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.

Hormones In Action

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

The body contains several endocrine glands and cells that secrete hormones that carry out certain functions.

Pineal gland

The pineal gland is located deep in the brain. It secretes melatonin, which is known as the "darkness hormone" because it is secreted at night while we sleep. The body's circadian rhythm, or light-dark cycles, are dependent on the levels of melatonin in the blood. The hormone is classified as an amine hormone derived from tryptophan (amino acid). The specific target of melatonin is unclear.

Hypothalamus

The hypothalamus, a cluster of neurons found in the brain, controls the body's homeostasis and behavorial drives, such as thirst, temperature, food intake, and osmolarity. The hypothalamus secretes trophic hormones which control the release of other hormones in the pituitary gland, specifically in the anterior pituitary. The trophic hormones released by the hypothalamus include:

• Prolactin- releasing hormone (PRH)
• Prolactin- inhibiting hormone (PIH; dopamine)
• Thyrotropin- releasing hormone (TRH)
• Corticotropin- releasing hormone (CRH)
• Growth hormone- inhibiting hormone (GHIH; somatostatin)
• Growth hormone- releasing hormone (GHRH)
• Gonadotropin- releasing hormone (GnRH)

Once released, these hormones, which are present in both males and females, travel to the anterior pituitary through the hypothalamic- hypophyseal portal system and either stimulate or inhibit the release of the hormones of the anterior pituitary gland.

Anterior Pituitary

The anterior pituitary is located in the brain and is part of the larger pituitary gland. It secretes six peptide hormones (in both sexes), all of which are controlled by the trophic hormones of the hypothalamus.

• Prolactin- controls milk production in the breast
• Thyroid- stimulating hormone (TSH; thyrotropin)- acts on the thyroid gland as a trophic hormone
• Adrenocorticotrophid hormone (ACTH; corticotropin)- targets the adrenal cortex and affects the release of cortisol
• Growth hormone (GH; somatotrophin)- targets the liver consequently affecting the release of insulin-like growth factors (IGFs); also targets several other tissues
• Follicle- stimulating hormone (FSH)- acts upon the endocrine cells of the gonads in both males and females; also affects germ cells of the gonads
• Luteinizing hormone (LH)- acts upon the endocrine cells of the gonads in both males and females; also affects germ cells of the gonads

Posterior Pituitary

The posterior pituitary is an extension of hypothalamic neurons and is part of the larger pituitary gland. It does not make any hormones. Rather, it stores two peptide hormones, which are made in the hypothalamus and kept for storage in the posterior pituitary. Both hormones consist of nine amino acids each (Silverthorn 2004). They are released when an electrical signal from the hypothalamus passes to the posterior pituitary causing the storage vesicles to be let into the blood circulation.

• Arginine vasopressin (AVP; also called antidiuretic hormone, ADH)- targets kidney to control water balance in body
• Oxytocin (OT)- In females, acts on breast and uterus to affect milk ejection, smooth muscle contractions during labor and delivery, may play a part in controlling maternal behavior; in males, secreted from the testes and possilby other reproductive tissues, pulses of oxytocin releae can be detected during ejaculation, may play a part in sperm transport within the male reproductive system (Bowen 1998).

Thyroid gland

The thyroid gland, located in the neck, produces the thryoid hormones, which are amines consisting of tyrosine (an amino acid) and iodine. It also produces calcitonin, which is a peptide hormone. The thyroid hormones are not necessary for survival in adults; however, they play a crucial role for the growth and development of children. All three hormones are present in both sexes.

• Calcitonin (CT)- targets bone and decreases plasma calcium levels by increasing bone formation; plays larger role in lower animals
• Thyroid hormones
  • Triiodothyronine (T3)- potent form of thyroid hormone, affects metabolism, growth and development
  • Thyroxine (T4)- a less active form of thyroid hormone which is converted into T3

Parathyroid gland

The parathyroid gland consists of four smaller glands located on the dorsal side of the thyroid gland. It secretes one peptide hormone that plays an active role in humans, both male and female. The parathyroid gland is essential for life.

• Parathyroid hormone (PTH)- acts upon bone and kidneys to increase plasma calcium levels and increase phosphate excretion

Thymus gland

The thymus gland is the site for lymphocyte (white blood cell) production. It is an immune tissue located in the chest of both sexes. Two peptide hormones are produced and secreted there, both which target lymphocytes and aid in their development.

• Thymosin
• Thymopoietin

Heart

The heart, a major organ of the body and pump of the circulation system, releases one peptide hormone, atrial natriuretic peptide hormone. It's release from atrial myocytes, located in the atria, is stimulated by high blood pressure, atrial distention and stretching, sympathetic stimulation of β-adrenoceptors, raised sodium concentration, angiotensin-II, and endothelin (a potent vasodilator). The hormone is a 28 amino acid peptide with a 17 amino acid ring.

• Atrial Natriuretic peptide hormone (ANP; also called atriopeptin)- targets kidneys and increases sodium and water excretion in order to reduce blood pressure, reduces aldosterone production by the adrenal cortex, relaxes vascular smooth muscle in arterioles and venules

Liver

The liver is responsible for the release of the peptid hormone angiotensinogen and also for the secretion of the peptide hormones referred to as insulin- like growth factors. The former consists of 453 amino acid residues and is cleaved to form angiotensin, which is the active form of the hormone.

• Angiotensinogen/ Angiotensin- targets the adrenal cortex, brain, and blood vessels; main effects include aldosterone secretion, vasoconstriction, and increased blood pressure
• Insulin- like growth factors (IGFs)- act on many tissues and are involved in growth

• 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

• 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

Bowen, R. 1998. Oxytocin. Colorado State University. (See external link below.)

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

External Links

• Oxytocin- Colorado State University

See also

• Hormones
• Nervous system

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