadir around midnight. Although light is the main entrainer of the cortisol rhythm, other influences, for example, physical activity, also have a significant effect on the human circadian pacemaker (2).