Endocrine system

Major endocrine glands. (Male left, female on the right.) 1. Pineal gland 2. Pituitary gland 3. Thyroid gland 4. Thymus 5. Adrenal gland 6. Pancreas 7. Ovary 8. Testis

The endocrine system is a control system of ductless glands and single cells that secretes chemical messengers called hormones. The endocrine system consists of three main components: endocrine glands, hormones, and target cells. It is found in vertebrae animals.

Hormones regulate the body's growth, development, metabolism, function, sexual development, and reproduction. They are secreted directly into the blood. As the travel through the blood, they are transported to various tissues and organs, which are collectively called target cells. Because hormones function by binding to receptors, only cells having the appropriate receptor for a hormone can respond to the message being carried by that hormone. Hormones can execute their effect at minimum concentrations and their action is terminated through degradation in the blood. The rate of this degradation refers to a hormone's half life.

The endocrine system does not include exocrine glands such as the salivary glands, sweat glands, and glands within the gastrointestinal tract. Exocrine glands release their secretions through ducts into the external environment (Silverthorn 2004).

Physiology

Hormones released into the blood act upon their target cells by binding to receptors and initiating biochemical responses, known as the cellular mechanism of action of the hormone (Silverthorn 2004). These reponses can vary from tissue to tissue and not all cells may respond to a specific hormone. At times, however, one hormone can act on multiple tissues at once.

Once a hormone has exerted its effect, its action must be terminated. This is accomplished by enzymes, which degradate, or breakdown, the hormone into metabolites.

Signal transduction of some hormones with steroid structure involves nuclear hormone receptor proteins that are a class of ligand activated proteins that, when bound to specific sequences of DNA serve as on-off switches for transcription within the cell nucleus. These switches control the development and differentiation of skin, bone and behavioral centers in the brain, as well as the continual regulation of reproductive tissues.

The endocrine system regulates its hormones through negative feedback. Increases in hormone activity decrease the production of that hormone. The immune system and other factors contribute as control factors also, altogether maintaining constant levels of hormones.

The endocrine system works in close relation with the nervous system. It links the brain to the organs that control various aspects of the body, as mentioned above. In addition, neurohormones are released by specialized groups of neurons in the brain. These function similarly to hormones and are often categorized into three major group: catecholamines, hypothalamic neurohormones that monitor hormone release from the anterior pituitary, and hypothalamic neurohormones that monitor hormone release from the posterior pituitary.

Endocrine glands and the hormones secreted

In both sexes

• Hypothalamus
  • Thyrotropin-releasing hormone (TRH)
  • Gonadotropin-releasing hormone (GnRH)
  • Growth hormone-releasing hormone (GHRH)
  • Corticotropin-releasing hormone (CRH)
  • Somatostatin (SS; also GHIH, growth factor-inhibiting hormone)
  • Dopamine (DA)

• Pituitary gland
  • Anterior pituitary (adenohypophysis)
    • Growth hormone (GH)
    • Prolactin (PRL)
    • Adrenocorticotropic hormone (ACTH, corticotropin)
    • Thyroid-stimulating hormone (TSH, thyrotropin)
    • Follicle-stimulating hormone (FSH, a gonadotropin)
    • Luteinizing hormone (LH, a gonadotropin)
  • Posterior pituitary (neurohypophysis)
    • Oxytocin (ocytocin)
    • Arginine vasopressin (AVP; also ADH, antidiuretic hormone)

• Pineal gland
  • Melatonin

• Thyroid gland
  • Triiodothyronine (T3), the potent form of thyroid hormone
  • Thyroxine (T4), a less active form of thyroid hormone
  • Calcitonin

• Parathyroid gland
  • Parathyroid hormone (PTH)

• Heart
  • Atrial-natriuretic peptide (ANP)

• Stomach and intestines
  • Cholecystokinin (CCK)
  • Gastrin
  • Ghrelin
  • Neuropeptide Y (NPY)
  • Secretin
  • Somatostatin

• Liver
  • Insulin-like growth factor (IGF)
  • Angiotensinogen
  • Thrombopoietin

• Islets of Langerhans in the pancreas
  • Insulin
  • Glucagon
  • Somatostatin

• Adrenal glands
  • Adrenal cortex
    • Glucocorticoids (chiefly cortisol)
    • Mineralocorticoids (chiefly aldosterone)
    • Androgens (including DHEA and testosterone)
  • Adrenal medulla
    • Adrenaline (epinephrine)
    • Noradrenaline (norepinephrine)

• Kidney
  • Renin
  • Erythropoietin (EPO)
  • Calcitriol (the active form of vitamin D₃)

• Skin
  • Vitamin D₃ (calciferol)

• Adipose tissue
  • Leptin
  • Estrogens (mainly estrone)

In males only

• Testes
  • Androgens (chiefly testosterone)

In females only

• Ovarian follicle
  • Estrogens (mainly estradiol)
  • Testosterone

• Corpus luteum
  • Progesterone
  • Estrogens (mainly estradiol)

• Placenta (when pregnant)
  • Progesterone
  • Estrogens (mainly estriol)
  • Human chorionic gonadotrophin (HCG)
  • Human placental lactogen (HPL)

Role in disease

The field of medicine that deals with disorders of endocrine glands is endocrinology, a branch of the larger field of internal medicine. Diseases of the endocrine system are common and include diabetes mellitus and thyroid disease.

Endocrinopathies can occur with any of these. Hypofunction can occur as result of loss of reserve, hyposecretion, agenesis, atrophy, destruction, etc. Hyperfunction can occur as result of hypersecretion, loss of suppression, tumor, hyperplasia, etc.

Endocrinopathies are classified as primary, secondary, or tertiary.

Primary is target organ dysfunction and is normally associated with increased or decreased secretory hormones. Secondary is a dysfunction that originates elsewhere like the pituitary gland and is normally associated with increased or decreased production of trophic factors. Tertiary is associated with dysfunction of the hypothalamus and its releasing hormones.

Diffuse endocrine system

Organs aren't the sole way for hormones to be sent into the body; there are a host of specific cells which secrete hormones independently. These are called the "diffuse" endocrine system, and include myocytes in the heart (atria) and epithelial cells in the stomach and small intestines. In fact, if one were to classify any chemical excretions in the term "hormone," every cell in the human body could be considered a part of the endocrine system.

See also

• Hormones
• Releasing hormones
• Endocrinology
• Neuroendocrinology
• Nervous system
• Endocrine disruptor

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