The glands and other structures that consist of the adrenal glands (near the kidneys), gonads, pancreas, parathyroids (glands situated above or within the thyroid gland), pineal (small, conical endocrine gland), pituitary (main endocrine gland in the head), thymus (glandular organ at the base of the neck), and thyroids (glands near the base of the neck); all of which are involved with the secretion of hormones that are released directly into the circulatory system, influencing metabolism and other body processes.