The Endocrine System

Chapter 9

Overview• Function: coordinates

functions of body with nervous system• Hormones: chemical

substances that are secreted by endocrine cells into ECF• Mediator molecule released in

one part of the body but regulates activity in other parts of the body

• Regulate metabolic activity of other cells

• Affects target cells/organs

Endocrine vs. Nervous

Endocrine Nervous

Release hormones as chemical signal

Electrical impulse transmitted and releases neurotransmitters as chemical signal

Work more slowly Work more quickly

Effector: any cell in the body (many times produced in one part of the body and affects another)

Effectors are muscles/glands

Control of Hormone Release• Negative feedback mechanisms: chief means of

regulating blood levels of nearly all hormones• Stimulus triggers hormone secretion and rising levels

inhibit further release• Maintaining homeostasis! (levels of hormones within a

narrow range)

• Positive feedback can also control (i.e. oxytocin)• Stimuli that activate endocrine organs:

• Hormonal: stimulated by other hormones• i.e. hypothalamic hormones stimulate anterior pituitary

• Humoral: stimulated by changing blood levels of certain ions and nutrients• i.e. blood calcium levels stimulate release of parathyroid

hormone

• Neural: stimulated by nerve fibers• i.e. sympathetic nervous system stimulates adrenal medulla

Endocrine & Exocrine Glands• Glandular tissue is epithelial tissue!• Endocrine gland: secrete hormones directly

into internal environment• No ducts• Secrete hormones

• Exocrine glands: secrete substances into external environment• Have ducts• Secrete substances such as sweat, oil, wax,

enzymes, etc.

Hormone Receptors

• Receptor: protein on cell membrane of target cell• i.e. insulin (secreted by pancreas) – liver has

insulin-specific receptors that receive insulin and tell liver to take sugar in and transfer to glycogen for storage

• Two types of hormones:• Circulating: deposited into fluid/blood, circulates

through body• Local: acts on neighboring cells or itself

(paracrine – neighboring) or autocrine (targets itself)

Circulating Local

Water-soluble vs. Lipid-soluble• Chemical classes determine how easily the hormone

gets into and moves through the blood stream and the cell membrane

• Lipid-soluble: nonpolar• Hard to transport because fluids are water-based; need to

be bound to transport protein (“chaperone”) that allows it to move around

• Once it gets to target cell, can move across membrane without protein channel

• i.e. steroid hormones

• Water-soluble: polar• No help needed for transport• Cannot go through membrane of target cell – needs a

protein receptor; binds to receptor and activates second messenger system

• i.e. amines, peptide/protein, glycoproteins, eicosanoids

WaterSoluble:• amine• peptide• eicosanoid

LipidSoluble:• steroid• thyroid• NO

“free” transport Bound to transportprotein

Major Endocrine Organs• Hypothalamus

• Major link between nervous and endocrine system!)

• Pituitary• Thyroid• Parathyroid• Adrenal Gland• Pineal Glands• Thymus• Pancreas• Gonads

• Ovaries• Testes

Pituitary Gland• Hangs from inferior surface of

hypothalamus• Some hormones released are tropic hormones: stimulate other endocrine glands to release hormones• Two lobes:

• Anterior pituitary (glandular tissue)• “master endocrine gland” –

controls so many others!• Posterior pituitary (nervous

tissue)• stores 2 hormones for

hypothalamus and secretes them

Hormones of the Anterior Pituitary• Hormones secreted with non-endocrine

targets• Growth hormone (GH): general metabolic

hormone, growth of skeletal muscles and long bones of the body, promotes healing and increases ATP• Pituitary dwarfism: hyposecretion during childhood• Gigantism: hypersecretion during childhood• Acromegaly: hypersecretion after growth plates

closed

• Prolactin (PRL): targets mammary glands in females to stimulate and maintain milk production after childbirth

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Pygmy

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Anterior Pituitary Hormones• Tropic hormones

• Adrenocorticotropic hormone (ACTH): regulates endocrine activity of the adrenal cortex

• Thyroid-stimulating hormone (TSH): influences growth & activity of thyroid, stimulates release of thyroid hormones (aka thyrotropic hormone or TH)

• Gonadotropic hormones (hyposecretion causes sterility)• Follicle-stimulating hormone (FSH): stimulates follicle

development in ovaries and sperm development by testes

• Luteinizing hormone (LH): triggers ovulation of egg from ovary and production of progesterone and estrogen; in men, stimulates testosterone production by testes

Hormones of the Posterior Pituitary• Posterior pituitary does not make hormones – it stores

them for the hypothalamus• Hypothalamus makes two hormones that are

transported via neurosecretory cells to be stored in the posterior pituitary

• Oxytocin: released in significant amounts only during childbirth and in nursing women; contractions of uterus & milk ejection• Pitocin is synthetic form: used for inducing labor and to

control hemorrhage and increase uterine muscle tone

• Antidiuretic hormone (ADH): inhibits or prevents urine production and causes kidneys to reabsorb more water from the forming urine

Hypothalamus & the Pituitary Gland

Thyroid Gland• Located at the base of the throat, inferior to the Adam’s

apple• Thyroid hormone - two active iodine-containing

hormones thyroxine (T4) & triiodothyronine (T3)• targets all cells• controls rate at which glucose is converted to energy

(increases basal metabolic rate)• Maintains body temperature• important for normal tissue growth and development;

accelerates body growth• Goiter: enlarged thyroid gland resulting in diet deficient in

iodine• Cretinism (hypothyroidism)• Graves’ disease (hyperthyroidism)

• Calcitonin: inhibits osteoclasts and decreases calcium level in the blood; causes calcium to be deposited in the bone

Before

After

Goiter

Parathyroid Glands• Glandular tissue found on posterior surface

of thyroid gland (“piggybacks” the thyroid)• Parathyroid hormone (PTH): regulates

ions (calcium, magnesium, phosphate); antagonist to calcitonin• Most important regulator of blood calcium

levels!• Increases blood calcium levels by promoting

resorption of calcium from bone matrix into blood (increases activity of osteoclasts)• Also acts on kidneys – slows loss of ions from

blood to urine

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Adrenal Glands

• Two bean-shaped glands that curve over the top of the kidneys• Functionally two organs in one: • Adrenal cortex (glandular)

• Corticosteroids (mineralocorticoids, glucocorticoids, sex hormones)

• Adrenal medulla (neural)• When stimulated by sympathetic nervous system,

releases catecholamines• epinephrine (adrenaline) • norepinephrine (noradrenaline)

• Stimulated in times of stress (“fight or flight”)• Allows body to better deal with short-term stressors

Hormones of the Adrenal Cortex• Mineralocorticoids: regulate mineral (salt)

content of the blood (Na+/K+ ions) – target kidney tubules to regulate electolyte and water balance in body• Aldosterone

• Glucocorticoids: promote normal cell metabolism and help body to resist long-term stressors (increase blood glucose levels); anti-inflammatory properties• Cortisone• Cortisol

• Sex hormones• Androgens (male - testosterone)• Estrogens (female)

Stress Response1. Fight or flight (Sympathetic Nervous

System) - adrenaline2. Resistance reaction - release hormones to

fight stress (cortisols) but no adrenaline rush

3. Exhaustion – prolonged exposure to cortisol leads to problems• Cardiovascular: high blood pressure• immunosuppression

Adrenal Cortex Disorders

• Addison’s Disease (hyposecretion of adrenal cortex hormones)• Cushing’s Syndrome (hypersecretion of

cortisol)• Congenital Adrenal Hyperplasia (CAH):

genetic disorder (don’t make cortisol)

• PTSD – stress hormones produced after traumatic event (i.e. cortisol)

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Pancreatic Islets

• Pancreas is a mixed gland• Pancreatic islets (endocrine – also called the islets of

Langerhans) • Pancreas also produces enzymes for the digestive

system (exocrine)

• Insulin – released by beta cells in response to high levels of glucose (acts on all body cells) – decreases blood glucose levels• Diabetes mellitus (hyposecretion)

• Glucagon – antagonist of insulin; released by alpha cells; helps regulate blood glucose levels by increasing them (primarily targets liver, stimulating it to break down stored glycogen)

Pancreatic Islets

Pineal Gland• Small, cone-shaped gland that hangs from

roof of third ventricle of brain (diencephalon - epithalamus)• Melatonin – “sleep trigger” that plays an

important role in establishing the body’s day-night cycle; sets “biological clock” and rhythms• Light can affectgland – SeasonalAffective Disorder& Jet Lag

Thymus

• Located in upper thorax, posterior to sternum (lymphatic organ)• Thymosin –

promotes normal development of T lymphocytes (immunity); may slow aging process

Gonads• Produce sex hormones identical to those produced by

adrenal cortex• Release of hormones stimulated by anterior pituitary

gonadotropins• Hormones of the ovaries

• Estrogens: development of sex characteristics in women (growth and maturation of reproductive organs) and the appearance of secondary sex characteristics at puberty

• Progesterone: acts with estrogen to bring about menstrual cycle, prepares mammary tissue for lactation, maintains pregnancy

• Hormones of the testes• Androgens (testosterone): promotes growth and

maturation of the reproductive system organs at puberty; causes secondary sex characteristics to appear; necessary for production of male gametes

• Hyposecretion of these hormones causes sterility

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Other Tissues/Organs• Hormone-producing cells are found in fatty tissue and

in walls of small intestine, stomach, kidneys, and heart (Table 9.2, page 331)

• Placenta: organ formed temporarily in uterus of pregnant women that acts as a respiratory, excretory, and nutrition-delivery system for fetus• Produces several hormones that help to maintain the

pregnancy and prepare for delivery of the baby• Human chorionic gonadotropin (hCG): stimulates the

ovaries to continue producing estrogen and progesterone so that the lining of the uterus is not sloughed off in menses

• Human placental lactogen (hPL) – works with estrogen and progesterone in preparing the breasts for lactation

• Relaxin: causes mother’s pelvic ligaments and the pubic symphysis to relax and become more flexible, which eases birth passage