The<unk>  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 vertebrates, the hypothalamus is the neural control center for all<unk> s. In humans, the major<unk>  glands are the thyroid gland and the adrenal glands. The study of the<unk>  and its disorders is known as<unk> logy. Glands that signal each other in sequence are often referred to as an axis, such as the hypothalamic-pituitary-adrenal axis. In addition to the specialized<unk>  organs mentioned above, many other organs that are part of other body systems have secondary<unk>  functions, including bone, kidneys, liver, heart and gonads. For example, the kidney secretes the<unk>  hormone erythropoietin. Hormones can be amino acid complexes, steroids, eicosanoids, leukotrienes, or prostaglandins. The<unk>  can be contrasted to both exocrine glands, which secrete hormones to the outside of the body, and paracrine signalling between cells over a relatively short distance.<unk>  glands have no ducts, are vascular, and commonly have intracellular vacuoles or granules that store their hormones. In contrast, exocrine glands, such as salivary glands, sweat glands, and glands within the gastrointestinal tract, tend to be much less vascular and have ducts or a hollow lumen.<unk> logy is a branch of internal medicine.
Endocrine system
Reproductive system
Autonomic nervous system
Somatosensory system
Integumentary system
Immune system
Immune cycle
Female reproductive system