Endocrine system

The endocrine system is a network of glands that release hormones directly into the bloodstream, acting as a chemical messenger system. These hormones travel through the circulatory system to regulate the function of distant organs. In vertebrates, the hypothalamus in the brain serves as the main neural control center for the entire system. The medical field that studies this system and its disorders is called endocrinology, which is a specialty within internal medicine.

The major endocrine glands in humans include the thyroid, parathyroid, pituitary, pineal, and adrenal glands, along with the testes in males and ovaries in females. The hypothalamus, pancreas, and thymus also function as endocrine glands, with the hypothalamus and pituitary forming the neuroendocrine system. The hypothalamus, located in the brain next to the pituitary, links the endocrine system to the nervous system. Other organs, such as the kidneys, also have endocrine roles by secreting specific hormones. For example, the thyroid secretes thyroxine, the pituitary releases growth hormone, the pineal gland produces melatonin, the testes secrete testosterone, and the ovaries release estrogen and progesterone.

Glands that signal one another in a sequence are often called an axis, like the hypothalamic–pituitary–adrenal axis. Many organs that are part of other body systems also have secondary endocrine functions, including bone, kidneys, liver, heart, and gonads. The kidney, for instance, secretes the hormone erythropoietin. Hormones can be amino acid complexes, steroids, eicosanoids, leukotrienes, or prostaglandins.

The endocrine system is different from exocrine glands, which secrete hormones to the outside of the body, and from paracrine signaling, which occurs between nearby cells. Endocrine glands have no ducts, are well supplied with blood vessels, and often store hormones in intracellular vacuoles or granules. In contrast, exocrine glands, such as salivary glands, mammary glands, and submucosal glands in the gastrointestinal tract, are less vascular and have ducts or a hollow lumen.

The human endocrine system operates through several feedback loops, many of which involve the hypothalamus and pituitary. Key feedback systems include the TRH–TSH–T3/T4 axis, the GnRH–LH/FSH–sex hormone axis, the CRH–ACTH–cortisol axis, the renin–angiotensin–aldosterone system, and the leptin–ghrelin balance.

Endocrine glands release their products directly into interstitial spaces, where they are absorbed into the blood. The major glands include the pineal, pituitary, pancreas, ovaries, testes, thyroid, parathyroid, hypothalamus, and adrenal glands. The hypothalamus and pituitary are neuroendocrine organs. The hypothalamus is a key regulator of the autonomic nervous system and has three sets of endocrine outputs: the magnocellular system, the parvocellular system, and autonomic intervention. The magnocellular system is involved in releasing oxytocin or vasopressin. The anterior pituitary produces and secretes tropic hormones such as TSH, ACTH, GH, LH, and FSH, controlled by parvocellular neurosecretory cells.

Specialized endocrine cells make up the larger tissues and glands of the system, releasing over fifty different hormones in animals. The pituitary gland has two parts: the anterior and posterior. The posterior pituitary does not produce hormones but stores and releases antidiuretic hormone (ADH), synthesized by the supraoptic nucleus of the hypothalamus, and oxytocin, synthesized by the paraventricular nucleus. ADH helps the body retain water, maintaining a balance between blood solutions and water, while oxytocin induces uterine contractions, stimulates lactation, and allows for ejaculation. The pineal gland produces melatonin. The follicular cells of the thyroid produce T3 and T4 in response to elevated TRH from the hypothalamus and TSH from the anterior pituitary, regulating metabolic activity, cell growth, and tissue differentiation. The parathyroid glands produce parathyroid hormone in response to low calcium, acting on bone, kidneys, and the GI tract to increase calcium reabsorption and phosphate excretion, and stimulating the conversion of vitamin D to its active form. The pancreas contains about 1 to 2 million pancreatic islets, which are its endocrine cells that secrete hormones, along with acini that secrete digestive enzymes.