The endocrine system integrates other body functions through its hormonal control. Endocrine glands have liberal blood supplies to help distribute released hormones. Endocrine cells are generally located close to capillary beds, where they can easily get into the bloodstream for bodywide distribution. As previously stated, most hormones circulate through the body in the bloodstream.
Cells have receptors which attract only a specific hormone.
Hormones act only as they are programmed by their target cells. In other words, hormones are triggers, but carry no information. Hormone secretion occurs in three ways. One is from endocrine glands precipitated by changing ion or nutrient levels in the blood. More simply put, hormone levels are allowed to reach a predetermined level, at which point, further secretion is strictly controlled in order to remain within a specific range in the blood.