The endocrine system helps regulate and maintain various body functions by synthesizing (making) and releasing hormones, chemical messengers.
The endocrine system is composed of glands that release their hormones directly into the bloodstream for chemical signaling of target cells. Endocrine glands do not have a duct system and are called ductless glands. These glands release hormones directly into the blood or lymph.
These glands include the pituitary gland, the pineal gland, the hypothalamus, the thyroid gland, the parathyroid glands, the thymus, the adrenal glands, the ovaries (in females) or testes (in males), and the pancreas.
Endocrine glands have a rich blood supply through which hormones travel to reach their target organs. Hormones alter the metabolism of target organs by increasing or decreasing their activity. One of the major tasks of hormones is to coordinate the activities of organ systems. Typically, the body synthesizes hormones in one part and transports it to another through the bloodstream or lymph. These changes in activity are strictly balanced to maintain homeostasis (a stable internal environment).
What the Endocrine System Does
Once a hormone is secreted, it travels from the endocrine gland through the bloodstream to target cells designed to receive its message. Along the way to the target cells, special proteins bind to some of the hormones. The special proteins act as carriers that control the amount of hormone that is available to interact with and affect the target cells.
Also, the target cells have receptors that latch only onto specific hormones, and each hormone has its own receptor, so that each hormone will communicate only with specific target cells that possess receptors for that hormone. When the hormone reaches its target cell, it locks onto the cell's specific receptors and these hormone-receptor combinations transmit chemical instructions to the inner workings of the cell.
When hormone levels reach a