Biology - Endocrine System
Terms
undefined, object
copy deck
- 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?
- Somatostatin
- 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?
- Gigantism
- 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?
- Suckling
- 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?
- Hypothyroidism
- 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?
- Cretinism
- 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?
- Aldosterone
- 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?
- Yes
- 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?
- Somatostatin
- 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?
- 3
- 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?
- Yes
- Why?
- 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