Glossary of Biology - Endocrine System

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What is the endocrine system?
It acts as a means of internal communication, coordinating the activities of the organ systems
What do the endocrine glands do?
They synthesize and secrete chemical substances called hormones directly into the circulatory system
What do exocrine glands do?
They secrete substances that are transported by ducts
What do hormones do?
They regulate the function of target organs or tissues
What are the glands that synthesize and or secrete hormones?
The pituitary, hypothalamus, thyroid, parathyroids, adrenals, pancreas, testes, ovaries, pineal, kidneys, gastrointestinal glands, heart, and thymus
Do all hormones regulate a single type of cell or organ?
No, some do, while others have more widespread actions
What is the specificity of hormonal action determined by?
The presence of specific receptors on or in the target cells
What is the pituitary gland?
It is a small tri-lobed gland lying at the base of the brain
How many lobes does it have?
2, the anterior and posterior
Do they do the same thing?
No, they are functionally distinct
What does the anterior pituitary do?
It synthesizes both direct hormones, which directly stimulate their target organs, and tropic hormones, which stimulate other endocrine glands to release hormones
What re the hormonal secretions of the anterior pituitary regulated by?
The hypothalamic secretions called releasing/inhibiting hormones or factors
What does growth hormone do?
Growth hormone promotes bone and muscle growth, inhibits the uptake of glucose by certain cells, and stimulates the breakdown of fatty acid, thus conserving glucose
What is GH secretion stimulated by?
The hypothalamic releasing hormone GHRH
What is GH secretion inhibited by?
What other control is secretion under?
Neural and metabolic control
In children, what can GH deficiency lead to?
Stunted growth (dwarfism)
What does overproduction of GH lead to in children?
What does overproduction of GH in adults cause?
Acromegaly, a disorder characterized by a disproportionate overgrowth of bone, localized especially in the skull, jaw, feet, and hands
What is prolactin?
A hormone that stimulates milk production and secretion in female mammary glands
What are endorphins?
They inhibit the perception of pain
What is Adrenocorticotropic hormone (ACTH)?
ACTH stimulates the adrenal cortex to synthesize and secrete glucocorticoids
What is ACTH regulated by?
The release of the hormone corticotrophin releasing factor (CRF)
What does thyroid stimulating hormone (TSH) do?
It stimulates the thyroid gland to absorb iodine and then synthesize and release thyroid hormone
What is TSH regulated by?
It is regulated by releasing the hormone thyroid regulating hormone (TRH)
What is luteinizing hormone (LH)?
In females, LH stimulates ovulation and formation of the corpus luteum
What does it do in males?
It stimulates the maturation of the seminiferous tubules and sperm production
What is LH regulated by?
Estrogen, progesterone, and gonadotropin releasing hormone (GnRH)
What is follicle-stimulating hormone (FSH)?
In females, FSH causes maturation of ovarian follicles
What is FSH regulated by?
Estrogen and by GnRH
What does the posterior pituitary do?
It does not synthesize hormones
It does not synthesize them, where are oxytocin and ADH made?
They are produced by neurosecretory cells of the hypothalamus
What is hormone secretion stimulated by?
Action potential descending from the hypothalamus
What is oxytocin?
It is secreted after childbirth
What is secretion also induced by?
What does it stimulate?
It stimulates milk secretion in mammary glands
What is Antidiuretic Hormone (ADH, vasopressin)?
ADH increases the permeability of the nephron’s collecting duct to water
What does this do?
It inhibits CRF and ACTH release
When is ADH secreted?
When plasma osmolarity increases
How is plasma osmolarity sensed?
By osmoreceptors in the hypothalamus
How else is it sensed?
When blood volume decreases, baroreceptors sense it in the circulatory system
What is the hypothalamus?
It is part of the forebrain and is located directly above the pituitary gland
What does it do?
It stimulates the uptake of glucose by muscle and adipose cells and the storage of glucose as glycogen in muscle and liver cells, thus lowering blood glucose levels
What do the transmissions trigger?
They trigger specific responses from the neurosecretory cells
What do neurosecretory cells regulate?
They regulate pituitary gland secretions via negative feedback mechanisms and through the actions of inhibiting and releasing hormones
What are hypothalamic releasing hormones?
They are hormones that stimulate or inhibit the secretions of the anterior pituitary
What is an example?
GnRH stimulates the anterior pituitary to secrete FSH and LH
Where are releasing hormones secreted into?
The hypothalamic-hypophyseal portal system
What happens in this circulatory pathway?
Blood from the capillary bed in the hypothalamus flows through a portal vein into the anterior pituitary, where it diverges into a second capillary network
What does this accomplish?
It allows the release of hormones to immediately reach the anterior pituitary
What does oversecretion of hormones do?
It is potentially harmful to an organism
What stops it from occurring?
A preventative mechanism called negative feedback has evolved
How does it work for hormones?
A high hormone level inhibits further production of that hormone
What is an example of this?
When plasma levels of adrenal cortical hormones reach a critical level, the hormones themselves exert an inhibitory effect on the pituitary and on the hypothalamus
What happens in the absence of CRF?
The anterior pituitary stops ACTH secretion, and the adrenal cortex stops secreting adrenal cortical hormones
What happens when adrenal hormone levels are too low?
The hypothalamus is stimulated to release CRF
What does this stimulate?
It stimulates the anterior pituitary to secrete ACTH, which in turn, stimulates the adrenal cortex to release adrenal cortical hormones
How do neurosecretory cells in the hypothalamus work?
They synthesize both oxytocin and ADH and transport them via their axons into the posterior pituitary for storage and secretion
What is the thyroid gland?
It is a bi-lobed structure located on the ventral surface of the trachea
What are thyroxine and triiodothyronine?
They are derived from the iodination of the amino acid tyrosine they are necessary for growth an neurological development in children
What do they do?
They synthesize and secrete parathyroid hormone (PTH), which together with calcitonin and vitamin D, regulates plasma Ca2+ concentration
What do high plasma levels of thyroid do?
They inhibit TRH and TSH secretion, thereby returning plasma levels to normal
What does inflammation of the thyroid or iodine deficiency cause?
What is that?
It is characterized by hyperglycemia (high blood glucose levels)
What are common symptoms?
Slowed heart rate and respiratory rate, fatigue, cold intolerance, and weight gain
What is hypothyroidism in newborn infants called?
What is it characterized by?
Mental retardation and short stature
What is hyperthyroidism?
The thyroid is overstimulated, resulting in the oversecretion of thyroid hormones
What are symptoms of it?
Increased metabolic rate, feelings of excessive warmth, profuse sweating, palpitations, weight loss, and protruding eyes
What happens to the thyroid in both disorders?
It often enlarges, forming a bulge in the neck called a goiter
What does calcitonin do?
It decreases plasma Ca2+ concentration by inhibiting the release of Ca2+ from the bone
What is its secretion regulated by?
Plasma Ca2+ levels
What are the parathyroid glands?
They are four small pea-shaped structures embedded in the posterior surface of the thyroid
What does plasma Ca2+ concentration regulate?
It regulates PTH secretion by means of negative feedback
What does PTH do to Ca2+?
It raises the Ca2+ concentration in the blood by stimulating Ca2+ release from the bone and decreasing Ca2+ excretion in the kidneys
What does PTH convert Vitamin D into?
It converts it into its active form, which stimulates intestinal calcium absorption
What are the adrenal glands?
They are situated on top of the kidneys and consist of the adrenal cortex and the adrenal medulla
What does ACTH stimulate the adrenal cortex to do in response to stress?
It stimulates it to synthesize and secrete steroid hormones, which are collectively known as corticosteroids
What are corticosteroids?
They are derived from cholesterol, and they include glucocorticoids, mineralcorticoids, and cortical sex hormones
What are glucocorticoids?
They are involved in glucose regulation and protein metabolism.
What are some examples?
Cortisol and cortisone
What do they do to glucose?
They raise blood glucose levels by promoting gluconeogenesis and decrease protein synthesis
What do they do to the body’s immune system?
They reduce the body’s immunological and inflammatory responses
What is Cortisol secretion governed by?
Negative feedback
What are mineralcorticoids?
They regulate plasma levels of sodium and potassium, and ultimately the total extracellular water volume
What is an example of a mineralcorticoid?
What does aldosterone do?
It causes the active reabsorption of sodium and the passive reabsorption of water in the nephron
What does this result in?
An increased basal metabolic rate
What does aldosterone stimulate?
It stimulates the secretion of potassium ion and hydrogen ion into the nephron and their subsequent excretion in urine
What is aldosterone secretion regulated by?
The rennin-angiotensin system
How does it work?
When blood volume falls, the juxtaglomerular cells of the kidney produce rennin, an enzyme that converts the plasma protein angiotensinogen to angiotensin I
What happens to angiotensin I?
It is converted to angiotensin II, which stimulates the adrenal cortex to secrete aldosterone
What does aldosterone help to do?
It helps to restore blood volume by increasing sodium reabsorption at the kidney
This leads to what?
An increase in water reabsorption
What does this remove?
The initial stimulus for rennin production
What are cortical sex hormones?
The adrenal cortex secretes small quantities of androgens (male sex hormones) in both males and females
In males, where are most androgens produced?
In the testes
What does this mean?
It relays messages from the extracellular peptide hormone to cytoplasmic enzymes, initiating a series of successive reactions within the cell
In females however, what does this do?
Overproduction of the adrenal androgens may have masculinizing effects, such as excessive facial hair
What can the secretory cells of the adrenal medulla be viewed as?
Specialized sympathetic nerve cells that secrete hormones into the circulatory system
What does the adrenal medulla produce?
Epinephrine and norepinephrine
What do they belong to?
A class of amino acid-derived compounds called catecholamines
What does epinephrine cause?
An increased conversion of glycogen to glucose in the liver and muscle tissue
What does this to do glucose levels in the blood?
It causes them to rise
What do epinephrine and norepinephrine do to the heart?
They both increase the rate and strength of the heartbeat
What do they do to blood vessels?
They dilate and constrict blood vessels in such a way as to increase the blood supply to skeletal muscle, the heart, and the brain, while decreasing the blood supply to the kidneys, skin, and digestive tract
What are these effects known as?
The fight or flight response
What are they elicited by?
The sympathetic nervous stimulation in response to stress
Are these hormones neurotransmitters?
What is the pancreas?
It is both an exocrine and endocrine organ
What is the exocrine function?
The exocrine function is performed by cells that secrete digestive enzymes into the small intestine via a series of ducts
What is the endocrine function performed by?
Small glandular structures called the islets of Langerhans
What are they composed of?
Alpha, beta, and delta cells
What do alpha cells produce?
They secrete glucagons
What do beta cells produce?
They produce and secrete insulin
What do delta cells produce and secrete?
What does glucagons do?
It stimulates protein and fat degradation, the conversion of glycogen to glucose, and gluconeogenesis
What do they serve to do?
They serve to increase blood glucose levels
What is glucagons secretion stimulated by?
It is stimulated by a decrease in blood glucose and by gastrointestinal hormones, CCK and gastrin
What is it inhibited by?
It is inhibited by high plasma glucose levels
What are glucagon’s actions in relations to those of insulin?
They are largely antagonistic
What is insulin?
It is a protein hormone secreted in response to a high blood glucose concentration
What else does it stimulate?
It stimulates the synthesis of fats from glucose and the uptake of amino acids
What is insulin secretion regulated by?
Blood glucose levels
What does overproduction of insulin cause?
Hypoglycemia (low blood glucose levels)
What does underproduction of insulin or insensitivity to insulin lead to?
Diabetes mellitus
What do high blood glucose levels lead to?
Excretion of glucose and water loss
What else is diabetes associated with?
Weakness and fatigue
What may it lead to?
Ketoacidosis, which is a dangerous lowering of the blood pH due to excess keto acids and fatty acids in the plasma
What is somatostatin?
Pancreatic somatostatin secretion is increased by high blood glucose or high amino acid levels, leading to both decreased insulin and glucagons secretion
What is somastatin regulated by?
CCK and GH levels
What do the interstitial cells produce?
They produce and secrete androgens
What does testosterone induce?
It induces embryonic sexual differentiation and male sexual development at puberty
What does it maintain?
Secondary sex characteristic
What is testosterone secretion controlled by?
A negative feedback mechanism involving FSH and LH
What is insensitivity to testosterone result in?
A syndrome called testicular feminization, in which a genetic male has female secondary sexual characteristics
What do the ovaries do?
They synthesize and secrete estrogens and progesterone
What is the secretion of both estrogens and progesterone regulated by?
LH and FSH, which in turn are regulated by GnRH
What are estrogens?
They are steroid hormones necessary for normal female maturation
What do they stimulate?
They stimulate the development of female reproductive tract and contribute to the development of secondary sexual characteristics and sex drive
What are estrogens responsible for thickening?
The endometrium
What are estrogens secreted by?
Ovarian follicles and the corpus luteum
What is progesterone?
It is a steroid hormone secreted by the corpus luteum during the luteal phase of the menstrual cycle
What does progesterone stimulate?
The development and maintenance of the endometrial walls in preparation for implantation
What is the menstrual cycle?
It is a monthly cyclical pattern resulting from interactions between hormones
What can the menstrual cycle be divided into?
The follicular phase, ovulation, the luteal phase, and menstruation
What is the follicular phase?
It begins with the cessation of the menstrual flow from the previous cycle
During this phase, what hormones act together?
FSH and LH act together to promote the development of several ovarian follicles, which grow and begin secreting estrogen
What do rising levels of estrogen in the latter half of this phase stimulate?
GnRH secretion, which in turn further stimulates LH secretion
What is ovulation?
Midway through the cycle, it occurs- a mature ovarian follicle bursts and releases an ovum
What is ovulation caused by?
It is caused by the surge in LH which is preceded, and in part caused, by a peak in estrogen levels
What is the luteal phase?
LH induces the ruptured follicle to develop into the corpus luteum, which secretes estrogen and progesterone
What does the progesterone do?
It causes the glands of the endometrium to mature and produce secretions that prepare it for the implantation of an embryo
What are progesterone and estrogen essential for?
The maintenance of the endometrium
What do they together inhibit?
They inhibit secretion of GnRH
What does that ultimately stop?
It inhibits LH and FSH secretion
What does this prevent?
FSH and LH secretion and the onset of a new menstrual cycle
What is menstruation?
It is what occurs if the ovum is not fertilized
What occurs?
The corpus luteum atrophies
What happens next?
The resulting drop in progesterone and estrogen levels causes the endometrium to slough off
What does this give rise to?
The flow
What happens to hormone levels from here?
Progesterone and estrogen levels decline and GnRH is no longer inhibited
What does GnRH do?
It restimulates LH and FSH secretion, and so the cycle begins anew
What happens during the first trimester of pregnancy?
The corpus luteum is preserved by human chorionic gonadotropin, HCG
What is HCG?
A hormone produced by the blastocyst and the developing placenta
What does this mean for Progesterone and Estrogen?
It means their levels of secretion by the corpus luteum are maintained during the first trimester
What happens during the second trimester?
HCG levels decline, but progesterone and estrogen levels rise
How come?
Because they are now secreted by the placenta itself
What do high levels of progesterone and estrogen do?
They inhibit GnRH secretion
What is menopause?
It is the period in a woman’s life, between 45-55, when menstruation first becomes irregular, and eventually stops
What is it the result of?
It results from the progressive decline in the functioning of the ovaries with advancing age
What happens?
Some follicles fail to rupture, ovulation does not occur, and less estrogen is produced by the ovaries, thereby disrupting the hormonal regulation of other glands
What symptoms do woman have during menopause?
Bloating, hot flashes, and headaches
What is the pineal gland?
It is a tiny structure at the base of the brain that secretes the hormone melatonin
What is melatonin’s purpose?
It role in humans in unclear, but it is believed to play a role in the regulation of circadian rhythms
What are those?
They are the physiological cycles lasting 24 hours
What is melatonin secretion regulated by?
Light and dark cycles in the environment
What does glandular tissue found in the mucosa of the stomach and intestines do?
The primary stimulus for gastrointestinal hormone release is the presence of food in the gut, though neural input and exposure to other hormones also affect their release
How many gastrointestinal peptides have been isolated?
Over 20
What are some important examples?
Gastrin, secretin, and CCK
What does Renin, an enzyme secreted by the kidney do?
It is involved in the regulation of aldosterone secretion
What is erythropoietin?
It is secreted by the kidney in response to decreased renal oxygen levels and stimulates bone marrow to produce red blood cells
How is the heart an endocrine gland?
It releases atrial natriuretic hormone
What does that do?
It is involved in the regulation of salt and water balance
What is the thymus gland?
It is located in the front of the neck region and secretes hormones such as thymosin during childhood
What does thymosin do?
It stimulates T lymphocyte development and differentiation
What happens to the thymus by adulthood?
It atrophies, after the immune system has fully developed
How are hormones classified?
By the basis of their chemical structure
How many groups are there?
What are they?
Peptide hormones, steroid hormones, and amino acid-derived hormones
What are the two ways in which hormones affect the activities of their target cells?
Via extracellular receptors or intracellular receptors
What are peptides?
They range from simple short peptides (amino acid chains) such as ADH, to complex polypeptides such as insulin
How does synthesis of peptide hormones begin?
With the synthesis of a large polypeptide
What happens to the polypeptide?
It is cleaved into smaller protein units and transported to the Golgi apparatus, where it is further modified into the active hormone
How is the hormone packaged?
It is packaged into secretory vesicles and stored until it is released by the cell via exocytosis
How do peptide hormones act as messengers?
They are first messengers
What does that mean?
It means they bind to specific receptors on the surface of their target cells, triggering a series of enzymatic reactions within the each cell
What is usually the first reaction?
The conversion of ATP to cAMP
What is this reaction catalyzed by?
The membrane-bound enzyme adenylate cyclase
How does Cyclic AMP act?
It acts as a second messenger
What are these reactions called?
A cascade effect
What happens to the hormone’s effects with each step?
They are amplified
How is cyclic AMP activity inactivated?
By the cytoplasmic enzyme phosphodiesterase
What are steroids?
They belong to a class of lipid-derived molecules with a characteristic ring structure
Where are they produced?
By the testes, ovaries, placenta, and adrenal cortex
What happens to precursors already present in the cell such as cholesterol, in the synthesis of steroid molecules?
They undergo enzymatic reactions that convert them into active hormones
Do steroid hormones pass through the membrane?
They are lipid soluble
Are they stored anywhere?
No, they are secreted at a rate determined by their rate of synthesis
How do steroids work?
They enter their target cells directly and bind to specific receptor proteins in the cytoplasm
What happens to this receptor-hormone complex?
It enters the nucleus and directly activates the expression of specific genes by binding to receptors on the chromatin
What does this induce?
It induces change in mRNA transcription and protein synthesis
What are amino acid derivatives?
They are hormones composed of one or two modified amino acids
Where are they synthesized?
In the cytoplasm of glandular cells
Are any modified further?
Some are further modified and stored in granules until the cell is stimulated to release them, while others are initially synthesized as component parts of larger molecules and stored
How do some work?
Some, like epinephrine, activate their target cells as peptide hormones do, via second messengers
What about the others?
Others, like thyroxine, act in the same manner as steroid hormones, entering the nucleus of their target cells and regulating gene expression

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