Ducts are tubes leading from a gland to its target organ. Exocrine glands such as the sudoriferous (sweat) glands contain ducts. These glands release hormones directly into the blood or lymph. Endocrine glands do not have a duct system and are called ductless glands. These changes in activity are strictly balanced to maintain homeostasis (a stable internal environment). Hormones alter the metabolism of target organs by increasing or decreasing their activity. Endocrine glands have a rich blood supply through which hormones travel to reach their target organs. Typically, the body synthesizes hormones in one part and transports it to another through the bloodstream or lymph. These glands include the pituitary gland, the pineal gland, the hypothalamus, the thyroid gland, the parathyroid glands, the thymus, the suprarenal (adrenal) glands, the ovaries (in females) or testes (in males), and the pancreas. The endocrine system is composed of glands that release their hormones directly into the bloodstream for chemical signaling of target cells. The major areas of control and integration include responses to stress and injury, growth and development, absorption of nutrients, energy metabolism, water and electrolyte balance, reproduction, birth, and lactation. The endocrine system helps regulate and maintain various body functions by synthesizing (making) and releasing hormones, chemical messengers.
