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Human Physiology/The endocrine system
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Introduction To The Endocrine System
The endocrine system is a control system of ductless glands that secrete hormones within specific organs. Hormones act as "messengers," and are carried by the bloodstream to different cells in the body, which interpret these messages and act on them.
It seems like a far fetched notion or idea that a small chemical can enter the bloodstream and cause an action at a distant location in the body. Yet this occurs in our bodies everyday of our lives. The ability to maintain homeostasis and respond to stimuli is largely due to hormones secreted within the body. Without hormones, you could not grow, maintain a constant temperature, produce offspring, or perform the basic actions and functions that are essential for life.
The endocrine system provides an electrochemical connection from the hypothalamus of the brain to all the organs that control the body metabolism, growth and development, and reproduction.
There are two types of hormones secreted in the endocrine system: Steroidal and non-steroidal, (or protein based) hormones.
The endocrine system regulates its hormones through negative feedback, except in very specific cases like childbirth. 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.
Types of Glands
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
Exocrine Glands are those which release their cellular secretions through a duct which empties to the outside or into the lumen (empty internal space) of an organ. These include certain sweat glands, salivary and pancreatic glands, and mammary glands. They are not considered a part of the endocrine system.
Endocrine Glands are those glands which have no duct and release their secretions directly into the intercellular fluid or into the blood. The collection of endocrine glands makes up the endocrine system.
The main endocrine glands are the pituitary (anterior and posterior lobes), thyroid, parathyroid, adrenal (cortex and medulla), pancreas and gonads. The pituitary gland is attached to the hypothalamus of the lower forebrain. The thyroid gland consists of two lateral masses, connected by a cross bridge, that are attached to the trachea. They are slightly inferior to the larynx. The parathyroid glands are four masses of tissue, two embedded posterior in each lateral mass of the thyroid gland. One adrenal gland is located on top of each kidney. The cortex is the outer layer of the adrenal gland. The medulla is the inner core. The pancreas is along the lower curvature of the stomach, close to where it meets the first region of the small intestine, the duodenum. The gonads are found in the pelvic cavity.
Hormones and Types
A hormone is a type of chemical signal. They are a means of communication between cells.
The endocrine system produces hormones that are instrumental in maintaining homeostasis and regulating reproduction and development. A hormone is a chemical messenger produced by a cell that effects specific change in the cellular activity of other cells (target cells). Unlike exocrine glands (which produce substances such as saliva, milk, stomach acid and digestive enzymes), endocrine glands do not secrete substances into ducts (tubes). Instead, endocrine glands secrete their hormones directly into the surrounding extra cellular space. The hormones then diffuse into nearby capillaries and are transported throughout the body in the blood.
The endocrine and nervous systems often work toward the same goal. Both influence other cells with chemicals (hormones and neurotransmitters). However, they attain their goals differently. Neurotransmitters act immediately (within milliseconds) on adjacent muscle, gland, or other nervous cells, and their effect is short-lived. In contrast, hormones take longer to produce their intended effect (seconds to days), may affect any cell, nearby or distant, and produce effects that last as long as they remain in the blood, which could be up to several hours.
In the following table there are the major hormones, their target and their function once in the target cell.
- Steroids : Hormones that are lipids synthesized from cholesterol. Steroids are characterized by four interlocking carbohydrate rings.
- Eicosanoids : Are lipids synthesized from the fatty acid chains of phospholipids found in plasma membrane.
Hormones circulating in the blood diffuse into the interstitial fluids surrounding the cell. Cells with specific receptors for a hormone respond with an action that is appropriate for the cell. Because of the specificity of hormone and target cell, the effects produced by a single hormone may vary among different kinds of target cells.
Hormones activate target cells by one of two methods, depending upon the chemical nature of the hormone.
- Lipid-soluble hormones (steroid hormones and hormones of the thyroid gland) diffuse through the cell membranes of target cells. The lipid-soluble hormone then binds to a receptor protein that, in turn, activates a DNA segment that turns on specific genes. The proteins produced as result of the transcription of the genes and subsequent translation of mRNA act as enzymes that regulate specific physiological cell activity.
- Water-soluble hormones (polypeptide, protein, and most amino acid hormones) bind to a receptor protein on the plasma membrane of the cell. The receptor protein, in turn, stimulates the production of one of the following second messengers:
Cyclic AMP (cAMP) is produced when the receptor protein activates another membrane-bound protein called a G protein. The G protein activates adenylate cyclase, the enzyme that catalyzes the production of cAMP from ATP. Cyclic AMP then triggers an enzyme that generates specific cellular changes.
Inositol triphosphate (IP3) is produced from membrane phospholipids. IP3, in turn, triggers the release of CA2+ from the endoplasmic reticulum, which then activates enzymes that generate cellular changes.
Endocrine glands release hormones in response to one or more of the following stimuli:
- Hormones from other endocrine glands.
- Chemical characteristics of the blood (other than hormones).
- Neural stimulation.
Most hormone production is managed by a negative feedback system. The nervous system and certain endocrine tissues monitor various internal conditions of the body. If action is required to maintain homeostasis, hormones are released, either directly by an endocrine gland or indirectly through the action of the hypothalamus of the brain, which stimulates other endocrine glands to release hormones. The hormones activate target cells, which initiate physiological changes that adjust the body conditions. When normal conditions have been recovered, the corrective action - the production of hormones - is discontinued. Thus, in negative feedback, when the original (abnormal) condition has been repaired, or negated, corrective actions decrease or discontinue. For example, the amount of glucose in the blood controls the secretion of insulin and glucagons via negative feedback.
The production of some hormones is controlled by positive feedback. In such a system, hormones cause a condition to intensify, rather than decrease. As the condition intensifies, hormone production increases. Such positive feedback is uncommon, but does occur during childbirth, where hormone levels build with increasingly intense labor contractions. Also in lactation, hormone levels increase in response to nursing, which causes an increase in milk production. The hormone produced by the hypothalamus causing the milk let down and uterine contraction is oxytocin.
Endocrine Glands
Pituitary gland
The hypothalamus makes up the lower region of the diencephalons and lies just above the brain stem. The pituitary gland (hypophysis) is attached to the bottom of the hypothalamus by a slender stalk called the infundibulum. The pituitary gland consists of two major regions, the anterior pituitary gland (anterior lobe or adenohypophysis) and the posterior pituitary gland (posterior lobe or neurohypophysis). The hypothalamus also controls the glandular secretion
of the pituitary gland.
The hypothalamus oversees many internal body conditions. It receives nervous stimuli from receptors throughout the body and monitors chemical and physical characteristics of the blood, including temperature, blood pressure, and nutrient, hormone, and water content. When deviations from homeostasis occur or when certain developmental changes are required, the hypothalamus stimulates cellular activity in various parts of the body by directing the release of hormones from the anterior and posterior pituitary glands. The hypothalamus communicates directives to these glands by one of the following two pathways: The Pituitary gland is found in the inferior part of the brain and is connected by the Pituitary Stalk. It can be referred to as the master gland because it is the main place for everything that happens within the endocrine system. It is divided into two sections: the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis). The Anterior pituitary is involved in sending hormones that control all other hormones of the body.
Posterior pituitary
Communication between the hypothalamus and the posterior pituitary occurs through neurosecretory cells that span the short distance between the hypothalamus and the posterior pituitary. Hormones produced by the cell bodies of the neurosecretory cells are packaged in vesicles and transported through the axon and stored in the axon terminals that lie in the posterior pituitary. When the neurosecretory cells are stimulated, the action potential generated triggers the release of the stored hormones from the axon terminals to a capillary network within the posterior pituitary. Two hormones, oxytocin and antidiuretic hormone (ADH), are produced and released this way. Decreased ADH release or decreased renal sensitivity to ADH produces a condition known as diabetes insipidus. Diabetes insipidus is characterised by polyuria (excess urine production), hypernatremia (increased blood sodium content) and polydipsia (thirst).
The posterior lobe is composed of neural tissue [neural ectoderm] and is derived from hypothalamus. Its function is to store oxytocin and antidiuretic hormone. When the hypothalamic neurons fire these hormones are release into the capillaries of the posterior lobe.
The posterior pituitary is, in effect, a projection of the hypothalamus. It does not produce its own hormones, but only stores and releases the hormones oxytocin and antidiuretic hormone. ADH is also known as arginine vasopressin (AVP) or simply vasopressin.
Anterior pituitary
The anterior lobe is derived from oral ectoderm and is composed of glandular epithelium. Communication between the hypothalamus and the anterior pituitary occurs through hormones(releasing hormones and inhibiting hormones) produced by the hypothalamus and delivered to the anterior pituitary via a portal network of capillaries. The releasing and inhibiting hormones are produced by specialized neurons of the hypothalamus called neurosecretory cells. The hormones are released into a capillary network or primary plexus, and transported through veins or hypophyseal portal veins, to a second capillary network or secondary plexus that supplies the anterior pituitary. The hormones then diffuse from the secondary plexus into the anterior pituitary, where they initiate the production of specific hormones by the anterior pituitary. Many of the hormones produced by the anterior pituitary are tropic hormones or tropins, which are hormones that stimulate other endocrine glands to secrete their hormones.
The anterior pituitary lobe receives releasing hormones from the hypothalamus via a portal vein system known as the hypothalamic-hypophyseal portal system.
The anterior pituitary secretes:
- thyroid-stimulating hormone (TSH)
- adrenocorticotropic hormone (ACH)
- prolactin
- follicle-stimulating hormone (FSH)
return to normal, glucagon secretion discontinues through negative feedback.
Another example of antagonistic hormones occurs in the maintenance of Ca2+^ ion concentration in the blood. Parathyroid hormone (PTH) from the parathyroid glands increases Ca2+^ in the blood by increasing Ca2+^ absorption in the intestines and reabsorption in the kidneys and stimulating Ca2+^ release from bones. Calcitonin (CT) produces the opposite effect by inhibiting the breakdown of bone matrix and decreasing the release of calcium in the blood.
Thyroid gland
The Thyroid gland is one of the largest endocrine glands in the body. It is positioned on the neck just below the Larynx and has two lobes with one on either side of the trachea. It is involved in the production of the hormones T (triiodothyronine) and T4 (thyroxine). These hormones increase the metabolic activity of the body‘s cells. The thyroid also produces and releases the hormone calcitonin (thyrocalcitonin) which contributes to the regulation of blood calcium levels. Thyrocalcitonin or calcitonin decreases the concentration of calcium in the blood. Most of the calcium removed from the blood is stored in the bones.
The thyroid hormone consists of two components, thyroxine and iodine. This hormone increases the metabolism of most body cells. A deficiency of iodine in the diet leads to the enlargement of the thyroid gland, known as a simple goiter. Hypothyroidism during early development leads to cretinism. In adults, it produces myxedema, characterized by obesity and lethargy. Hyperthyroidism leads to a condition known as exophthalmic goiter, characterized by weight loss as well as hyperactive and irritable behavior.
The thyroid gland is a two-lobed gland that manifests a remarkably powerful active transport mechanism for up-taking iodide ions from the blood. As blood flows through the gland, iodide is converted to an active form of iodine. This iodine combines with an amino acid called tyrosine. Two molecules of iodinated tyrosine then combine to form thryroxine. Following its formation, the thyroxine becomes bound to a polysaccharide-protein material called thyroglobulin. The normal thyroid gland may store several weeks supply of thyroxine in this bound form. An enzymatic splitting of the thyroxine from the thyroglobulin occurs when a specific hormone is released into the blood. This hormone, produced by the pituitary gland, is known as thyroid-stimulating hormone (TSH). TSH stimulates certain major rate-limiting steps in thyroxine secretion, and thereby alters its rate of release. A variety of bodily defects, either dietary, hereditary, or disease induced, may decrease the amount of thyroxine released into the blood. The most popular of these defects is one that results from dietary iodine deficiency. The thyroid gland enlarges, in the continued presence of TSH from the pituitary, to form a goiter. This is a futile attempt to synthesize thyroid hormones, for iodine levels that are too low. Normally, thyroid hormones act via a negative feedback loop on the pituitary to decrease stimulation of the thyroid. In goiter, the feedback loop cannot be in operation - hence continual stimulation of the thyroid and the inevitable protuberance on the neck. Formerly, the principal source of iodine came from seafood. As a result, goiter was prevalent amongst inland areas far removed from the sea. Today, the incidence of goiter has been drastically reduced by adding iodine to table salt.
Thyroxine serves to stimulate oxidative metabolism in cells; it increases the oxygen consumption and heat production of most body tissues, a notable exception being the brain. Thyroxine is also necessary for normal growth. The most likely explanation being that thyroxine promotes the effects of growth hormone on protein synthesis. The absence of thyroxine significantly reduces the ability of growth hormone to stimulate amino acid uptake and RNA synthesis. Thyroxine also plays a crucial role in the closely related area of organ development, particularly that of the central nervous system.
If there is an insufficient amount of thyroxine, a condition referred to as hypothyroidism results. Symptoms of hypothyroidism stem from the fact that there is a reduction in the rate of oxidative energy-releasing reactions within the body cells. Usually the patient shows puffy skin, sluggishness, and lowered vitality. Other symptoms of hypothyroidism include weight gain, decreased libido, inability to tolerate cold, muscle pain and spasm, insomnia and brittle nails. Hypothyroidism in children, a condition known as cretinism, can result in mental retardation, dwarfism, and permanent sexual immaturity. Sometimes the thyroid gland produces too much thyroxine, a condition
known as hyperthyroidism. This condition produces symptoms such as an abnormally high body temperature, profuse sweating, high blood pressure, loss of weight, irritability, and muscular pain and weakness. It also causes the characteristic symptom of the eyeballs protruding from the skull called exopthalmia. This is surprising because it is not a symptom usually related to a fast metabolism. Hyperthyroidism has been treated by partial removal or by partial radiation destruction of the gland. More recently, several drugs that inhibit thyroid activity have been discovered, and their use is replacing the former surgical procedures. Unfortunately thyroid conditions require lifetime treatment and because of the body's need for a sensitive balance of thyroid hormone both supplementing and suppressing thyroid function can take months or even years to regulate.
T3 and T4 Function within the body
The Production of T3 and T4 are regulated by thyroid stimulating hormone (TSH), released by the pituitary gland. TSH Production is increased when T3 and T4 levels are too low. The thyroid hormones are released throughout the body to direct the body's metabolism. They stimulate all cells within the body to work at a better metabolic rate. Without these hormones the body's cells would not be able to regulate the speed at which they performed chemical actions. Their release will be increased under certain situations such as cold temperatures when a higher metabolism is needed to generate heat. When children are born with thyroid hormone deficiency they have problems with physical growth and developmental problems. Brain development can also be severely impaired
The significance of iodine
Thyroid hormone cannot be produced without an abundant source of iodine. The iodine concentration within the body, although significant, can be as little as 1/25th the concentration within the thyroid itself. When the thyroid is low on iodine the body will try harder to produce T3 and T4 which will often result in a swelling of the thyroid gland, resulting in a goiter.
Extrathyroidal iodine
Iodine accounts for 65% of the molecular weight of T4 and 59% of the T3. 15–20 mg of iodine is concentrated in thyroid tissue and hormones, but 70% of the body's iodine is distributed in other tissues, including mammary glands, eyes, gastric mucosa, the cervix, and salivary glands. In the cells of these tissues iodide enters directly by sodium-iodide symporter (NIS). Its role in mammary tissue is related to fetal and neonatal development, but its role in the other tissues is unknown.[]^ It has been shown to act as an antioxidant in these tissues.[]
The US Food and Nutrition Board and Institute of Medicine recommended daily allowance of iodine ranges from 150 micrograms /day for adult humans to 290 micrograms /day for lactating mothers. However, the thyroid gland needs no more than 70 micrograms /day to synthesize the requisite daily amounts of T4 and T3. These higher recommended daily allowance levels of iodine seem necessary for optimal function of a number of body systems, including lactating breast, gastric mucosa, salivary glands, oral mucosa, thymus, epidermis, choroid plexus, etc.[1]^ [2] [3] (^) Moreover, iodine can add to double bonds of docosahexaenoic acid and arachidonic acid of cellular membranes,
making them less reactive to free oxygen radicals.[4]
Calcitonin
Calcitonin is a 32 amino acid polypeptide hormone. It is an additional hormone produced by the thyroid, and contributes to the regulation of blood calcium levels. Thyroid cells produce calcitonin in response to high calcium levels in the blood. This hormone will stimulate movement of calcium into the bone structure. It can also be used therapeutically for the treatment of hypercalcemia or osteoporosis. Without this hormone calcium will stay within the blood instead of moving into bones to keep them strong and growing. Its importance in humans has not been as well established as its importance in other animals.
If parathyroid glands are removed accidentally during surgery on the thyroid, there would be a rise in the phosphate concentration in the blood. There would also be a drop in the calcium concentration as more calcium is excreted by the kidneys and intestines, and more incorporated into the bone. This can produce serious disturbances, particularly in the muscles and nerves, which use calcium ions for normal functioning. Over activity of the parathyroid glands, which can result from a tumor on the glands, produces a weakening of the bones. This is a condition that makes them much more vulnerable to fracturing because of excessive withdrawal of calcium from the bones.
Adrenal glands
Adrenal glands are a pair of ductless glands located above the kidneys. Through hormonal secretions, the adrenal glands regulate many essential functions in the body, including biochemical balances that influence athletic training and general stress response. The glucocorticoids include corticosterone, cortisone, and hydrocortisone or cortisol. These hormones serve to stimulate the conversion of amino acids into carbohydrates which is a process known as gluconeogenesis, and the formation of glycogen by the liver. They also stimulate the formation of reserve glycogen in the tissues, such as in the muscles. The glucocorticoids also participate in lipid and protein metabolism. The cortex of the adrenal gland is known to produce over 20 hormones, but their study can be simplified by classifying them into three categories: glucocorticoids, mineralcorticoids, and sex hormones.
They are triangular-shaped glands located on top of the kidneys. They produce hormones such as estrogen, progesterone, steroids, cortisol, and cortisone, and chemicals such as adrenalin (epinephrine), norepinephrine, and dopamine. When the glands produce more or less hormones than required by the body, disease conditions may occur.
The adrenal cortex secretes at least two families of hormones, the glucocorticoids and mineral corticoids. The adrenal medulla secretes the hormones epinephrine (adrenalin) and norepinephrine (noradrenalin).
Adrenal Cortex: The hormones made by the Adrenal Cortex supply long-term responses to stress. The two major hormones produced are the Mineral Corticoids and the Glucocorticoids. The Mineral Corticoids regulate the salt and water balance, leading to the increase of blood volume and blood pressure. The Glucocorticoids are monitoring the ACTH, in turn regulating carbohydrates, proteins, and fat metabolism. This causes an increase in blood glucose. Glucocorticoids also reduce the body's inflammatory response.
Cortisol is one of the most active glucocorticoids. It usually reduces the effects of inflammation or swelling throughout the body. It also stimulates the production of glucose from fats and proteins, which is a process referred to as gluconeogenesis.
Aldosterone is one example of a mineralcorticoid. It signals the tubules in the kidney nephrons to reabsorb sodium while secreting or eliminating potassium. If sodium levels are low in the blood, the kidney secretes more renin , which is an enzyme that stimulates the formation of angiotensin from a molecule made from the liver. Angiotensin stimulates aldosterone secretion. As a result, more sodium is reabsorbed as it enters the blood.
Aldosterone, the major mineralcorticoid, stimulates the cells of the distal convoluted tubules of the kidneys to decrease re-absorption of potassium and increase re-absorption of sodium. This in turn leads to an increased re-absorption of chloride and water. These hormones, together with such hormones as insulin and glucagon, are important regulators of the ionic environment of the internal fluid.
The renin-angiotensin-aldosterone mechanism can raise blood pressure if it tends to drop. It does this in two ways. Angiotensin is a vasoconstrictor, decreasing the diameter of blood vessels. As vessels constrict, blood pressure increases. In addition, as sodium is reabsorbed, the blood passing through the kidney becomes more hypertonic. Water follows the sodium into the hypertonic blood by osmosis. This increases the amount of volume in the blood and also increases the blood pressure.
Adrenal Medulla The hypothalamus starts nerve impulses that travel the path from the bloodstream, spinal cord, and sympathetic nerve fibers to the Adrenal Medulla, which then releases hormones. The effects of these hormones
provide a short-term response to stress
Excessive secretion of the glucocorticoids causes Cushing's syndrome , characterized by muscle atrophy or degeneration and hypertension or high blood pressure. Under secretion of these substances produces Addison's disease , characterized by low blood pressure and stress.
Epinephrine and norepinephrine produce the "fight or flight" response, similar to the effect from the sympathetic nervous system. Therefore, they increase heart rate, breathing rate, blood flow to most skeletal muscles, and the concentration of glucose in the blood. They decrease blood flow to the digestive organs and diminish most digestive processes.
Suprarenal glands viewed from the front.
Suprarenal glands viewed from behind.
The adrenal sex hormones consist mainly of male sex hormones (androgens) and lesser amounts of female sex hormones (estrogens and progesterone). Normally, the sex hormones released from the adrenal cortex are insignificant due to the low concentration of secretion. However, in cases of excess secretion, masculine or feminine effects appear. The most common syndrome of this sort is "virilism" of the female. Should there be an insufficient supply of cortical hormones, a condition known as Addison's disease would result. This disease is characterized by an excessive excretion of sodium ions, and hence water, due to lack of mineralcorticoids. Accompanying this is a decreased blood glucose level due to a deficient supply of glucocorticoids. The effect of a decreased androgen supply cannot be observed immediately. Injections of adrenal cortical hormones promptly relieve these symptoms.
Hormonal production in the adrenal cortex is directly controlled by the anterior pituitary hormone called adrenocorticotropic hormone (ACTH).
The two adrenal glands lie very close to the kidneys. Each adrenal gland is actually a double gland, composed of an inner core like medulla and an outer cortex. Each of these is functionally unrelated.
The adrenal medulla secretes two hormone, adrenalin or epinephrine and noradrenalin or norepinephrine, whose functions are very similar but not identical. The adrenal medulla is derived embriogically from neural tissue. It has been likened to an overgrown sympathetic ganglion whose cell bodies do not send out nerve fibers, but release their active substances directly into the blood, thereby fulfilling the criteria for an endocrine gland. In controlling epinephrine secretion, the adrenal medulla behaves just like any sympathetic ganglion, and is dependent upon stimulation by sympathetic preganglionic fibers.
Epinephrine promotes several responses, all of which are helpful in coping with emergencies: the blood pressure rises, the heart rate increases, the glucose content of the blood rises because of glycogen breakdown, the spleen contracts and squeezes out a reserve supply of blood, the clotting time decreases, the pupils dilate, the blood flow to skeletal muscles increase, the blood supply to intestinal smooth muscle decreases and hairs become erect. These adrenal functions, which mobilize the resources of the body in emergencies, have been called the fight-or-flight
promotes glycogenesis (glycogen formation), stimulating the breakdown of glycogen into glucose for release into the blood.
Insulin deficiency leads to the development of diabetes mellitus , specifically type I , juvenile diabetes. As the pancreas does not produce sufficient insulin, it is treated by insulin injections. In type II or maturity onset diabetes, the pancreas does produce enough insulin, but the target cells do not respond to it.
As already stated, the pancreas is a mixed gland having both endocrine and exocrine functions. The exocrine portion secretes digestive enzymes into the duodenum via the pancreatic duct. The endocrine portion secretes two hormones, insulin and glucagon, into the blood.
Insulin is a hormone that acts directly or indirectly on most tissues of the body, with the exception of the brain. The most important action of insulin is the stimulation of the uptake of glucose by many tissues, particularly the liver, muscle and fat. The uptake of glucose by the cells decreases blood glucose and increases the availability of glucose for the cellular reactions in which glucose participates. Thus, glucose oxidation, fat synthesis, and glycogen synthesis are all accentuated by an uptake of glucose. It is important to note that insulin does not alter glucose uptake by the brain, nor does it influence the active transport of glucose across the renal tubules and gastrointestinal epithelium.
As stated, insulin stimulates glycogen synthesis. In addition, it also increases the activity of the enzyme that catalyzes the rate-limiting step in glycogen synthesis. Insulin also increases triglyceride levels by inhibiting triglyceride breakdown, and by stimulating production of triglyceride through fatty acid and glycerophosphate synthesis. The net protein synthesis is also increased by insulin, which stimulates the active membrane transport of amino acids, particularly into muscle cells. Insulin also has effects on other liver enzymes, but the precise mechanisms by which insulin induces these changes are not well understood.
Insulin is secreted by beta cells, which are located in the part of the pancreas known as the islets of Langerhans. These groups of cells, which are located randomly throughout the pancreas, also consist of other secretory cells called alpha cells. It is these alpha cells that secrete glucagon. Glucagon is a hormone that has the following major effects: it increases hepatic synthesis of glucose from pyruvate, lactate, glycerol, and amino acids (a process called gluconeogenesis, which also raises the plasma glucose level); and it increases the breakdown of adipose tissue triglyceride, thereby raising the plasma levels of fatty acids and glycerol. The glucagon secreting alpha cells in the pancreas, like the beta cells, respond to changes in the concentration of glucose in the blood flowing through the pancreas; no other nerves or hormones are involved.
It should be noted that glucagon has the opposite effects of insulin. Glucagon elevates the plasma glucose, whereas insulin stimulates its uptake and thereby reduces plasma glucose levels; glucagon elevates fatty acid concentrations, whereas insulin converts fatty acids and glycerol into triglycerides, thereby inhibiting triglyceride breakdown.
The alpha and beta cells of the pancreas make up a push-pull system for regulating the plasma glucose level.
Sex organs
The Sex organs (Gonads) are the testes in the male, and the ovaries in the female. Both of these organs produce and secrete hormones that are balanced by the hypothalamus and pituitary glands.
The main hormones from the reproductive organs are:
Testosterone is more prominent in males. It belongs to the family of androgens, which are steroid hormones producing masculine effects. Testosterone stimulates the development and functioning of the primary sex organs. It also stimulates the development and maintenance of secondary male characteristics, such as hair growth on the face and the deep pitch of the voice.
Estrogen In females, this hormone stimulates the development of the uterus and vagina. It is also responsible for the development and maintenance of secondary female characteristics, such as fat distribution throughout the body and the width of the pelvis.
Male
The testes produce androgens (i.e., "testosterone"). Testosterone is classified as a steroid and is responsible for many of the physical characteristics in males like.
- Broad shoulders
- Muscular body
- Hair
Testosterone increases protein production. Hormones that build up protein are called anabolic steroids. Anabolic steroids are available commercially and are being used by athletes because they help improve their physical ability, however, they do have major side effects such as:
- Liver and kidney disorders
- Hypertension (high blood pressure)
- Decreased sperm count and impotency
- Aggressive behavior ("roid mania)
- Balding
- Acne
Female
Schematic frontal view of female anatomy.
The ovaries produce estrogen and progesterone. Estrogen increases at the time of puberty and causes the growth of the uterus and vagina. Without estrogen egg maturation would not occur. Estrogen is also responsible for secondary sex characteristics such as female body hair and fat distribution. Estrogen and Progesterone are responsible for the development of the breast and for the uterine cycle. Progesterone is a female hormone secreted by the corpus luteum after ovulation during the second half of the menstrual cycle. It prepares the lining of the uterus for implantation of a fertilized egg and allows for complete shedding of the endometrium at the time of menstruation. In the event of pregnancy, the progesterone level remains stable beginning a week or so after conception.
Water-soluble Hormones: bind to a receptor protein on the plasma membrane of the cell
Chapter Review Questions
Answers for these questions can be found here [5]
- My child just fell and was hurt, the anxious feeling that I feel is caused by:
A) glucagon B) insulin C) epinephrine D) adrenocorticotropic E) None of these
- All of Bob’s life he has had to take insulin shots, this is caused because
A) his beta cells don’t function correctly B) his alpha cells don’t function correctly C) his DA hormone isn’t functioning correctly D) his GHRH hormone isn’t functioning correctly
- The reason iodine is in salt is
A) to prevent diabetes B) to prevent simple goiters C) to prevent Addison’s disease D) to prevent Cushing's syndrome
- All hormones react to a negative feedback except
A) progesterone B) estrogen C) prolactin D) oxytocin E) none of these
- If I have a high blood calcium level it may be due to
A) calcitonin B) parathyroid C) glucocorticoids D) glucagon
- Hormones that are lipids that are synthesized from cholesterol
A) protein B) amino acid-derived C) polypeptide D) steroids E) eicosanoids
- This type of hormone must bind to a receptor protein on the plasma membrane of the cell
A) water soluble B) lipid soluble
C) steroid D) polypeptide E) a and d F) b and c
- Endocrine glands release hormones in response to
A) Hormones from other endocrine glands B) Chemical characteristics of the blood C) Neural stimulation D) All of the above
- The anterior pituitary secretes
A) oxytocin B) endorphins C) ADH D) TRH
- Chief cells produce
A) epinephrine B) glucagon C) insulin D) mineralocoticoids E) parathyroid hormone
- Name the eight major endocrine glands.
- Name the four major groups hormones can be chemically classified into.
References
[4] Cocchi, M. and Venturi, S. Iodide, antioxidant function and Omega-6 and Omega-3 fatty acids: a new hypothesis of a biochemical cooperation? Progress in Nutrition, 2000, 2, 15- [5] http:/ / en. wikibooks. org/ wiki/ Human_Physiology/ Appendix_1:_answers_to_review_questions#The_Endocrine_System
