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By Levi Clancy for Student Reader on

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The endocrine system is composed of endocrine glands located throughout the body. These ductless glands secrete hormones (chemical messengers) into the bloodstream, in response to an appropriate signal.

The endocrine system acts by releasing hormones that in turn trigger actions in specific target cells. Hormones are long-range chemical messengers. They are carried in the blood to other sites in the body where they exert their effects on target cells some distance from their site of release. Hormones generally produce their effects by altering intracellular protein activity. Hormones act on target cells, which must have appropriate receptors to bind the hormone and bring about a physiological response. Hormones generally regulates activities that require duration, rather than speed.

Receptors on target cell membranes bind only to one type of hormone. More than fifty human hormones have been identified; all act by binding to receptor molecules. The binding hormone changes the shape of the receptor causing the response to the hormone.

There are two mechanisms of hormone action on all target cells.
Nonsteroid HormonesThese include hydrophilic peptides and catecholamines, which are poorly soluble in lipid and thus unable to cross the lipid bilayer. Thus their receptors on the cell surface. Nonsteroid hormones (water soluble) do not enter the cell but bind to plasma membrane receptors, generating a chemical signal (second messenger) inside the target cell. Five different second messenger chemicals, including cyclic AMP have been identified. Second messengers activate other intracellular chemicals to produce the target cell response.
Steroid HormonesThe second mechanism involves steroid (and thyroid) hormones, which are lipophilic and cross the lipid bilayer to bind to specific intracellular receptors. After passing through the plasma membrane they act in a two step process. Steroid hormones bind, once inside the cell, to the nuclear membrane receptors, producing an activated hormone-receptor complex. The activated hormone-receptor complex binds to DNA and activates specific genes, increasing production of proteins.
Hormones can also be classified into three categories based on their structure:
PeptidesComprise most hormones, including those secreted by the hypothalamus, anterior and posterior pituitary, pancreas and parathyroid.
AminesDerived from the amino acid, tyrosine, and include the hormones secreted by the thyroid gland, and adrenal medulla. Adrenomedullary hormones are called catecholamines.
SteroidsNeutral lipids derived from cholesterol; include hormones secreted by the adrenal cortex, ovaries and testes. Steroid and thyroid hormones are lipid soluble (lipophilic).

Several important hormones are insulin, growth hormone, antidiuretic hormone, prolactin, gonadotropins and endorphins.


Hormone responses can differ based on the target cell. A hormone acting on its target cell receptor, produces a characteristic response in the target cell, which is different for different hormones, and differs between different target cells responding to the same hormone; eg. an adrenal medullary catecholamine, epinephrine, can produce the following effects: contraction of vascular smooth muscle; relaxation of respiratory airway smooth muscle; breakdown of liver glycogen.

Hormones ultimately affect their target cells by altering activity of proteins within the cell, generally by one of the following mechanisms:

  1. most hydrophilic hormones binding to cell surface receptors, produce a second messenger molecule within the target cell (hormone is the “first” messenger)

  2. a few hydrophilic hormones following binding to cell surface receptors, alter cell permeability by opening or closing particular ion channels

  3. lipophilic hormones through binding to intracellular receptors, activate specific genes (through transcriptional regulation within the nucleus), leading to the formation of new intracellular proteins which produce a characteristic physiological effect

Hormone receptors

Types of cell-surface receptors
Ligand-gated ion channelsEg., acetylcholine receptor
G-protein-linked receptorsEg., guanyl nucleotide binding proteins (G proteins) act as molecular switches; active when GTP is bound, inactive with GDP due to action of intrinsic GTPase.
Enzyme-linked receptorsEg., insulin receptor
Each hormone has a specific shape that binds to receptors on target cells. These binding sites are called hormone receptors.

Many hormones come in antagonistic pairs, where each has opposite effects on the target organs. Hormonal regulation relies heavily on feedback loops to maintain balance and homeostasis.

Most animals with well-developed nervous and circulatory systems have an endocrine system.

The endocrine systems of crustaceans, arthropods, and vertebrates are very similar due to convergent evolution. The vertebrate endocrine system consists of glands (pituitary, thyroid, adrenal), and diffuse cell groups scattered in epithelial tissues.

More than fifty different hormones are secreted.

Endocrine glands arise during development for all three embryologic tissue layers (endoderm, mesoderm, ectoderm).

The type of endocrine product is determined by which tissue layer a gland originated in. Glands of ectodermal and endodermal origin produce peptide and amine hormones; mesodermal-origin glands secrete hormones based on lipids.

There are two classes of hormones: steroids (derived from cholesterol) and peptides (derived from amino acids).

A hormone is a specific messenger molecle synthesized and secreted by a group of specialized cells called an endocrine gland. Hormones are secreted into the bloodstream and reach many cells, but only target cells respond.

Endocrine glands are ductless, meaning their secretions (hormones) are released directly into the bloodstram and travel elsewhere in the body to target organs upon which they act.

In contrast to other body systems, the endocrine system maintains homeostasis and long-term control using chemical signals.

Our digestive glands have ducts for releasing digestive enzymes. The nervous system coordinates rapid and precise responses to stimuli using action potentials. However,the endocrine system works in parallel with the nervous system to control growth and maturation along with homeostasis.

Hormone-secreting glands

The primary hormone-secreting glands are:
  • Pituitary Gland (anterior pituatiary secrets GH which i a protein stimulating protein synthesis and growth) (depression at bottom of skull just over back of roof of mouth)

  • Hypothalamus

The other hormone-secreting glands are:
  • Sex Organs (ovaries: endrogens & progesterones; testes: androgens) (gonads)

  • Adrenal Glands (adrenal medulla secrets epinephrine, norepinephrine, modified amino acids stimualte flight-or-flight...increase heart, redistriubute blood to muscles, raise blood surgar.) (Adrenal cortex secrets glucocorticoids (cortisol) sterioids targeting muscles, immune system, and other tissues to mediate response to stress, reduce metabolism of glucose, increase metabolism of proeins and fats, reduce inflammation and immune responses.) (adrenal gland sits above each kindey.)

  • Thyroid Gland (secretes calcitonin, which is a peptide acting on bones to stimulate bone formaiton and lower blood calcium.) neck (parathyroid raises blood calcium)

  • Pancreas (insulin)

  • Pineal Gland (secretes melatonin a modified amino acid to hypothalamaus for slep et al)

Other chemical messengers

Interferons are proteins released when a cell has been attacked by a virus. They cause neighboring cells to produce antiviral proteins. Once activated, these proteins destroy the virus.

Prostaglandins are fatty acids that behave in many ways like hormones. They are produced by most cells in the body and act on neighboring cells.

Pheromones are chemical signals that travel between organisms, rather than between cells within an organism. In the animal world, pheromones are heavily used to mark territory, signal prospective mates and to communicate. The presence of a pheromone as a human sex attractant has not been established conclusively.

Endocrine diseases

  • Overproduction of a hormone

  • Underproduction of a hormone

  • Nonfunctional receptors that cause target cells to become insensitive to hormones