Chapter 18: The Endocrine System Flashcards
compare control of body functions by the nervous system and endocrine systems.
Hormone – a secretion of endocrine cells that alters the physiological activity of target cells in the body. Is a mediator cell that is released from one part of the body but regulates the activity of cells in other parts of the body.
TABLE 18.1 Comparison of Control by the Nervous and Endocrine Systems
- CHARACTERISTIC Mediator molecules
NERVOUS SYSTEM Neurotransmitters released locally in response to nerve impulses.
ENDOCRINE SYSTEM Hormones delivered to tissues throughout body by blood.
- CHARACTERISTIC Site of mediator action
NERVOUS SYSTEM Close to site of release, at synapse; binds to receptors in postsynaptic membrane.
ENDOCRINE SYSTEM Far from site of release (usually); binds to receptors on or in target cells.
- CHARACTERISTIC Types of target cells
NERVOUS SYSTEM Muscle (smooth, cardiac, and skeletal) cells, gland cells, other neurons.
ENDOCRINE SYSTEM Cells throughout body.
- CHARACTERISTIC Time to onset of action
NERVOUS SYSTEM Typically within milliseconds (thousandths of a second).
ENDOCRINE SYSTEM Seconds to hours or days.
- CHARACTERISTIC Duration of action
NERVOUS SYSTEM Generally briefer (milliseconds).
ENDOCRINE SYSTEM Generally longer (seconds to days).
distinguish between exocrine and endocrine glands.
exocrine gland – a gland that secretes its product into ducts that carry the secretions into body cavities, into the lumen of an organ, or to the outer surface of the body.
- Include sudoriferous (sweat), sebaceous (oil), mucous, and digestive glands
endocrine gland – a gland that secretes hormones into interstitial fluid and then blood; a ductless gland.
- From interstitial fluid, hormones diffuse into blood capillaries and blood carries them to target cells throughout the body.
- Endocrine glands are highly vascular.
- Include the pituitary, thyroid, parathyroid, adrenal, and pineal glands
- Also, many organs that are not specifically glands contain cells that secrete hormones, including the hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart, adipose tissue, and placenta.
describe how hormones interact with target-cell receptors.
hormone activity
receptor – specific protein that recognizes and binds to a particular ligand (hormone)
- only the target cells for a given hormone have receptors that bind and recognize that hormone.
down regulation – phenomenon in which there is a decrease in the number of receptors in response to an excess or a hormone or neurotransmitter.
- Receptors are constantly being synthesized and broken down.
- If a hormone is present in excess, the receptors may be broken down to regulate the amount of response to that hormone.
- Down-regulation makes a target cell less sensitive to a hormone.
up regulation – when a hormone is deficient, the number of receptors may increase
- Up-regulation makes a target cell more sensitive to a hormone.
circulating hormone – pass from the secretory cells that make them to interstitial fluid and then into the blood.
- Most endocrine hormones are circulating
local hormone – act locally on neighboring cells or on the same cell that secreted them without entering the blood stream.
- Two kinds: paracrine, autocrine
- Paracrine – local hormones that act on neighboring cells
- Autocrine – local hormones that act on the same cell that secreted them.
compare the two chemical classes of hormones based on their solubility.
chemical classes of hormones – lipid soluble, water soluble
lipid soluble hormones
a. steroid hormones – derived from cholesterol.
* Each steroid hormone is unique due to different chemical groups attached at various sites on the for core structural rings
b. thyroid hormones (T3 and T4) – synthesized by attaching iodine to the amino acid tyrosine
* the presence of two benzene rings with a T3 or T4 molecule makes these molecules very lipid-soluble.
c. nitric oxide (NO) – synthesis is catalyzed by the enzyme nitric oxide synthase
* both a hormone and a neurotransmitter
water soluble hormones
a. amine hormones – synthesized by decarboxylating and otherwise modifying certain amino acids
- called amines because they retain an amino group.
- Catecholamines – epinephrine, norepinephrine, and dopamine, involve modifying tyrosine
- Histamine is synthesized from the amino acid histidine by mast cells and platelets
- Serotonin and melatonin are derived from tryptophan
b. peptide hormones and protein hormones – amino acid polymers
- smaller peptide hormones consist of chains of 3-49 amino acids 1. ex. Antidiuretic hormone, oxytocin
- larger protein hormones include 50-200 amino acids. 1. Ex. Human growth hormone, insulin
c. glycoprotein hormones – protein hormones with an attached carbohydrate group i. ex. Thyroid-stimulating hormone
d. eicosanoid hormones – derived from arachidonic acid (a 20 carbon fatty acid)
- Two major types of eicosanoids: prostaglandins and leukotrienes
- Important local hormones, may act as circulating hormones also.
hormone transport in the blood
- most water-soluble hormone molecules circulate in blood plasma in “free” form – not attached to other molecules.
- Most Lipid-soluble hormones are bound to transport proteins
transport proteins – synthesized in the liver, have 3 functions:
- make lipid-soluble hormones temporarily water-soluble, thus increasing their solubility in the blood
- retard passage of small hormone molecules through the filtering mechanism of the kidneys, slowing the rate of hormone loss in the urine
- provide a ready reserve of hormone, already present in the blood stream.
- This portion diffuses out of capillaries, binds to receptors, and triggers responses.
free fraction – the portion (0.1-10%) of the molecules of a lipid-soluble hormone are not bound to a transport protein.
- As free hormone molecules leave the blood and bind to their receptors, transport proteins release new ones to replenish the free fraction.
describe the two general mechanisms of hormone action.
mechanisms of hormone action – the response depends on both the hormone itself and the target cell. Various target cells respond differently to the same hormone. For example, insulin stimulates synthesis of glycogen in liver cells and synthesis of triglycerides in adipose cells.
action of lipid soluble hormones – receptors are located within the cell
- a free lipid-soluble hormone molecule diffuses from the blood, through interstitial fluid, and through the lipid bilayer of the plasma membrane into a cell
- if the cell is a target cell, the hormone binds to and activates receptors located within the cytosol or nucleus. The activated receptor-hormone complex then alters gene expression: it turns specific genes of the nuclear DNA on or off
- as the DNA is transcribed, new messenger RNA forms, leaves the nucleus, and enters the cytosol. There, it directs synthesis of a new protein (often an enzyme) on the ribosomes
- the new proteins alter the cell’s activity and cause the responses typical of that hormone.
action of water soluble hormones – receptors are integral transmembrane proteins located in the plasma membrane.
- first messenger – the hormone that binds to its receptor at the outer surface of the plasma membrane
- second messenger – produced inside the cell by the first messenger binding to the receptor outside
- cyclic AMP (cAMP) – common second messenger
- G protein – membrane protein, once activated by the hormone-receptor complex, it activates adenylate cyclase
- adenylate cyclase – an enzyme that converts ATP into cyclic AMP. This process occurs in the cytosol
hormone interactions
permissive effect – when a second hormone requires the recent or simultaneous presence of a first hormone to fully act
synergistic effect – when the effect of two hormones acting together is greater or more extensive than the effect of each hormone acting alone.
antagonistic effects – when one hormone opposes the actions of another hormone. Ex of an antagonistic pair: insulin which promotes synthesis of glycogen by liver cells and glucagon which stimulates breakdown of glycogen in the liver.
describe the mechanisms of control of hormone secretion.
control of hormone secretion
- release of most hormones occurs in short bursts, with little or no secretion between bursts.
- When stimulated, an endocrine gland will release its hormone in more frequent bursts, increasing blood concentration of the hormone.
- Blood level drops in absence of stimulation
- Regulation of secretion normally prevents overproduction or underproduction of any given hormone to help maintain homeostasis
Hormone secretion is regulated by:
- Signals from the nervous system a. Ex. Nerve impulses to the adrenal medullae regulate the release of epinephrine
- Chemical changes in the blood a. Ex. Blood Ca2+ level regulates the secretion of parathyroid hormone
- Other hormones a. Adrenocorticotropic hormone (from the anterior pituitary) stimulates the release of cortisol by the adrenal cortex
describe the locations of and relationships between the hypothalamus and pituitary gland.
hypothalamus and pituitary gland or hypophysis
pituitary gland – AKA hypophysis
- Small endocrine gland occupying the hypophyseal fossa of the sphenoid bone and attached to the hypothalamus by the infundibulum
- Thought to be the “master” endocrine gland, but now known that the hypothalamus is the real master.
- Secretes 7 different hormones
anterior pituitary or anterior lobe or adenohypophysis – 75% total weight of the pituitary gland
a. Composed of epithelial tissue
b. Consists of two parts in an adult: pars distalis (larger portion), pars tuberalis (shealth around the infundibulum)
Posterior pituitary – AKA neurohypophysis
a. composed of neural tissue
b. Consists of two parts: pars nervosa (larger bulbar portion) and the infundibulum
pars intermedia – third region of the pituitary gland
- A small avascular zone between the anterior and posterior pituitary glands
- Atrophies during fetal development but some cells migrate into the anterior pituitary.
Hypothalamus – a portion of the diencephalon, lying beneath the thalamus and forming the floor and part of the wall of third ventricle
- The major link between the nervous and endocrine systems
- Cells in the hypothalamus synthesize at least 9 different hormones
Infundibulum – stalklike structure that attaches the pituitary gland to the hypothalamus of the brain.
releasing hormones – hormone secreted by the hypothalamus that can stimulate secretion of hormones of the anterior pituitary
inhibiting hormones – hormone secreted by the hypothalamus that suppresses secretion of hormones by the anterior pituitary
Portal system: in a portal system, blood passes through a network of capillaries into a portal vein and then into a secondary capillary network. The name of the portal system indicates the location of the second capillary network.
hypophyseal portal system – blood flows from capillaries in the hypothalamus into portal veins that carry blood to capillaries of the anterior pituitary.
- Superior hypophyseal arteries (branches of the internal carotid arteries) bring blood into the hypothalamus
- Primary plexus of the hypophyseal portal system – capillary network at the junction of the median eminence of the hypothalamus and infundibulum
- Hypophyseal portal veins – from the primary plexus, down the outside of the infundibulum to the anterior pituitary
- Secondary plexus of the hypophyseal portal system – capillary network formed by hypophyseal portal veins dividing
neurosecretory cell – specialized neuron
- a neuron that secretes a hypothalamic releasing hormone or inhibiting hormone into blood capillaries of the hypothalamus The hormone diffuses into the primary plexus of the hypophyseal portal system and into the portal veins and the secondary plexus, thus acting immediately on the pituitary
- a neuron that secretes oxytocin or antidiuretic hormone into blood capillaries of the posterior pituitary.
tropic hormones – anterior pituitary hormones that act on other endocrine glands
describe the location, histology, hormones, and functions of the anterior and posterior pituitary.
anterior and posterior pituitary
Control of secretion by the anterior pituitary:
- Secretion regulated in 2 ways: neurosecretory cells in the hypothalamus secrete 5 releasing hormones and 2 inhibiting hormones
- Negative feedback in the form of hormones released by target glands decreases secretions of 3 types of anterior pituitary cells
Activity of Thyrotrophs, gonadotrophs, and corticotrophs decreases when blood levels of target gland hormones rise.
5 types of pituitary cells:
- Somatotrophs – secrete human growth hormone hGH i. Most numerous cells in anterior pituitary
- Thyrotrophs – secrete thyroid stimulating hormone TSH
- Gonadotrophs – secrete two gonadotrophins: follicle stimulating hormone FSH, and luteinizing hormone LH
- Lactotrophs – secrete prolactin PRL
- Corticotrophs – secrete adrenocorticotropic hormone ACTH
anterior pituitary hormones (several are tropic hormones or tropins, see also Table 18.3)
a. human growth hormone (hGH) or somatotropin – most plentiful anterior pituitary hormone.
- Main function is to promote synthesis and secretion of small protein hormones called insulinlike growth factors or somatomedins.
- GHRH – growth hormone-releasing hormone 1. Promotes secretion of human growth hormone
- GHIH – growth hormone-inhibiting hormone 1. Suppresses secretion of hGH 1. Both GHRH and GHIH regulated by blood glucose level
- insulinlike growth factors (IGFs) - secreted by cells in the liver, skeletal muscles, cartilage, bones, and other tissues in response to human growth hormone
- Functions of IGFs:
- Cause cells to grow and multiply by increasing uptake of amino acids into cells and accelerating protein synthesis. Also decrease breakdown of proteins and the use of amino acids for ATP production. Due to these effects of IGFs, hGH increases growth rate of body during childhood and muscle and bone mass and healing in adult body
- Enhance lipolysis in adipose tissue, which results in increased use of the released fatty acids for ATP production by body cells
- Influence carbohydrate metabolism by decreasing glucose uptake, which decreases the use of glucose for ATP production by most body cells. This action spares glucose so it is available to neurons for ATP production in times of glucose scarcity. IGFs and hGH may also stimulate liver cells to release glucose into the blood.
Hypoglycemia and hyperglycemia
Hypoglycemia – stimulates hypothalamus to secrete GHRH which stimulates somatotrophs to release hGH which stimulates secretion of IGFs which speed breakdown of liver glycogen into glucose, causing glucose to enter the blood more rapidly
Hyperglycemia – stimulates hypothalamus to secrete GHIH which inhibits secretion of hGH in the pituitary which lowers level of hGH and IGFs and slows breakdown of glycogen in the liver, slowing release of glucose into the blood, lowering blood glucose level
thyroid stimulating hormone (TSH) or thyrotropin –
stimulates the synthesis and secretion of the two thyroid hormones: triiodothyronine (T3) and thyroxine (T4)
- TRH – thyrotropin-releasing hormone
- From the hypothalamus
- Control TSH secretion
- Release of TRH depends on blood levels of T3 and T4; high levels inhibit secretion of TRH via negative feedback.
- There is no thyrotropin-inhibiting hormone
Gonadotropins
i. follicle-stimulating hormone (FSH)
- Females: ovaries are the target
- Initiates the development of several ovarian follicles
- Also stimulates follicular cells to secrete estrogens
- Males: stimulates sperm production in the testes
ii. luteinizing hormone (LH)
- Females: triggers ovulation, stimulates formation of the corpus luteum in the ovary and the secretion of progesterone by the corpus luteum
- Together, FSH and LH also stimulate secretion of estrogens by ovarian cells.
- Estrogens and progesterone prepare the uterus for implantation of a fertilized ovum and help prepare the mammary glands for milk secretion.
- Males: LH stimulates cells in the testes to secrete testosterone.
iii. gonadotropin-releasing hormone (GnRH) - from the hypothalamus
- Stimulates FSH and LH release
- Release of GnRH and FSH is suppressed by estrogens in females and by testosterone in males through negative feedback.
- Weak by itself. Other hormones exert permissive effects: estrogens, progesterone, glucocorticoids, hGH, thyroxine, and insulin prime the mammary glands.
iv. There is no gonadotropin-inhibiting hormone
prolactin (PRL) - together with other hormones, initiates and maintains milk production by the mammary glands. Function in males unknown
- Ejection of milk from the mammary glands depends on oxytocin, released by the posterior pituitary.
-
PRH – prolactin-releasing hormone 1. During pregnancy, PRH released by the hypothalamus increases the prolactin level. iv. PIH – prolactin-inhibiting hormone
- In females, PIH is dopamine and inhibits release of prolactin from the anterior pituitary most of the time
- Level drops just before menstruation, which might account for breast tenderness.
Hypersecretion of prolactin: in females, causes galactorrhea (inappropriate lactation) and amenorrhea; in males, erectile dysfunction.
e. adrenocorticotropic hormone (ACTH) or corticotropin – secreted by corticotrophs.
i. ACTH controls the production and secretion of cortisol and other glucocorticoids by the cortex (outer portion) of the adrenal glands.
ii. corticotropin-releasing hormone (CRH) - from the hypothalamus
- Stimulates secretion of ACTH by corticotrophs.
- Stress-related stimuli, such as low blood sugar or trauma, and interleukin-1 also stimulate release of ACTH.
- Little circulating MSH in humans
iii. Glucocorticoids inhibit CRH and ACTH release via negative feedback.
melanocyte-stimulating hormone – exact role in humans unknown but presence of MSH receptors in the brain suggests it may affect brain activity.
- Continued administration of MSH over several days produces darkening of the skin
- Excessive levels of CRH can stimulate MSH release
- Dopamine inhibits MSH release
- Produced by corticotrophs
posterior pituitary or neurohypophysis – does not synthesize hormones but stores and releases 2 hormones. Consists of axons and axon terminals of more than 10,000 hypothalamic neurosecretory cells. Secretes Oxytocin and antidiuretic hormone
- The cell bodies of the neurosecretory cells are in the paraventricular and supraoptic nuclei of the hypothalamus
- hypothalamohypophyseal tract – a bundle of axons containing secretory vesicles filled with oxytocin or antidiuretic hormone that extends from the hypothalamus to the posterior pituitary
- The neuronal cell bodies of both the paraventricular and supraoptic nuclei synthesize oxytocin and antidiuretic hormone.
- oxytocin (OT) - secreted by neurosecretory cells in the paraventricular and supraoptic nuclei of the hypothalamus that stimulates contraction of smooth muscle in the pregnant uterus and myoepithelial cells around the ducts of mammary glands.
-
antidiuretic hormone (ADH) or vasopressin –secretion varies with blood osmotic pressure hormone produced by neurosecretory cells in the paraventricular and supraoptic nuclei of the hypothalamus that stimulates water reabsorption from kidney tubule cells into the blood and vasoconstriction of arterioles “antidiuretic” = retain fluid, decrease urine production
- In absence of ADH, urine production increases 10 fold, to 20 liters per day
- Alcohol inhibits secretion of ADH so drinking alcohol causes frequent and copious urination
- ADH also decreases water lost through sweating
- Both oxytocin and ADH are packaged into secretory vesicles after produced in the cell bodies of neurosecretory cells. They move by fast axonal transport to the axons terminals in the posterior pituitary, where they are stored until nerve impulses trigger exocytosis and release of the hormone.
- Pituicytes – supporting cell of the posterior pituitary Associated with the axon terminals in the posterior pituitary
control of secretion by the posterior pituitary
- Osmoreceptors – receptor in the hypothalamus that is sensitive to changes in blood osmolarity and, in response to high osmolarity (low water concentration), stimulates synthesis and release of ADH. i. Low osmotic pressure inhibits osmoreceptors which reduces or stops ADH secretion
- Secretion of ADH can be altered in other ways: pain, stress, trauma, anxiety, acetylcholine, nicotine, and some drugs such as morphine, tranqs, and some anesthetics stimulate ADH secretion.
- Hyposecretion of ADH or non functional ADH receptors cause diabetes insipidus
describe the location, histology, hormones, and functions of the thyroid gland.
thyroid gland – endocrine gland with right and left lateral lobes on either side of trachea connected by an isthmus, located anterior to the trachea just inferior to the cricoid cartilage. Secretes T4, T3, and calcitonin
- lateral lobes – left and right, one on either side of the trachea
- Isthmus – narrow strip of tissue connecting two larger parts. In the thyroid, connects the left and right lateral lobes
- Pyramidal lobe – in about 50% of thyroids, a third small lobe extends superiorly from the isthmus
thyroid follicles – microscopic spherical sacs that form the parenchyma of the thyroid gland and consists of follicular cells that produce T4 and T3.
follicular cells – found in the wall of each thyroid follicle.
- When inactive, shape is squamous to low cuboidal. When active, range from cuboidal to low columnar
- Produce two hormones: thyroxine, and triiodothyronine
thyroid hormones
- thyroxine or tetraiodothyronine or T4 – contains 4 atoms of iodine i. Hormone secreted by the thyroid gland that regulates metabolism, growth and development, and the activity of the nervous system.
- triiodothyronine or T3 – contains 3 atoms of iodine. i. Hormone produced by the thyroid gland that regulates metabolism, growth and development, and the activity of the nervous system.
parafollicular cells or C cells – lie between follicles
- Few in number.
- Produce the hormone calcitonin
Calcitonin – hormone produced by the parafollicular cells of the thyroid gland
- Can lower the amount of blood calcium and phosphates by inhibiting bone resorption and by accelerating uptake of calcium and phosphates into bone matrix.
formation, storage and release of thyroid hormones – the thyroid is the only endocrine gland that stores its secretory products in large quantities (normally about a 100 day supply)
- Traps iodine; the thyroid houses most of the body’s iodine supply
- thyroglobulin (TGB) - a large glycoprotein that is produced in the rough ER, modified in the Golgi complex, and packaged into secretory vesicles which undergo exocytosis and release TGB into the lumen of the follicle
- Oxidation of iodine, iodination of tyrosine, coupling of T1 and T2 to form T3 and T4. TGB with attached iodine atoms is a sticky material stored in the lumen of the thyroid follicle and is called colloid.
- Pinocytosis and digestion of colloid, secretion of thyroid hormones, transport in the blood.
actions of thyroid hormones – most body cells have receptors for T3 and T4, so they have effects throughout the body.
1. basal metabolic rate (BMR) - increased by thyroid hormones.
- The rate of oxygen consumption under standard or basal conditions (awake, at rest, and fasting)
- Thyroid hormones stimulate the use of cellular oxygen to produce ATP.
- When BMR is increased, cellular metabolism of carbohydrates, lipids, and proteins increases.
2. calorigenic effect – phenomenon in which cells produce and use more ATP, more heat is given off, and body temperature rises
- Thyroid hormones stimulate synthesis of additional Na-K pumps which use large amounts of ATP to continually eject Na and import K
- So thyroid hormones play an important role in the maintenance of normal body temperature.
control of thyroid hormone secretion – thyrotropin-releasing hormone from the hypothalamus and TSH from the anterior pituitary stimulate synthesis and release of thyroid hormones.
- Low blood levels of T3 and T4 or low metabolic rate stimulates the hypothalamus to secrete TRH.
- TRH enters the hypophyseal portal veins and flows to the anterior pituitary, where it stimulates thyrotrophs to secrete TSH.
- TSH stimulates virturally all aspects of thyroid follicular cell activity, including iodine trapping, hormone synthesis and secretion, and growth of follicular cells.
- Thyroid follicular cells release T3 and T4 into the blood until metabolic rate returns to normal
- An elevated level of T3 inhibits release of TRH and TSH
- Conditions that increase ATP demand- cold, hypoglycemia, high altitude, pregnancy, also increase the secretion of the thyroid hormones.
Calcitonin – produced by the parafollicular cells of the thyroid.
- Can decrease the level of calcium in the blood by inhibiting the action of osteoclasts.
- The secretion of CT is controlled by a negative feedback system.
- When blood level of CT is high, CT inhibits bone resorption, thus lowering the amount of blood calcium and phosphates.
- Miacalcin – a calcitonin extract derived from salmon, 10x more potent
- that human calcitonin, prescribed for osteoporosis.insipidus
describe the location, histology, hormone, and functions of the parathyroid glands.
parathyroid glands - usually 4, small endocrine glands embedded in the posterior surfaces of the lateral lobes of the thyroid gland.
Contain 2 kinds of cells: chief cells and oxyphil cell
chief (principal) cells - more numerous; produce parathyroid hormone PTH
parathyroid hormone (PTH) or parathormone - hormone secreted by the chief cells of the parathyroid glands that increases blood calcium level and decreased blood phosphate level.
- Major regulator of Ca2+, Mg2+, and phosphate (HPO4 2-) ions in the blood
- Specificially, PTH increases the number and activity of osteoclasts resulting in elevated bone resorption releasing Ca and phosphates into the blood.
- Also acts on the kidneys, slowing the rate at which Ca 2+ ad Mg2+ are lost in the urine and increasing loss of phosphates from blood into the urine.
- Finally, also promotes formation of hormone calcitriol in the kidneys.
Oxyphil cell – normal function unknown. Helps identify the parathyroid glands because of its unique staining characteristics In cancer of parathyroid glands, oxyphil cells secrete excess PTH.
Calcitriol - active form of vitamin D
- Formed in the kidneys in presence of PTH
- Increases rate of Ca2+, Mg2+ and phosphate absorption in the GI tract into the blood.
control of secretion of calcitonin and parathyroid hormone - controlled directly by the blood calcium level via negative feedback loops that do not involve the pituitary gland
- Higher than normal level of calcium ions in the blood stimulates parafollicular cells of the thyroid to release more calcitonin
- Calcitonin inhibits the activity of osteoclasts, thereby decreasing blood Ca2+ level.
- Lower-than-normal level of Ca2+ in the blood stimulates chief cells of the parathyroid gland to release more PTH
- PTH promotes bone resorption of bone extracellular matrix, which releases Ca2+ into the blood and slows loss of Ca2+ in the urine, raising the blood Ca2+ level
- PTH also stimulates the kidneys to synthesize calcitriol, the active form of Vitamin D
- Calcitriol stimulates increased absorption of Ca2+ from foods in the GI tract, which helps increase the blood level of Ca2+.
describe the location, histology, hormones, and functions of the adrenal glands.
adrenal glands - paired glands located superior to each kidney.
adrenal glands or suprarenal glands- superior to each kidney in the retroperitoneal space
- Have a flattened pyramidal shape
- During embryonic development, the adrenal glands differentiate into two distinct regions: large peripheral adrenal cortex (80- 90% of the gland) and a small centrally located adrenal medulla.
- Connective tissue capsule covers the gland
- The adrenal glands are highly vascularized
adrenal cortex - the outer portion of an adrenal gland.
- Produces steroid hormones essential for life.
- Divided into 3 zones:
1. zona glomerulosa - the outer zone, just deep to the connective tissue
capsule.
- Consists of closely packed cells arranged in spherical clusters and arched columns which secrete mineralocorticoids
- Mineralocorticoids - a group of hormones of the adrenal cortex that help regulate sodium and potassium balance. Aldosterone = major mineralocorticoid
-
zona fasciculate - middle zone, widest of the 3 zones, consists of cells arranged in long, straight columns. Secrete glucocorticoids
* Glucocorticoids - hormones secreted by the adrenal cortex, especially cortisol, that influence glucose metabolism. - zona reticularis - inner zone consisting of cords of branching cells that secrete sex hormones, mostly androgens
Androgens - masculinizing sex hormones produced by the testes in males and the adrenal cortex in both sexes, responsible for libido
- The two main androgens are testosterone and dihydrotestosterone.
Aldosterone - the major mineralocorticoid
- Regulates homeostasis of sodium and potassium ions
- Helps adjust blood pressure and blood volume
- Promotes excretion of H+ in the urine, helping prevent acidosis
renin angiotensin aldosterone (RAA) pathway - controls the secretion of aldosterone
- Initiating stimuli include dehydration, Na+ deficiency, or hemorrhage.
- These conditions cause a decrease in blood volume
- Decreased blood volume leads to decreased blood pressure
- Lowered blood pressure stimulates juxtaglomerular cells of the kidneys to excrete renin
- Renin - enzyme secreted by juxtaglomerular cells
- Level of renin in the blood increases
- Angiotensinogen - a plasma protein produced by the liver. Renin converts angiotensinogen into angiotensin I
- Blood containing increased levels of angiotensin I circulates the body
- Angiotensin-converting enzyme (ACE) converts angiotensin I to angiotensin II as blood flows through capillaries, particularly those of the lungs
- angiotensin II blood levels increase
- Angiotensin II stimulates the adrenal cortex to secrete aldosterone.
- Blood containing increased levels of aldosterone circulates to the kidneys
- In the kidneys, aldosterone increases reabsorption of Na+ which in turn causes reabsorption of water by osmosis. Therefore, less water is lost in the urine. Also stimulates the kidneys to increase K+ and H+ secretion into the urine.
- With increased water reabsorption by the kidneys, blood volume increases
- As blood volume increases, blood pressure increases to normal
- Angiotensin II also stimulates contraction of smooth muscle in the walls of arterioles. Vasoconstriction increases blood pressure also
- A second stimulator of aldosterone secretion is an increase in the K+ concentration of blood or interstitial fluid. A decrease in K+ level has the opposite effect.
Glucocorticoids - regulate metabolism and resistance to stress
- Include cortisol, corticosterone, and cortisone.
- cortisol or hydrocortisone - most abundant, 95% of glucocorticoid activity.
- corticosterone
- cortisone
- effects of glucocorticoids:
- Protein breakdown – increase the rate of protein breakdown, mainly in muscle fibers, and thus increase the liberation of amino acids into the blood stream. The amino acids may be used by body cells for synthesis of new proteins or for ATP production
- Glucose formation – on stimulation by glucocorticoids, liver cells may convert certain amino acids or lactic acid to glucose, which neurons and other cells can use for ATP production.
- 4.. Gluconeogenesis – conversion of a substance other than glycogen or another monosaccharide into glucose
- Lipolysis – glucocorticoids stimulate lipolysis – the breakdown of triglycerides and release of fatty acids from adipose tissue into the blood.
- Resistance to stress – glucocorticoids work in many ways to provide resistance to stress.
- The additional glucose supplied by the liver cells provides tissues with a ready source of ATP to
- combat a range of stresses including exercise, fasting, fright, temperature extremes, high altitude, bleeding, infection, surgery, trauma, and disease.
- Because glucocorticoids make blood vessels more sensitive to other hormones that cause vasoconstriction, they raise BP.
- This effect would be an advantage in cases of severe blood loss which causes BP to drop.
- Anti-inflammatory effects – inhibit white blood cells that participate in inflammatory responses.
- For this reason, glucocorticoids are prescribed for organ transplant recipient to retard tissue rejection by the immune system.
Unfortunately, glucocorticoids also retard tissue repair, as a result, they slow wound healing.
High doses can cause severe mental disturbances but glucocorticoids are useful in the treatment of chronic inflammatory disorders such as RA.
Depression of immune responses – high doses of glucocorticoids depress immune responses.
control of glucocorticoid secretion involves corticotropin releasing hormone (CRH) - occurs via negative feedback
- Low blood levels of glucocorticoids, mainly cortisol, stimulate neurosecretory cells in the hypothalamus to secrete corticotropin-releasing hormone (CRH)
- CRH together with low cortisol level, promotes the release of ACTH from the anterior pituitary
- ACTH flows in the blood to the adrenal cortex, where it stimulates glucocorticoid secretion
Androgens - in both sexes, the adrenal cortex secretes small amounts of weak androgens. The major androgen secreted by the adrenal gland isdehydroepiandrosterone (DHEA)
- In males, after puberty, the testes secrete lots of testosterone so the effects of the adrenal gland is insignificant
- In females, adrenal androgens play important roles:
- Promote libido
- Are converted into estrogens by other body tissues
- After menopause, all female estrogens come from conversion of adrenal androgens
- Also stimulate growth of axillary and pubic hair in boys and girls and contribute to the prepubertal growth spurt.
- The main hormone that stimulates adrenal androgen secretion is ACTH.
- A modified sympathetic ganglion of the autonomic nervous system
adrenal medulla
- Develops from the same embryonic tissue as all other sympathetic ganglia but its cells, which lack axons, form clusters around large blood vessels.
- Rather than releasing a neurotransmitter, the cells of the adrenal medulla secrete hormones.
- Produces 3 catecholamine hormones: norepinephrine, epinephrine, and a small amount of dopamine.
- chromaffin cells - the hormone producing cells
- Innervated by sympathetic preganglionic neurons of the ANS
- Because the ANS exerts direct control over the chromaffin cells, hormone release can occur very quickly.
epinephrine and norepinephrine (NE) - the two major hormones synthesized by the adrenal medulla
- Chromaffin cells secrete 80% epinephrine and 20% norepinephrine.
- These hormones intensify sympathetic responses that occur in other parts of the body
control of secretion of epinephrine and norepinephrine
- In stressful situations and during exercise, impulses from the hypothalamus stimulate sympathetic preganglionic neurons which in turn stimulate chromaffin cells to secrete epinephrine and norepinephrine
- These two hormones greatly augment the fight or flight response (ch 15)
- Epi and norepi increase the output of the heart by increasing heart rate and force of contraction. They also increase blood flow to the heart, liver, skeletal muscles, adipose tissue, dilate airways to the lungs, and increase blood levels of glucose and fatty acids.
describe the location, histology, hormones, and functions of the pancreatic islets.
pancreatic islets
I. Pancreas - soft, oblong organ lying along the greater curvature of the stomach, connected by a duct to the duodenum
- Both an exocrine (secreting pancreatic juice) and an endocrine (secreting insulin, glucagon, somatostatin, and pancreatic polypeptide) gland
- Consists of a head, body, and tail
- Roughly 99% of the exocrine cells of the pancreas are arranged in clusters called acini
- Highly vascular both endocrine and exocrine portions.
Acini - groups of cells in the pancreas that secrete digestive enzymes
- The enzymes flow into the GI tract through a network of ducts
pancreatic islets or islets of Langerhans - a cluster of endocrine gland cells in the pancreas that secretes insulin, glucagon, somatostatin, and pancreatic polypeptide.
- Scattered among the acini
- 1-2 million tiny clusters of endocrine tissue
Cell types in the pancreatic islets:
-
alpha or A cells - secrete glucagon
* Glucagon - a hormone produced by the alpha cells of the pancreatic islets that increases blood glucose level -
beta or B cells - secrete insulin
* Insulin - a hormone produced by the beta cells of the pancreatic islets that decreases blood glucose level -
delta or D cells - secretes somatostatin
* Somatostatin - hormone secreted by delta cells; acts in a paracrine manner to inhibit both insulin and glucagon release from neighbouring beta and alpha cells. Slows absorption of nutrients from GI tract
4. F cells - secretes pancreatic polypeptide
- pancreatic polypeptide - inhibits somatostatin secretion, gallbladder contraction, and secretion of digestive juices by the pancreas.
control of secretion of glucagon and insulin - level of blood glucose controls secretion of both glucagon and insulin via negative feedback
- Low blood glucose level stimulates secretion of glucagon from alpha cells of the pancreatic islets
- Glucagon acts on hepatocytes (liver cells) to accelerate the conversion of glycogen into glucose (glycogenolysis) and to promote formation of glucose from lactic acid and certain amino acids (gluconeogenesis)
- As a result, hepatocytes release glucose into the blood more quickly, raising blood glucose level
- If blood glucose level rises above normal, release of glucagon is inhibited (negative feedback)
- Insulin acts on various cells in the body to:
- accelerate facilitated diffusion of glucose into cells,
- to speed conversion of glucose into glycogen (glycogenesis),
- to increase uptake of amino acids by cells and to increase protein synthesis,
- to speed synthesis of fatty acids (lipogenesis),
- to slow the conversion of glycogen to glucose (glycogenolysis),
- and to slow the formation of glucose from lactic acid and amino acids (gluconeogenesis)
- If blood glucose level drops below normal, low blood glucose inhibits release of insulin (neg feedback) and stimulates release of glucagon
Insulin secretion is also stimulated by:
- Acetylcholine – neurotransmitter liberated from axon terminals of parasympathetic vagus nerve fibers that innervate the pancreatic islets
- The amino acids arginine and leucine, which would be present in blood at high levels after a protein-containing meal
- Glucose-dependent insulinotropic peptide (GIP) - a hormone released by enteroendocrine cells of the small intestine in response to the presence of glucose in the GI tract
-
Indirectly, hGH and ACTH stimulate secretion of insulin because they act to elevate blood glucose
- Digestion and absorption of food containing both carbohydrates and proteins provides strong stimulation for insulin release
Glucagon secretion is stimulated by:
- Increased activity of the sympathetic division of the ANS, as during exercise
- A rise in blood amino acids if blood glucose level is low, which could occur after a meal that contained only protein
describe the location, hormones, and functions of the male and female gonads.
Gonads – the organs that produce gametes; a gland that produces gametes and hormones.
Ovaries – female gonad that produces oocytes and the estradiol, estrogen, progesterone, inhibin, and relaxin hormones.
Estrogens - feminizing sex hormones produced by the ovaries.
- Govern development of oocytes
- Maintenance of female reproductive structures
- Appearance of secondary sex characteristics
- Affect fluid and electrolyte balance and protein anabolism
Progesterone - a female sex hormone produced by the ovaries
a. Helps prepare the endometrium of the uterus for implantation of a fertilized ovum and mammary glands for milk secretion
Inhibin - hormone secreted by the gonads that inhibits release of FSH by the anterior pituitary.
Relaxin - produced by the ovaries and placenta during pregnancy
- Increases flexibility of the pubic symphysis and helps dilate the cervix to ease delivery of baby.
testis (plural is testes) - male gonad that produces sperm and the hormones testosterone and inhibin
Testosterone - main hormones produced and secreted by the testes.
- Needed for development of sperm
- Stimulates descent of the testes before birth
- Together with a second androgen termed dihydrotestosterone (DHT), controls the growth and development of male reproductive organs, secondary sex characteristics, and body growth
Inhibin - produced by the testes
- Inhibits secretion of FSH
describe the properties of the pineal gland and thymus.
pineal gland - a cone-shaped endocrine gland located midline in the roof of the third ventricle that secretes melatonin
- Part of the epithalamus, positioned between the two superior colliculi
- Covered by a capsule formed by the pia mater
- Consists of masses of neuroglia and secretory cells
Pinealocytes - secretory cell of the pineal gland that releases melatonin
Melatonin - an amine hormone derived from serotonin that is secreted by the pineal gland and helps set the timing of the body’s biological clock.
- More is liberated during darkness than in light, thought to promote sleepiness
- During sleep, plasma levels of melatonin increase 10-fold and then decline to a low level before awakening
- Also a potent antioxidant that may provide some protection against damaging oxygen free radicals
Thymus - a bilobed organ, located in the superior mediastinum posterior to the sternum
and between the lungs. Not specifically an endocrine gland
- T cells develop immunocompetence here.
- thymosin, thymic humoral factor, thymic factor and thymopoietin – hormones produced by the thymus. Promote the maturation of T cells and may retard the aging process
outline the functions of each of the hormones secreted by cells in tissues and organs other than endocrine glands.
GASTROINTESTINAL TRACT
HORMONE Gastrin PRINCIPAL ACTIONS Promotes secretion of gastric juice; increases movements of the stomach.
HORMONE Glucose-dependent insulinotropic peptide (GIP) PRINCIPAL ACTIONS Stimulates release of insulin by pancreatic beta cells.
HORMONE Secretin PRINCIPAL ACTIONS Stimulates secretion of pancreatic juice and bile.
HORMONE Cholecystokinin (CCK) PRINCIPAL ACTIONS Stimulates secretion of pancreatic juice; regulates release of bile from gallbladder; causes feeling of fullness after eating.
PLACENTA
HORMONE Human chorionic gonadotropin (hCG) PRINCIPAL ACTIONS Stimulates corpus luteum in ovary to continue production of estrogens and progesterone to maintain pregnancy.
HORMONE Estrogens and progesterone PRINCIPAL ACTIONS Maintain pregnancy; help prepare mammary glands to secrete milk.
HORMONE Human chorionic somatomammotropin (hCS) PRINCIPAL ACTIONS Stimulates development of mammary glands for lactation.
KIDNEYS
HORMONE Renin PRINCIPAL ACTIONS Part of reaction sequence that raises blood pressure by bringing about vasoconstriction and secretion of aldosterone.
HORMONE Erythropoietin (EPO) PRINCIPAL ACTIONS Increases rate of red blood cell formation.
HORMONE Calcitriol*(active form of vitamin D)PRINCIPAL ACTIONS Aids in absorption of dietary calcium and phosphorus.
HEART
HORMONE Atrial natriuretic peptide (ANP) PRINCIPAL ACTIONS Decreases blood pressure.
ADIPOSE TISSUE
HORMONE Leptin PRINCIPAL ACTIONS Suppresses appetite; may increase FSH and LH activity.
