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. Testes

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. The system is found in vertebrae animals.

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).

Hormones

Overview

Hormones regulate the body's growth, development, metabolism, function, sexual development, and reproduction. They are often divided into three main groups according to their chemical makeup: peptide/ protein hormones containing three or more amino acids, steroid hormones derived from cholesterol, and amine hormones derived from one amino acid (Silverthorn 2004). Hormones are secreted directly into the blood by endocrine glands, which are mostly of mesodermal or entodermal origin (Wyngaarden 1982). As hormones 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.

Hormone Characteristics

The endocrine system depends heavily upon the action of hormones, which are released according to one or more of the following types of influences:

• Spontaneous release at a constant rate or under circadian rhythm
• Various physiological or pathological stimuli
• "Sensor" mechanisms that monitor hormone levels and their suitability to the body's needs (Wyngaarden 1982)

Once hormones are released into the blood, they 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. A hormone can also stimulate or inhibit the release of other hormones, in which case it is called a tropic hormone. The anterior pituitary and hypothalamus release several such hormones.

It is not uncommon for more than one hormone to be released at the same time. In such cases, different types of hormone interactions can occur. If the hormones yield a result that is more than additive, a synergistic interaction is said to have taken place. Conversely, if one hormone counteracts the action of another hormone, they are said to be antagonistic to one another. A permissive interaction can occur if one hormone is needed for a second hormone to fully exert its effects. In this situation, the first hormone is said to be permissive to the second one.

Hormones are often classified into three major groups: peptide/ protein hormones, steroid hormones, and amine hormones. Each of these groups of hormones share several similarities as well as several differences. Peptide hormones are made and stored in vesicles within cells until they receive a signal for secretion. Initially, peptide hormones are made as preprohormones, which are inactive, and then are converted into prohormones. These prohormones are then cut into active hormones and peptide pieces, which are all secreted together. Because of their hydrophilic (water loving) nature, peptide hormones travel freely in the blood as they dissolve. They experience short half lives and tend to bind to surface cell receptors to initiate quick cellular responses. Peptide hormones can cause the synthesis of new proteins.

Steroid hormones, on the other hand, are made on demand. Because they are derived from cholesterol they are hydrophobic (water fearing) and tend to travel in the blood with protein carriers. Consequently, they have a longer half life. Receptors for steroid hormones are traditionally found inside the target cell. Responses include the turning on and off of genes and the direct synthesis of fresh proteins. Overall, the cell responses with steroid hormones are slower than those with peptide hormones.

Amine hormones are those which are derived from a single amino acid. They can behave like a peptide hormone or as a combination of a peptide hormone and steroid hormone.

After a hormone has been secreted and has exerted its effect, its action must be terminated. This is accomplished by enzymes, which degradate, or breakdown, the hormone into metabolites. The metabolites are excreted along with bile and/or urine. Enzymes may be present in the blood or within the cell itself. Endocytosis of the receptor- hormone complex can also terminate hormone action. The rate at which a hormone is broken down in the bloodstream is called the half life of the hormone. It is the amount of time needed to reduce the hormone concentration by one- half. This rate indicates the period of time a hormone is active in the body.

The endocrine system regulates hormone release and concentration through the negative feedback loop. Increases in hormone activity decrease the production and secretion of that hormone. Similarly, a decrease in activity of a hormone prompts an increase in the production and release of that hormone. The immune system as well as other factors contribute as control factors of hormone secretion. Together these various mechanisms of control maintain constant levels of hormones within the body.

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. 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 groups: catecholamines, hypothalamic neurohormones that monitor hormone release from the anterior pituitary, and hypothalamic neurohormones that monitor hormone release from the posterior pituitary. Neuroendocrinology is an area of medicine that focuses on the overlapping fields between the nervous and endocrine systems.

Endocrine Glands and Hormones of the Human Body

• Pineal gland (gland)
• Hypothalamus (groups of neurons)
• Anterior Pituitary (gland)
• Posterior Pituitary (extension of hypothalamic neurons)
• Thyroid (gland)
• Parathyroid (gland)
• Thymus (gland)
• Heart (cells)
• Liver (cells)
• Stomach and Small Intestines (cells0
• Pancreas (gland)
• Adrenal Cortex (gland)
• Adrenal Medulla (gland)
• Kidney (cells)
• Skin (cells)
• Testes (male) (glands)
• Ovaries (female) (glands)
• Adipose tissue (fat) (cells)
• Placenta (pregnant females only) (gland)

Endocrine Glands and 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 (also called the adenohypophysis)
    • Growth hormone (GH)
    • Prolactin (PRL)
    • Adrenocorticotropic hormone (ACTH, a corticotropin)
    • Thyroid-stimulating hormone (TSH, a thyrotropin)
    • Follicle-stimulating hormone (FSH, a gonadotropin)
    • Luteinizing hormone (LH, a gonadotropin)
  • Posterior pituitary (also called the 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 the endocrine glands is endocrinology, a branch of the larger field of internal medicine. Diseases of the endocrine system are common and can result when the sensitivity of target cells to hormones has varied. Other causes of endocrinopathies include hyper or hypo secretion of hormone, ectopic production of hormone, or iatrogenic factors (physician induced). Some common pathologies include diabetes mellitus and thyroid disease.

Endocrinopathies are classified as primary, secondary, or tertiary. Primary is target cell dysfunction and is normally associated with increased or decreased secretory hormones. Secondary refers to a dysfunction that originates elsewhere, like the pituitary gland, and is normally associated with increased or decreased production of trophic hormones. Tertiary is associated with dysfunction of the hypothalamus and its releasing hormones.

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.

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.

References

ISBN links support NWE through referral fees

Silverthorn, D. 2004. Human Physiology, An Integrated Approach (3rd Edition). San Francisco: Benjamin Cummings. ISBN 013102153

Wyngaarden, J. B., and L. H. Smith. 1982. Cecil Textbook of Medicine (16th Edition). Philadelphia: W.B. Saunders Company. ISBN 072169621X

See also

• Hormones
• Nervous system

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