The Endocrine System - Collin College

The Endocrine System - Collin College

The Endocrine System Part A 1 Endocrine System: Overview Endocrine system the bodys second great controlling system which influences metabolic

activities of cells by means of hormones Endocrine glands pituitary, thyroid, parathyroid, adrenal, pineal, and thymus The pancreas and gonads produce both hormones and exocrine products Endocrine System: Overview The hypothalamus Secretes hormones Secretes regulatory hormones

Has autonomic centers that regulate adrenal medulla Other tissues and organs that produce hormones adipose cells, pockets of cells in the walls of the small intestine, stomach, kidneys, and heart Major Endocrine Organs Figure 16.1 Intercellular communication

Direct communication Through gap junctions Ions, small solutes, etc Limited to adjacent cells of the same type Synaptic communication Neurotransmitters Used in crises management Intercellular communication

Autocrine chemicals that exert effects on the same cells that secrete them Paracrine locally acting chemicals that affect cells other than those that secrete them Cytokines or local hormones These are not considered hormones since hormones are long-distance chemical signals Intercellular communication

Endocrine Hormones chemical substances secreted by cells into the extracellular fluids Regulate the metabolic function of other cells Have lag times ranging from seconds to hours Tend to have prolonged effects Types of Hormones Amino acid based

Amines, Thyroxine, ADH Oxytocin GH Prolactin Types of Hormones Lipid derivatives Steroids gonadal and adrenocortical hormones Eicosanoids leukotrienes and

prostaglandins Function as paracrine and autocrine factors Function as hormones Hormone Action Hormones that bind to receptors in the cell membrane:

First and second messengers Regulatory G proteins Water-soluble hormones Hormones that bind to intracellular receptors Direct gene activation Steroid and thyroid hormones The precise response depends on the type of the target cell Mechanism of Hormone Action Hormones produce one or more of the following cellular changes in target cells

Alter plasma membrane permeability Stimulate protein synthesis Activate or deactivate enzyme systems Induce secretory activity Stimulate mitosis Hormones that bind to receptors in the cell membrane: cAMP mechanism

Hormone (first messenger) binds to its receptor, which then binds to a G protein The G protein is then activated as it binds GTP, displacing GDP Activated G protein activates the enzyme adenylate cyclase Adenylate cyclase generates cAMP (second messenger) from ATP cAMP activates protein kinases, which then cause cellular effects Hormones that bind to receptors in the cell membrane : cAMP mechanism Extracellular fluid

Hormone A Adenylate cyclase Hormone B 1 1 2 GTP Receptor Gs Catecholamines ACTH

FSH LH Glucagon PTH TSH Calcitonin GTP GDP 3 GTP ATP


Gi Receptor cAMP 5 Inactive protein kinase A Active protein kinase A

Triggers responses of target cell (activates enzymes, stimulates cellular secretion, opens ion channels, etc.) Cytoplasm 14 Hormones that bind to receptors in the cell membrane: PIP-Calcium mechanism

Hormone binds to the receptor and activates G protein G protein binds and activates phospholipase Phospholipase splits the phospholipid PIP2 into diacylglycerol (DAG) and IP3 (both act as second messengers)

DAG activates protein kinases; IP3 triggers release of Ca2+ stores Ca2+ (second messenger) alters cellular responses Hormones that bind to receptors in the cell membrane: PIPCalcium mechanism Extracellular fluid Hormone DAG 1 2 Receptor Gq

GTP Catecholamines (1 receptors) TRH ADH GnRH Oxytocin Cytoplasm GTP 3

GTP GDP Phospholipase C PIP2 4 5 Active protein

kinase C Inactive protein kinase C IP3 Triggers responses of target cell 5 Endoplasmic reticulum

6 Ca2+ Ca2+- calmodulin Hormones that bind to intracellular receptors Steroid Hormones This interaction prompts DNA transcription to produce mRNA

The mRNA is translated into proteins, which bring about a cellular effect Steroid hormone Cytoplasm Steroid hormone Receptorchaperonin complex Receptor-hormone

complex Molecular chaperones Binding Hormone response elements Chromatin Transcription mRNA

mRNA Nucleus Ribosome Translation New protein 19 Target Cell Specificity

Hormones circulate to all tissues but only activate cells referred to as target cells Target cells must have specific receptors to which the hormone binds These receptors may be intracellular or located on the plasma membrane Target Cell Specificity

Examples of hormone activity ACTH receptors are only found on certain cells of the adrenal cortex Thyroxin receptors are found on nearly all cells of the body Target Cell Activation Target cell activation depends on three factors Blood levels of the hormone Relative number of receptors on the target cell The affinity of those receptors for the

hormone Target Cell Activation Up-regulation when a hormone causes the target cells to form more receptors to it. It causes the tissue to become more sensitive to the hormone. Down-regulation when a hormone cause the target cells to lose receptors to it. It causes the tissue to become less sensitive to the specific

hormone Hormone Concentrations in the Blood Hormones circulate in the blood in two forms: free or bound. Bound: Attached to plasma proteins Bound to their own carriers Hormone Concentrations in the Blood

Concentrations of circulating hormone reflect: Rate of release Speed of inactivation and removal from the body Hormones are removed from the blood by: Degrading enzymes The kidneys Liver enzyme systems Interaction of Hormones at Target Cells Permissiveness one hormone cannot exert its effects without another hormone being present.

Estrogen and thyroid hormone Synergism more than one hormone produces the same effects on a target cell Glucagon and epinephrine Antagonism one or more hormones opposes the action of another hormone. Glucagon and insulin Control of Hormone Release Blood levels of hormones:

Are controlled by negative feedback systems Vary only within a narrow desirable range Hormones are synthesized and released by glands in response to: Humoral stimuli Neural stimuli Hormonal stimuli Humoral Stimuli

Humoral stimuli secretion of hormones in direct response to changing blood levels of ions and nutrients Example: concentration of calcium ions in the blood Declining blood Ca2+ concentration stimulates the parathyroid glands to secrete PTH (parathyroid hormone) PTH causes Ca2+ concentrations to rise and the stimulus is removed Humoral Stimuli

Neural Stimuli Neural stimuli nerve fibers stimulate hormone release Preganglionic sympathetic nervous system (SNS) fibers stimulate the adrenal medulla to secrete catecholamines

Hormonal Stimuli Hormonal stimuli release of hormones in response to hormones produced by other endocrine organs The hypothalamic hormones stimulate the anterior pituitary In turn, pituitary hormones stimulate targets to secrete still more hormones Hormonal Stimuli

Nervous System Modulation The nervous system modifies the stimulation of endocrine glands and their negative feedback mechanisms Nervous System Modulation The nervous system can override normal endocrine controls For example, control of blood glucose levels

Normally the endocrine system maintains blood glucose Under stress, the body needs more glucose The hypothalamus and the sympathetic nervous system are activated to supply ample glucose Major Endocrine Organs: Pituitary (Hypophysis)

Pituitary gland two-lobed organ that secretes nine major hormones Neurohypophysis posterior lobe (neural tissue) and the infundibulum Receives, stores, and releases hormones from the hypothalamus Adenohypophysis anterior lobe, made up of glandular tissue Synthesizes and secretes a number of hormones Pituitary (Hypophysis)

Pituitary-Hypothalamic Relationships: Posterior Lobe The posterior lobe is a downgrowth of hypothalamic neural tissue Has a neural connection with the hypothalamus (hypothalamic-hypophyseal

tract) Nuclei of the hypothalamus synthesize oxytocin and antidiuretic hormone (ADH) These hormones are transported to the posterior pituitary Pituitary-Hypothalamic Relationships: Anterior Lobe The anterior lobe of the pituitary is an outpocketing of the oral mucosa

There is no direct neural contact with the hypothalamus Pituitary-Hypothalamic Relationships: Anterior Lobe There is a vascular connection, the hypophyseal portal system, consisting of: The primary capillary plexus The hypophyseal portal veins The secondary capillary plexus Pituitary-Hypothalamic Relationships:

Anterior Lobe Adenophypophyseal Hormones The six hormones of the adenohypophysis: Abbreviated as GH, TSH, ACTH, FSH, LH, and PRL Regulate the activity of other endocrine glands In addition, pro-opiomelanocortin (POMC): Has been isolated from the pituitary

Is split into ACTH, opiates, and MSH Activity of the Adenophypophysis The hypothalamus sends a chemical stimulus to the anterior pituitary Releasing hormones stimulate the synthesis and release of hormones Inhibiting hormones shut off the synthesis and release of hormones Activity of the Adenophypophysis

The tropic hormones that are released are: Thyroid-stimulating hormone (TSH) Adrenocorticotropic hormone (ACTH) Follicle-stimulating hormone (FSH) Luteinizing hormone (LH) Thyroid Stimulating Hormone (Thyrotropin)

Stimulates the normal development and secretory activity of the thyroid Triggered by hypothalamic peptide thyrotropinreleasing hormone (TRH) Rising blood levels of thyroid hormones act on the pituitary and hypothalamus to block the release of TSH Adrenocorticotropic Hormone (Corticotropin)

Stimulates the adrenal cortex to release corticosteroids Triggered by hypothalamic corticotropinreleasing hormone (CRH) in a daily rhythm Internal and external factors such as fever, hypoglycemia, and stressors can trigger the release of CRH Gonadotropins Gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH)

Regulate the function of the ovaries and testes FSH stimulates gamete (egg or sperm) production Absent from the blood in prepubertal boys and girls Triggered by the hypothalamic gonadotropin-releasing hormone (GnRH) during and after puberty Functions of Gonadotropins In females

LH works with FSH to cause maturation of the ovarian follicle LH works alone to trigger ovulation (expulsion of the egg from the follicle) LH promotes synthesis and release of estrogens and progesterone Functions of Gonadotropins In males LH stimulates interstitial cells of the testes to produce testosterone LH is also referred to as interstitial cellstimulating hormone (ICSH)

Growth Hormone (GH) Produced by anterior pituitary gland and: Stimulate most cells, but target bone and skeletal muscle Promote protein synthesis and encourage the use of fats for fuel Most effects are mediated indirectly by somatomedins or IGFs

Growth Hormone (GH) Antagonistic hypothalamic hormones regulate GH Growth hormonereleasing hormone (GHRH) stimulates GH release Growth hormoneinhibiting hormone (GHIH) or somatostatin inhibits GH release Metabolic Action of Growth Hormone

GH stimulates liver, skeletal muscle, bone, and cartilage to produce insulin-like growth factors (IGFs) Direct action promotes lipolysis and inhibits glucose uptake Metabolic Action of Growth Hormone (GH) Prolactin (PRL)

In females, stimulates milk production by the breasts Triggered by the hypothalamic prolactinreleasing hormone (PRH) Inhibited by prolactin-inhibiting hormone (PIH) Blood levels rise toward the end of pregnancy Suckling stimulates PRH release and encourages continued milk production The Posterior Pituitary and

Hypothalamic Hormones Posterior pituitary made of axons of hypothalamic neurons, stores antidiuretic hormone (ADH) and oxytocin ADH and oxytocin are synthesized in the hypothalamus ADH decreases urine formation

Oxytocin stimulates smooth muscle contraction in breasts and uterus Both use PIP-calcium second-messenger mechanism The Endocrine System PART B 56 Oxytocin

Oxytocin is a strong stimulant of uterine contraction Regulated by a positive feedback mechanism to oxytocin in the blood This leads to increased intensity of uterine contractions, ending in birth Oxytocin triggers milk ejection (letdown reflex) in women producing milk

Oxytocin Synthetic and natural oxytocic drugs are used to induce or hasten labor Plays a role in sexual arousal and satisfaction in males and nonlactating females Antidiuretic Hormone (ADH)

ADH helps to avoid dehydration or water overload Prevents urine formation Osmoreceptors monitor the solute concentration of the blood With high solutes, ADH preserves water With low solutes, ADH is not released, thus causing water loss Alcohol inhibits ADH release and causes copious urine output

Thyroid Gland The largest endocrine gland, located in the anterior neck, consists of two lateral lobes connected by a median tissue mass called the isthmus Composed of follicles that produce the

glycoprotein thyroglobulin Colloid (thyroglobulin + iodine) fills the lumen of the follicles and is the precursor of thyroid hormone Other endocrine cells, the parafollicular cells, produce the hormone calcitonin Thyroid Gland Thyroid Hormone Thyroid hormone major metabolic hormone

Consists of two related iodine-containing compounds T4 thyroxine; has two tyrosine molecules plus four bound iodine atoms T3 triiodothyronine; has two tyrosines with three bound iodine atoms Effects of Thyroid Hormone TH is concerned with: Glucose oxidation

Increasing metabolic rate Heat production TH plays a role in: Maintaining blood pressure Regulating tissue growth Developing skeletal and nervous systems Maturation and reproductive capabilities Synthesis of Thyroid Hormone

Thyroglobulin is synthesized and discharged into the lumen Iodides (I) are actively taken into the cell, oxidized to iodine (I2), and released into the lumen Iodine attaches to tyrosine, forming T1 (monoiodotyrosine, or MIT), and T2 (diiodotyrosine, or DIT) Synthesis of Thyroid Hormone Iodinated tyrosines link together to form T3 and

T4 Colloid is then endocytosed and combined with a lysosome, where T3 and T4 are cleaved and diffuse into the bloodstream Thyroid follicle cell Capillary 1 Thyroglobulin is synthesized

Colloid and discharged into the follicle lumen Colloid in lumen of follicle Golgi apparatus 3b Iodine is attached to tyrosine in colloid, forming DIT and MIT

Rough ER Iodine 2 Iodide (I) T3 T3 T4 oxidized to iodine Lysosome

T4 T3 DIT (T2) T4 MIT (T1) 4 Iodinated tyrosines are T3

linked together to form T3 and T4 5 Thyroglobulin colloid T4 6 Lysosomal enzymes cleave To peripheral tissues Thyroglobulin colloid

3a Iodide is is trapped (actively transported in) Iodide (I) T4 T3 T4 and T3 from thyroglobulin colloid and hormones diffuse from follicle cell into bloodstream is endocytosed and

combined with a lysosome 66 Transport and Regulation of TH T4 and T3 bind to thyroxine-binding globulins (TBGs) produced by the liver Both bind to target receptors, but T3 is ten times more active than T4

Peripheral tissues convert T4 to T3 Mechanisms of activity are similar to steroids Regulation is by negative feedback Hypothalamic thyrotropin-releasing hormone (TRH) can overcome the negative feedback

Calcitonin A peptide hormone produced by the parafollicular, or C cells Lowers blood calcium levels in children Antagonist to parathyroid hormone (PTH) Regulated by a humoral (calcium ion concentration in the blood) negative feedback

mechanism Calcitonin Calcitonin targets the skeleton, where it: Inhibits osteoclast activity (and thus bone resorption) and release of calcium from the bone matrix Increases calcium excretion by the kidneys Stimulates calcium uptake and incorporation into the bone matrix Parathyroid Glands

Tiny glands embedded in the posterior aspect of the thyroid Cells are arranged in cords containing oxyphil and chief cells Chief (principal) cells secrete PTH PTH (parathormone) regulates calcium balance in the blood

Parathyroid Glands Effects of Parathyroid Hormone PTH release increases Ca2+ in the blood as it: Stimulates osteoclasts to digest bone matrix Enhances the reabsorption of Ca2+ and the secretion of phosphate by the kidneys Increases absorption of Ca2+ by intestinal mucosal

Rising Ca2+ in the blood inhibits PTH release Effects of Parathyroid Hormone Adrenal (Suprarenal) Glands Adrenal glands paired, pyramid-shaped organs atop the kidneys Structurally and functionally, they are two glands in one Adrenal medulla neural tissue that acts as

part of the SNS Adrenal cortex glandular tissue derived from embryonic mesoderm Adrenal Cortex Synthesizes and releases steroid hormones called corticosteroids Different corticosteroids are produced in each of the three layers Zona glomerulosa mineralocorticoids (chiefly aldosterone)

Zona fasciculata glucocorticoids (chiefly cortisol) Zona reticularis gonadocorticoids (chiefly androgens) Adrenal Cortex Mineralocorticoids Regulate electrolytes in extracellular fluids Aldosterone most important mineralocorticoid

Maintains Na+ balance by reducing excretion of sodium from the body Stimulates reabsorption of Na+ and secretion of K+ by the kidneys Mineralocorticoids Aldosterone secretion is stimulated by: Rising blood levels of K+ Low blood Na+ Decreasing blood volume or pressure The Four Mechanisms of Aldosterone

Secretion Renin-angiotensin mechanism kidneys release renin, which is converted into angiotensin II that in turn stimulates aldosterone release Plasma concentration of sodium and potassium directly influences the zona glomerulosa cells ACTH causes small increases of aldosterone

during stress Atrial natriuretic peptide (ANP) inhibits activity of the zona glomerulosa Major Mechanisms of Aldosterone Secretion Glucocorticoids (Cortisol) Help the body resist stress by: Keeping blood sugar levels relatively

constant Maintaining blood volume and preventing water shift into tissue Cortisol provokes: Gluconeogenesis (formation of glucose from noncarbohydrates) Rises in blood glucose, fatty acids, and amino acids Excessive Levels of Glucocorticoids Excessive levels of glucocorticoids: Depress cartilage and bone formation

Inhibit inflammation Depress the immune system Promote changes in cardiovascular, neural, and gastrointestinal function Gonadocorticoids (Sex Hormones) Zona Reticularis produces mainly androgens that are converted to testosterone in the testes Androgens contribute to:

The onset of puberty The appearance of secondary sex characteristics Sex drive in females Androgens can be converted into estrogens after menopause Adrenal Medulla Made up of cells that secrete epinephrine and norepinephrine

Secretion of these hormones causes: Blood glucose levels to rise Blood vessels to constrict The heart to beat faster Blood to be diverted to the brain, heart, and skeletal muscle Adrenal Medulla Epinephrine is the more potent stimulator of the heart and metabolic activities

Norepinephrine is more influential on peripheral vasoconstriction and blood pressure Stress and the Adrenal Gland Pancreas

A triangular gland, which has both exocrine and endocrine cells, located behind the stomach Acinar cells produce an enzyme-rich juice used for digestion (exocrine product) Pancreatic islets (islets of Langerhans) produce hormones (endocrine products) The islets contain two major cell types: Alpha () cells that produce glucagon Beta () cells that produce insulin Pancreas Glucagon

Its major target is the liver, where it promotes: Glycogenolysis the breakdown of glycogen to glucose Gluconeogenesis synthesis of glucose from lactic acid and noncarbohydrates Release of glucose to the blood from liver cells Insulin

Synthesized as part of proinsulin and then excised by enzymes, releasing functional insulin Insulin: Lowers blood glucose levels Enhances transport of glucose into body cells Counters metabolic activity that would enhance blood glucose levels Effects of Insulin Binding

The insulin receptor is a tyrosine kinase enzyme After glucose enters a cell, insulin binding triggers enzymatic activity that: Catalyzes the oxidation of glucose for ATP production Polymerizes glucose to form glycogen Converts glucose to fat (particularly in adipose tissue) Regulation of Blood Glucose Levels

The hyperglycemic effects of glucagon and the hypoglycemic effects of insulin Diabetes Mellitus (DM) Results from hyposecretion or hypoactivity of

insulin The three cardinal signs of DM are: Polyuria huge urine output Polydipsia excessive thirst Polyphagia excessive hunger and food consumption Hyperinsulinism excessive insulin secretion, resulting in hypoglycemia Diabetes Mellitus (DM) Gonads: Female

Paired ovaries in the abdominopelvic cavity produce estrogens and progesterone They are responsible for: Maturation of the reproductive organs Appearance of secondary sexual characteristics Breast development and cyclic changes in the uterine mucosa Gonads: Male

Testes located in an extra-abdominal sac (scrotum) produce testosterone Testosterone: Initiates maturation of male reproductive organs Causes appearance of secondary sexual characteristics and sex drive Is necessary for sperm production Maintains sex organs in their functional state Pineal Gland

Small gland hanging from the roof of the third ventricle of the brain Secretory product is melatonin Melatonin is involved with: Day/night cycles Physiological processes that show rhythmic variations (body temperature, sleep, appetite) Thymus

Lobulated gland located deep to the sternum Major hormonal products are thymopoietins and thymosins These hormones are essential for the development of the T lymphocytes (T cells) of the immune system Other Hormone-Producing Structures

Heart produces atrial natriuretic peptide (ANP), which reduces blood pressure, blood volume, and blood sodium concentration Gastrointestinal tract enteroendocrine cells release local-acting digestive hormones Placenta releases hormones that influence the course of pregnancy Other Hormone-Producing Structures

Kidneys secrete erythropoietin, which signals the production of red blood cells Skin produces cholecalciferol, the precursor of vitamin D Adipose tissue releases leptin, which is involved in the sensation of satiety, and stimulates increased energy expenditure

Developmental Aspects Exposure to pesticides, industrial chemicals, arsenic, dioxin, and soil and water pollutants disrupts hormone function Sex hormones, thyroid hormone, and glucocorticoids are vulnerable to the effects of pollutants Interference with glucocorticoids may help

explain high cancer rates in certain areas Developmental Aspects Ovaries undergo significant changes with age and become unresponsive to gonadotropins Female hormone production declines, the ability to bear children ends, and problems associated with estrogen deficiency (e.g.,

osteoporosis) begin to occur Testosterone also diminishes with age, but effect is not usually seen until very old age Developmental Aspects GH levels decline with age and this accounts for muscle atrophy with age Supplemental GH may spur muscle growth, reduce body fat, and help physique

TH declines with age, causing lower basal metabolic rates PTH levels remain fairly constant with age, and lack of estrogen in women makes them more vulnerable to bone-demineralizing effects of PTH

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