Which body system in Selye's general adaptation syndrome (GAS) model is responsible for regulating various body processes, including digestion, immune-system responses, emotion, and energy expenditure?

Hypothalamic-pituitary-adrenal axis (HPA axis)

Explanation:

The three-stage process describing the physiological changes that take place in the body when it is under stress is known as general adaptation syndrome (GAS). The model describing the three-stage process was given by the scientist Hans Selye. According to this model, the responses in the body that occur during stress is regulated by the Hypothalamic-pituitary-adrenal axis (HPA axis) where the brain senses the stress and then the signals travel from the hypothalamus to the pituitary and finally the body releases cortisol from adrenal glands that contributes to the various changes.