Final answer:
The major stress axis of an organism is the Hypothalamus Pituitary Adrenal (HPA) Axis, which regulates the stress response by secreting cortisol from the adrenal cortex.
Step-by-step explanation:
The major stress axis of an organism is the Hypothalamus Pituitary Adrenal (HPA) Axis. This system is crucial for the regulation of the body's response to stress. It involves a sequence where the hypothalamus releases corticotropin-releasing hormone (CRH) that stimulates the pituitary gland to release adrenocorticotropic hormone (ACTH). ACTH, in turn, activates the adrenal cortex to secrete cortisol, known as the stress hormone. The cortisol affects nearly every organ in the body and helps provide the energy boost during the initial stress response, although continually high levels can impair the immune system. Additionally, CRH has multiple roles in the stress response, including the activation of the sympathetic nervous system and inhibition of behaviors not crucial during stress, such as exploration, food consumption, and reproduction.