The endocrine system, found in almost all species of the animal kingdom, is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In humans, the major endocrine glands are the thyroid gland and the adrenal glands. In response to environmental stimuli, the endocrine system secretes hormones and uses them as chemical messengers to orchestrate physiological, developmental and reproductive changes that affect the entire body for a long period of time. In order to maintain the proper functioning of the body through its entire life cycle, the endocrine system utilizes a complex feedback mechanism to fine-tune the balance of hormones in the bloodstream.