The hypothalamic-pituitary-arenal axis (HPA or HTPA axis), also known as the limbic-hypothalamic-pituitary-adrenal axis (LHPA axis), is a complex set of direct influences and feedback interactions between: the hypothalamus, a hollow, funnel-shaped part of the brain; the pituitary gland, a pea-shaped structure located below the hypothalamus; and the adrenal or suprarenal gland, a small, paired, pyramidal organ located at the top of each kidney. The fine, homeostatic interactions between these three organs constitute the HPA axis, a major part of the neuroendocrine system that controls reactions to stress and regulates various body processes including digestion, the immune system, mood and sexuality, and energy usage. Species from humans to the most ancient organisms share components of the HPA axis. It is the mechanism for a set of interactions among glands, hormones and parts of the mid-brain that mediate a general adaptation syndrome (GAS).