Chapter 17: The Endocrine System

Endocrine system?

Consists of glands, tissues, and cells that secrete → hormones

Hormones?

Are chemical messengers that are transported by the bloodstream

Stimulate physiological responses in cells of another tissue or organ

Endocrine glands?

No ducts

Contain dense, porous capillary networks, which allow for easy uptake of hormones into bloodstream

“Internal secretions”

Intracellular effects such as altering target cell metabolism

Nervous system and Endocrine system both do what?

Both systems serve for internal communication

Nervous? (Speed and persistence of response)

Reacts quickly (ms timescale), stops quickly

Endocrine? (Speed and persistence of response)

Reacts slowly (seconds or days), effect may continue for days or longer

Nervous? (Adaptation to long-term stimuli)

Response declines (adapts quickly)

Endocrine? (Adaptation to long-term stimuli)

Response persists (adapts slowly)

Nervous? (Area of effect)

Targeted and specific (one organ)

Endocrine? (Area of effect)

General, widespread effects (many organs)

Several chemicals function as what?

As both hormones and neurotransmitters

Norepinephrine, dopamine, and antidiuretic hormone

Both systems can have similar what?

Similar effects on target cells

Both norepinephrine and glucagon cause glycogen hydrolysis in liver

The two systems can do what?

Regulate each other

Neurotransmitters can affect glands, and hormones can affect neurons

Neuroendocrine cells share what?

Share characteristics with both systems

Neuron-like cells that secrete oxytocin into blood

Target organs or cells?

Those organs or cells that have receptors for a hormone and can respond to it

Some target cells possess enzymes that convert a circulating hormone to its more active form

The hypothalamus forms what?

The floor and walls of the third ventricle of brain

Regulates primitive functions from water balance and thermoregulation to sex drive and childbirth

Many of its functions carried out by the → pituitary gland (Hypophysis)

The pituitary gland is suspended from hypothalamus by a stalk called the what?

Infundibulum

What is composed of two structures with independent origins and separate functions?

The pituitary gland (hypophysis)

Adenohypophysis (anterior pituitary)

Neurohypophysis (posterior pituitary)

Adenohypophysis (anterior lobe)?

Makes up the anterior 3/4th of the pituitary

Linked to hypothalamus by hypophyseal → portal system

Capillaries in the hypothalamus are connected to capillaries in adenohypophysis by portal venules

Hypothalamic hormones regulate what?

Adenohypophysis cells

Hypothalamic-releasing and - inhibiting hormones travel in hypophyseal portal system from hypothalamus to anterior pituitary

These hormones regulate what is produced by the cells of the anterior pituitary

Neurohypophysis (posterior lobe)?

Makes up the posterior 1/4th of the pituitary

Nervous tissue, not a true gland

Nerve axons in the hypothalamus do what?

Moves down the stalk in the hypothalamo–hypophyseal tract and ends in the posterior lobe

Hypothalamic neurons secrete what?

Secrete hormones that are stored in neurohypophysis until released into blood

How many hormones are produced in the hypothalamus?

Eight

How many hormones regulate the anterior pituitary?

Six

How many hormones are released into and stored in the posterior pituitary?

Two

Anterior lobe of the pituitary does what?

Synthesizes and secretes six hormones

Two gonadotropin hormones that target the what?

Gonads

Follicle-stimulating hormone (FSH)

Luteinizing hormone (LH)

Thyroid-stimulating hormone (TSH)?

Stimulates the thyroid gland to secrete→ thyroid hormone

Adrenocorticotropic hormone (ACTH)?

Stimulates adrenal cortex of the adrenal gland to secrete → glucocorticoids

Prolactin (PRL)?

After birth, stimulates mammary glands of females to synthesize → milk

Growth hormone (GH)?

Stimulates mitosis and cellular → differentiation

Effects fat, muscle, and bone

Two hormones are produced in the hypothalamus and transported to the posterior lobe of pituitary for?

Storage

The hormones are released when hypothalamic neurons are?

Stimulated

ADH (Antidiuretic Hormone)?

Increases water retention, which reduces urine volume, preventing dehydration

Also called vasopressin because it can cause vasoconstriction

Oxytocin (OT)?

Surge of hormone released during sexual → arousal and orgasm

Promotes feelings of sexual satisfaction and emotional bonding between partners

Stimulates labor contractions during childbirth

Stimulates flow of milk during lactation

May promote emotional bonding between lactating mother and infant

Rates of secretion are not what?

Constant

Regulated by hypothalamus, other brain areas, and feedback from target organs

Brain monitors conditions and influences what?

Anterior pituitary accordingly

In times of stress, hypothalamus triggers release of → ACTH

During pregnancy, hypothalamus triggers prolactin secretion

Posterior pituitary is controlled by?

Neuroendocrinere flexes

Hypothalamic osmoreceptors trigger release of?

ADH when they detect a rise in blood osmolarity (NaCl levels)

Infant suckling triggers hypothalamic response to release?

Oxytocin

Negative feedback inhibition?

Increased target organ hormone levels inhibit release of hypothalamic and/or pituitary hormones

Hypothalamus secretes what?

Thyrotropin- releasing hormone (TRH)

TRH stimulates the?

The anterior pituitary to secrete thyroid-stimulating hormone (TSH)

What gland does TSH stimulate?

Stimulates the thyroid gland to secrete thyroid hormone (TH)

TH stimulates?

Metabolism

TH can also inhibit what?

TRH and TSH release

Stretching of uterus increases what?

Oxytocin release, causes contractions, causing more stretching of uterus until delivery

Positive feedback

Pineal gland?

Attached to roof of the third ventricle beneath the posterior end of corpus callosum

May synchronize physiological function with 24-hour circadian rhythms of daylight and darkness

Synthesizes melatonin from serotonin during the night

Fluctuates seasonally with changes in day length

Pineal gland may influence timing of puberty in humans

What three systems does the thymus play a role in?

Endocrine, lymphatic, and immune

Bilobed gland in the mediastinum superior to the heart

Site of maturation of T cells important in immune defense

Secretes hormones (thymopoietin, thymosin, and thymulin) that stimulate development of other lymphatic organs and activity of T lymphocytes

Thyroid gland?

Largest gland that is 100% endocrine

Composed of two lobes and an isthmus, below the larynx

Thyroid follicles?

Sacs that make up most of thyroid

Contain protein-rich → colloid

Follicular cells: simple cuboidal epithelium that lines follicles

Secretes thyroxine (T4) and triiodothyronine (T3) in response to → TSH

Increases metabolic rate, O2 consumption, heat production (calorigenic effect), appetite, growth hormone secretion, alertness, reflex speed

Parafollicular (C or clear)?

Cells secrete calcitonin when blood calcium levels are → high

Stimulates osteoblast activity and bone formation in children

Parathyroid glands?

On average consist of four glands partially embedded in posterior surface of thyroid gland

Parathyroid glands secrete what?

Parathyroid hormone (PTH) when calcium levels are → low

Increases blood Ca2+ levels

Promotes synthesis of → calcitriol

Increases absorption of Ca2+

Decreases urinary excretion

Increases bone resorption

Adrenal (suprarenal) glands?

Cap on top of each → kidney

Inner core – adrenal → medulla

Outer surrounding – adrenal → cortex

Adrenal medulla?

Inner core, it acts as an endocrine gland and a ganglion of the sympathetic nervous system

Innervated by sympathetic preganglionic fibers

Consists of modified sympathetic postganglionic neurons called → chromaffin cells

When stimulated, release catecholamines (epinephrine and norepinephrine) and a trace of dopamine directly into the bloodstream

As hormones, catecholamines have multiple effects?

Increase alertness and prepare body for physical activity

Mobilize high-energy fuels, lactate, fatty acids, and glucose

Glycogenolysis (breakdown of glycogen to glucose) and gluconeogenesis (creation of glucose by non- carbohydrate sources) by liver

Boosts glucose levels

Epinephrine inhibits what?

Inhibits insulin secretion

Muscles use fatty acids, saving glucose for the → brain

Increase blood pressure, heart rate, blood flow to muscles, pulmonary airflow, and metabolic rate

Decrease digestion and urine production

Adrenal Cortex?

Cortex surrounds medulla and secretes several corticosteroids (hormones) from three layers of glandular tissue

Mineralocorticoids?

Steroid hormones that regulate electrolyte balance

Aldosterone stimulates Na+ retention and K+ excretion

Water is retained with sodium by osmosis, so blood volume and blood pressure are → maintained

Part of the renin-angiotensin-aldosterone (RAA) system

Glucocorticoids?

Secreted in response to → ACTH

Regulate metabolism of glucose and other fuels

Cortisol and corticosterone stimulate fat and protein catabolism, gluconeogenesis (glucose from amino acids and fatty acids) and release of fatty acids and glucose into → blood

Help body adapt to stress and repair tissues

Anti-inflammatory effect causes immunosuppression with

long-term use

Pancreas?

Elongated gland, below and behind the → stomach

Exocrine digestive gland with endocrine cell clusters of pancreatic → islet cells

Glucagon?

Secreted by → alpha (α) cells

Released between meals when blood glucose concentration is → falling

In liver, stimulates gluconeogenesis, glycogenolysis, and the release of glucose into circulation, raising blood glucose level

In adipose tissue, stimulates fat catabolism and release of free fatty acids

Insulin?

Secreted by → beta (β) cells

Secreted during and after meal

When glucose and amino acid blood levels are → rising

Stimulates cells to absorb nutrients and store or metabolize them, lowering blood glucose levels

Promotes synthesis glycogen, fat, and protein

Suppresses use of already-stored fuels

Brain, liver, kidneys, and RBCs absorb glucose without insulin, but other tissues require insulin

Insufficiency or inaction is the cause of diabetes mellitus

Gonads?

Ovarian hormones

Estradiol (estrogens), progesterone, and inhibin

Testicular hormones

Testosterone and inhibin

What are the Endocrine functions of other Tissues?

Skin, Liver, Kidneys, Heart, Stomach, Adipose tissue, Osseous tissue, Placenta

Steroids?

Made from → cholesterol

Sex steroids (estrogen) from gonads and corticosteroids (cortisol) from adrenals

Monoamines (biogenic amines)?

Made from → amino acids

Catecholamines (dopamine, epinephrine, norepinephrine), melatonin, thyroid hormone

Peptides and glycoproteins?

Created from chains of amino acids

Examples include hormones from both lobes of the pituitary, and releasing and inhibiting hormones from hypothalamus

Insulin is a large peptide hormone

Hormones are not what?

Are not secreted at steady rates

Levels in the bloodstream can fluctuate throughout the day

Some are secreted in some cases on a daily (circadian) rhythm

Some on a monthly rhythm (in a woman’s ovarian cycle)

Others under the influence of stimuli that signify a need for them:

Neural stimuli, Hormonal stimuli, Humoral stimuli

Neural Stimuli?

Nerve fibers supply some endocrine glands and cause the release of their hormones

The sympathetic nervous system stimulates the adrenal medulla to secrete epinephrine and norepinephrine in situations of → stress

In childbirth, nerve signals originate from stretch receptors in the uterus, travel up the spinal cord and brainstem to the hypothalamus, and stimulate the release of → oxytocin

Hormonal Stimuli?

Hormones from the hypothalamus regulate secretions of the anterior pituitary gland

Pituitary hormones stimulate other endocrine glands to release thyroid hormone, sex hormones, and cortisol

Humoral Stimuli?

Refers to blood-borne stimuli

Rising blood glucose concentration stimulates the release of → insulin

Low blood osmolarity (NaCl) stimulates the secretion of → aldosterone

Low blood calcium levels stimulates the secretion of → parathyroid hormone (PTH)

Peptide hormones and catecholamines are what?

Are stored in secretory vesicles of the endocrine cell until → needed

Released by exocytosis when the cell receives a → stimulus

Steroid hormones are not what?

Are not stored in vesicles or released by exocytosis

They are released as fast as they are synthesized by diffusion through the cell surface

Most monoamines and peptides are?

Hydrophilic

Mix easily with blood plasma

Steroids and thyroid hormone are?

Hydrophobic

Must bind to transport proteins (albumins and globulins synthesized by the liver)

Hormones stimulate cells that have?

Receptors for them

Receptors are what?

Protein or glycoprotein molecules

On plasma membrane, in the cytoplasm, or in the nucleus

Receptors act like switches, turning on metabolic pathways when hormone binds to them

Usually each target cell has a few thousand receptors for a hormone

Receptor–hormone interactions have specificity and saturation

Specific receptor for each hormone

Saturated when all receptor molecules are occupied by hormone molecules

Peptides and catecholamines?

Cannot penetrate target cell

Bind to surface receptors and activate intracellular processes through second messengers

Steroids and thyroid hormone?

Penetrate the plasma membrane and bind to internal receptors (usually in nucleus)

Influence expression of genes of the target cell

Take several hours to days to show effect due to lag for protein synthesis

Target-cell sensitivity adjusted by changing the?

Number of receptors

Up-regulation means number of receptors is?

Increased

Sensitivity is → increased

Down-regulation reduces number of?

Receptors

Cell less sensitive to hormone

Happens with long-term exposure to high hormone concentrations

Most cells are sensitive to more than what?

More than one hormone and exhibit interactive effects

Synergistic effects?

Multiple hormones act together for greater effect

Synergism between FSH and testosterone on sperm production

Permissive effects?

One hormone enhances the target organ’s response to a second later hormone

Estrogen prepares uterus for action of progesterone

Antagonistic effects?

One hormone opposes the action of another

Insulin lowers blood glucose and glycogen raises it