The endocrine system is a system that regulates hormones.
The endocrine system is a messenger system that regulates distant target organs via feedback loops of hormones produced directly into the circulatory system by internal glands of an organism. In mammals, the hypothalamus serves as the cerebral control centre for all endocrine systems. Humans have two basic endocrine glands: the thyroid gland and the adrenal glands. The study of the endocrine system and related pathologies is known as endocrinology.
The hypothalamic-pituitary-adrenal axis is an example of an axis, which is a group of glands that communicate with one another in a precise order. Many other organs that are part of other body systems, such as bone, kidneys, liver, heart, and gonads, exhibit secondary endocrine activity in addition to the specialised endocrine organs mentioned above. The kidney, for example, secretes the endocrine hormone erythropoietin. Hormones include amino acid complexes, steroids, eicosanoids, leukotrienes, and prostaglandins.
Exocrine glands, which send hormones to the outside of the body, and paracrine signalling, which occurs between cells over a short distance, are examples of the endocrine system. Endocrine glands lack ducts, are vascular, and have intracellular vacuoles or granules where hormones are stored. Exocrine glands, such as salivary glands, sweat glands, and gastrointestinal glands, are less vascular and have ducts or a hollow lumen. Internal medicine includes the field of endocrinology.