Sources: Dr. Farabee
What Is the Endocrine System?
Endocrine glands secrete hormones (chemical messengers) into the blood. Hormones act on target cells, which must have appropriate receptors to bind the hormone and bring about a physiological response. The endocrine system is composed of endocrine glands located throughout the body, and generally regulates activities that require duration, rather than speed.
What Is A Hormone?
Hormones are long-range chemical messengers secreted into the bloodstream by endocrine (ductless) glands in response to an appropriate signal, and 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 are classified into three categories based on their structure:
For a more in-depth analysis of what is a hormone, click here.
Several important hormones are insulin, growth hormone, antidiuretic hormone, prolactin, gonadotropins and endorphins.
Hormone-Secreting Glands
The primary hormone-secreting glands are:
The other hormone-secreting glands are:
For the entire chapter on hormone-secreting glands, click here.
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.
Potential Defects of the Endocrine System
Hormones generally produce their effects by altering intracellular protein activity.
Hormones bind with specific target cell receptors; starting a chain of events in the target cell which produce effects characteristic of that hormone. Based on the location of their receptors, hormones can be classified into 2 groups:
Water-soluble (hydrophillic) hormones dissolve into the cytoplasm; lipophilic hormones are bound to membrane-bound proteins.
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:
Cytoplasmic hormone receptor
Types of cell-surface receptors
Ligand-gated ion channels – eg. acetylcholine receptor
G-protein-linked receptors – 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 receptors – eg. 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). They are secreted into body fluids and reach many cells, but only target cells respond. The endocrine system is similar, using chemicals to communicate. These are known as hormones. A hormone is a specific messenger molecle synthesized and secreted by a group of espcialized cells called an endocrine gland. These are ducltless, meaning their secretions (hormones) are released directly into the bloodstram and travel elsewhere in the body to target organs upon which they act.Note that this is in contrast to our digestive glands, which have ducts for ereleasing digestive enzymes.The nervous system coordinates rapid and precise responses to stimuli using action potentials. The endocrine system maintains homeostasis and long-term control using chemical signals. The endocrine system works in parallel with the nervous system to control growth and maturation along with homeostasis.
Hypothalamus: Gonadotropin-releasing hormone (GRH)
Ant. Pituitary: Luteinizing hormone & follicle-stimulating hormone
Testes: Testosterone Ovaries: Estradiol and Progesterone
Leutinizing hormone (LH) causes the bursting of the follicle in a woman’s ovaries, and also facilitates the formation of a corpeus luteum from the remains of the follicle.
Polypeptides are short chains of amino acids; most hormones are peptides. They are secreted by the pituitary, parathyroid, heart, stomach, liver, and kidneys.
Peptide hormones are synthesized as precursor molecules and processed by the endoplasmic reticulum and Golgi where they are stored in secretory granules. When needed, the granules are dumped into the bloodstream. Different hormones can often be made from the same precursor molecule by cleaving it with a different enzyme.
Pituitary Hormones
Hypothalamic Hormones
Thyroid Hormones
Digestive Hormones
Pancreatic Hormones
Steroids are lipids derived from cholesterol. Testosterone is the male sex hormone. Estradiol, similar in structure to testosterone, is responsible for many female sex characteristics. Steroid hormones are secreted by the gonads, adrenal cortex, and placenta.
Steroid hormones are derived from cholesterol by a biochemical reaction series. Defects along this series often lead to hormonal imbalances with serious consequences. Once synthesized, steroid hormones pass into the bloodstream; they are not stored by cells, and the rate of synthesis controls them.
Amines are derived from the amino acid tyrosine and are secreted from the thyroid and the adrenal medulla. Solubility of the various hormone classes varies.
Amine hormones (notably epinephrine) are stored as granules in the cytoplasm until needed.
Melatonin promotes sleep and decreases activity of the gonads. In addition, melatonin affects thyroid and adrenal cortex functions and (in some animals) skin pigmentation. Because melatonin production is affected by the amount of light to which a person is exposed, this is tied to circadian rhythm (having an activity cycle of about 24 hours), annual cycles, and biological clock functions. SAD or seasonal affective disorder (syndrome) is a disorder in which too much melatonin is produced, especially during the long nights of winter, causing profound depression, oversleeping, weight gain, tiredness, and sadness. Treatment consists of exposure to bright lights for several hours each day to inhibit melatonin production. It has also been found that melatonin levels drop 75% suddenly just before puberty, suggesting the involvement of melatonin in the regulation of the onset of puberty. Studies have been done on blind girls (with a form of blindness in which no impulses can travel down the optic nerve and reach the brain and pineal gland), which showed that these girls tended to have higher levels of melatonin for a longer time, resulting in a delay in the onset of puberty. While some older people, who don’t make very much melatonin, thus don’t sleep well, might benefit from a melatonin supplement, I’m skeptical of the recent melatonin craze in this country. When so many people apparently are suffering from SAD, I question the wisdom of purposly ingesting more melatonin, especially since the pineal gland is one of the least-studied, least-understood of the endocrine glands.
Insulin is a peptide hormone secreted by the pancreas and results in the lowering of blood sugar levels. It acts on liver, fat, muscle and other tissues to stimulate uptake of glucose. Uptaken glucose is either metabolised or converted to glycogen and fat. After a meal, when blood glucose levels rise, insulin is released so that cells can uptake the elevated glucose levels.
Glucagon is a peptide hormone that acts upon the liver to stimulate breakdown of glycogen and raise blood sugar levels.
Relevant Articles
Pheromones are also communication chemicals used to send signals to other emmebers of same species. Queen bees, ants, and naked mole rats exert control of their respective colonies via pheromones. THey are commonly used to attract mates, for….Pheromones are widely studied in insects and are the basis for some kinds of Japanese beetle and gypsy moth traps. While pheromones have not been so widely studied in humans, some interesting studies have been done in recent years on pheromonal control of menstrual cycles in women. It has been found that pheromones in male sweat and/or sweat from another “dominant†female will both influence/regulate the cycles of women when smeared on their upper lip, just below the nose. Also, there is evidence that continued reception of a given man’s pheromone(s) by a woman in the weeks just after ovulation/fertilization can significantly increase the chances of successful implantation of the new baby in her uterus. Pheromones are also used for things like territorial markers (urine) and alarm signals.
Growth hormone (GH) is a peptide anterior pituitary hormone essential for growth. GH-releasing hormone stimulates release of GH. GH-inhibiting hormone suppresses the release of GH. The hypothalamus maintains homeostatic levels of GH. Cells under the action of GH increase in size (hypertrophy) and number (hyperplasia). GH also causes increase in bone length and thickness by deposition of cartilage at the ends of bones. During adolescence, sex hormones cause replacement of cartilage by bone, halting further bone growth even though GH is still present. Too little or two much GH can cause dwarfism or gigantism, respectively.
One non-sex hormone secreted by the posterior pituitary is antidiuretic hormone or ADH. This hormone helps prevent excess water excretion by the kidneys. Ethanol inhibits the release of ADH and can, thus, cause excessive water loss. That’s also part of the reason why a group of college students who go out for pizza and a pitcher of beer need to make frequent trips to the restrooms. Diuretics are chemicals which interfere with the production of or action of ADH so the kidneys secrete more water. Thus diuretics are often prescribed for people with high blood pressure, in an attempt to decrease blood volume.
Another group of non-sex hormones that many people have heard of is the endorphins, which belong to the category of chemicals known as opiates and serve to deaden our pain receptors. Endorphins, which are chemically related to morphine, are produced in response to pain. The natural response to rub an injured area, such as a pinched finger, helps to release endorphins in that area. People who exercise a lot and push their bodies “until it hurts†thereby stimulate the production of endorphins. It is thought that some people who constantly over-exercise and push themselves too much may actually be addicted to their own endorphins which that severe exercise regime releases.
the thyroid gland.
Gonadotropins and secreted by the anterior pituitary. Gonadotropins influence the gonads. Gonadotropins (which include follicle-stimulating hormone, FSH, and luteinizing hormone, LH) affect the gonads by stimulating gamete formation and production of sex hormones.
Prolactin are also secreted by the anterior pituitary. Prolactin is secreted near the end of pregnancy and prepares the breasts for milk production.
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