Chapter 16

The Endocrine System

J.F. Thompson, Ph.D. & J.R. Schiller, Ph.D. & G.R. Pitts, Ph.D.

Endocrine System: An OverviewThe body’s second homeostatic control system

Uses hormones as control agents

Hormones: chemical messengers released into the blood to regulate specific body functions

Hormones are secreted by endocrine (ductless) glands and tissues

EndocrinologyEndocrinology: the scientific study of hormones and the endocrine organs

Hormones Regulate: Volume & chemical composition of the extracellular fluid

(ECF) Metabolism and energy balance Contraction of smooth and cardiac muscle fibers and many

glandular secretions Homeostasis during normal and emergency conditions Some immune system activities Coordinated, sequential growth, development, and

maturation Reproduction by regulating:

gamete production fertilization nourishment of the embryo and fetus labor and delivery lactation for nourishment of the infant

Nervous vs. Endocrine Systems

rapid action potentials (nerve

impulses) propagated via nerve fibers

neurotransmitters released at specific effector(s)

nerve impulses are brief (msecs/seconds), although control can be sustained

response of effectors is of relatively short duration (seconds/minutes)

slower hormones released into

body fluids; circulated throughout the body in the blood

all body cells exposed; only target cells with receptors respond

hormones persist for seconds/hours/days

responses of target cells may last seconds/hours/days, even weeks/months

Endocrine versus Exocrine Glands All glands

have extensive capillary blood supply

form a discrete structure/organ Endocrine glands

secrete hormones into surrounding tissue fluid by exocytosis and the blood transports them to target cells

Exocrine glands secrete various compounds by

exocytosis into a duct system Mixed glands

both endocrine and exocrine functions

Six Pure Endocrine Glandspinealpituitarythyroidparathyroidadrenal

cortex/medullathymus

Other Endocrine System Componentsmixed glands:

pancreas gonads: ovaries & testes

other endocrine tissue: stomach and intestines skin and adipose tissue heart kidneys placenta

neuroendocrine “organs”: Hypothalamus/Pituitary gland

Types of Chemical RegulatorsCirculating hormones (endocrines): travel via

the blood to reach all tissues, and may affect distant target cells

Local hormones – diffuse into local interstitial fluid, reach and affect only local target cellsparacrine - acts on target cells close to the

site of releaseautocrine - acts on the same cell which

secreted itfor the various immune system local

hormones, see Chapter 21 (cytokines, lymphokines, etc.)

Circulating vs. Local HormonesLocal hormone molecules are usually short

lived, and inactivated quickly

Circulating hormone molecules linger in the bloodstream, and exert their effects for minutes or hours

inactivated by enzymes in the target tissues or in the bloodstream or in the liver; some hormones are also eliminated by the kidneys

kidney or liver disease – may cause problems due to increased hormone levels

The Chemistry of Hormones Two main chemical classes of circulating

hormones:I. Amino acid based:

aminesamines - from single amino acids peptidespeptides – short sequences of amino acids proteinsproteins - long chains of amino acids

II. Steroids: synthesized from cholesterol

A third category exists, if local hormones are included:

eicosanoidseicosanoids: synthesized from a cell membrane fatty acid (arachidonic acid)

Mechanisms of Hormone ActionHormones may alter cell activities and

metabolism by:Changing membrane permeability or membrane

potential by opening or closing gated ion channels

Synthesis of proteins, lipids, or carbohydrates or certain regulatory molecules within the cell

Enzyme activation or deactivation

Induction or suppression of secretory activities

Stimulation of mitosis (and meiosis in the stem cells in the gonads)

Second Messenger SystemsMost amino acid, peptide and

protein hormones:

Are water soluble/lipid insoluble (hydrophilic)

Cannot cross the cell membrane

Need a second messenger to exert their effects

Second Messenger SystemsSince amino acid based hormones cannot enter

cells, a 2nd messenger must convey the hormone signal to the inside of the cell (the hormone is the 1st messenger)

Molecules that serve as second messengers include: cyclic AMPcyclic AMP activates protein kinases cyclic GMPcyclic GMP inactivates protein kinases IPIP3 3 (inositol triphosphate)(inositol triphosphate) Ca2+ ions released CaCa2+ 2+ ionsions that may bind to calmodulincalmodulin

Cyclic AMP (cAMP)

1) Hormone A (excitatory) binds membrane receptor, activating Gs

2) Gs stimulates adenylate cyclase (AC)

3) AC forms cAMP from ATP4) cAMP activates Protein Kinase A 5) PKA: activates/deactivates other

enzymes; stimulates cell secretion; opens ion channels, etc.

1) Hormone B (inhibitory) binds its membrane receptor, activating Gi

2) Gi inhibits adenylate cyclase

3) Antagonistic control

Second Messengers (cont.)

Two second messengers may work together (e.g., IP3 & Ca2+)

Twice as much activation

Activate enzymes and trigger other intracellular activities

Amplification by Hormones

Hormones are in very low concentrations in body fluids

They bind reversibly to target cell membrane receptors

Second messengers initiate a cascade of events (a “snowball” effect) because they activate enzymes that act on other enzymes

This cascade effect amplifies the effect of small quantities of hormone binding to cells

Amplification: the Cascade Effect

For instance, consider a single hormone molecule binding to a specific receptor on a cell surface

It may activate 10 membrane proteins

Each membrane protein may activate 10 adenylate cyclase enzymes to produce 1000 cAMP’s

This produces a total of 100,000 second messengers in the cell which act on various cytoplasmic enzymes

Each enzyme may then activate hundreds/thousands of other protein molecules

Steroid Hormone Action Steroid hormones (derived from

cholesterol) are lipid soluble and penetrate the cell membrane

Bind to cytoplasmic receptors inside the cell

Hormone-receptor (h-r) complex enters the nucleus, binds to a DNA receptor protein

This causes transcription of certain genes, and thus produces specific proteins

This direct gene activation is a slower process, but with longer lasting effects

Target Cell SpecificityTarget cells have specific cell surface or

cytoplasmic receptors which bind to a specific hormone

A target cell has 2,000 to 100,000 receptors for each hormone to which they respond

down-regulation: reduction in the number of receptors when a hormone is present in excess so target tissues become less sensitive

up-regulation: increase in the number of receptors when hormone is deficient so that target tissues become more sensitive

Hormone Interactions at TargetsPermissvenessPermissveness: one hormone allows another

hormone to cause an effectex: thyroid hormone permits reproductive

hormones to cause their effects on reproductive development

SynergismSynergism: effect of two hormones acting together is greater than either acting aloneex: glucagon and epinephrine together cause

more increase in blood glucose than either alone

AntagonismAntagonism: one hormone has an opposite effect to another hormoneex: glucagon elevates blood glucose, insulin

lowers blood glucose

Control of Hormone Release1. Humoral

Control/Autocontrol: levels of substances in the blood regulate the release of the hormone, e.g.:

Ca2+ levels in blood regulate PTH release by the parathyroid gland

Glucose levels in blood regulate insulin and glucagon release by the pancreatic islets

Na+ and K+ levels in the blood regulate aldosterone release by the adrenal cortex

Control of Hormone Release2. Nervous System Control: neural

input stimulates the release of specific hormones, e.g.:

Sympathetic ANS stimulation of the adrenal glands cause them to release epinephrine and norepinephrine

Nerve impulses from the hypothalamus cause oxytocin release from the posterior pituitary during labor or breast feeding

Nerve impulses from hypothalamus cause ADH release from the posterior pituitary when water concentration of blood declines

Control of Hormone Release3. Hormonal Control:

hormones stimulate the release of other hormones

Neurohormones from the hypothalamus stimulate the anterior pituitary to release hormones which, in turn, stimulate the thyroid gland, the adrenal cortex, and the gonads, respectively, to release their hormones

What To Know About Every Endocrine Organ For The ExamName and location of each endocrine glandNames and acronyms of hormones secreted by

each endocrine glandChemical class of the hormone(s) (amine,

peptide/protein, or steroid)Release mechanisms for the hormone(s)Antagonistic control to reduce the release of

the hormone(s)Target tissues or cells for each hormoneMajor responses of the target tissues or cells to

each hormone

The Pituitary Gland Two structural components with different

embryological origins

Posterior Lobe(Neurohypophysis)

Anterior Lobe(Adenohypophysis)

“The Master Gland” The pituitary gland has

two functional components Anterior pituitary

Adenohypophysis Primarily glandular tissue Synthesizes protein

hormones

Posterior pituitary Neurohypophysis Primarily neuosecretory

cells (their cell bodies in the hypothalamus)

Secretes peptide hormones

Some support/glial cells

The Pituitary Gland Connected to the

hypothalamus by the infundibulum

Vascular linkage hypothalamus to

the anterior pituitary

two capillary beds – the hypophyseal portal system

Nervous linkage hypothalamus to

the posterior pituitary

hypothalamic neuron axons

Regulation of Pituitary Hormone ReleaseAnterior pituitary

hypothalamic releasing and inhibiting hormones/factorstransported via blood in the hypophyseal portal system

Posterior pituitary neuroendocrine

release from neurosecretory cells

hormones produced in hypothalamus and released from axon end bulbs in the posterior lobe

Anterior Lobe / AdenohypophysisGrowth Hormone = human growth hormone (hGH)

Release stimulated by GHRH from the hypothalamus negative feedback regulation by low blood levels of

GH inhibited by GHIH (somatostatin) from the

hypothalamus

Actions targets especially liver, muscle, bone, cartilage; also

most tissues stimulates growth, mobilizes fats, elevates blood

glucose (insulin antagonist)

Anterior Lobe / Adenohypophysis Growth Hormone

pathologies hyposecretion – pituitary dwarfism (normal trunk/limb

proportions) hypersecretion

• childhood – pituitary gigantism• adulthood - acromegaly

Anterior Lobe / AdenohypophysisThyroid Stimulating Hormone (TSH)

Release stimulated by:

• TRH from hypothalamus• indirectly by pregnancy and body temperature

inhibited by negative feedback from the thyroid hormones and GHIH (somatostatin)

Actions targets thyroid gland stimulates thyroid hormone release (T3 and T4)

Anterior Lobe / Adenohypophysis Thyroid Stimulating Hormone (TSH)

pathologies hyposecretion – hypothyroidism hypersecretion -- hyperthyroidism

thyroid cretinsim

myxedema

exophthalmia

Anterior Lobe / AdenohypophysisAdrenocorticotropic Hormone (ACTH)

Release stimulated by corticotropin releasing hormone

(CRH) from hypothalamus inhibited by negative feedback by glucocorticoids

from adrenal gland (and by chronic use of therapeutic anti-inflammatory steroids)

Actions targets adrenal cortex stimulates release of glucocorticoids (and to a

lesser degree -- gonadocorticoids, and mineralocorticoids)

Anterior Lobe / AdenohypophysisAdrenocorticotropic Hormone

(ACTH) pathologies

hyposecretion – Addison’s Disease

hypersecretion – Cushing’s Disease (pituitary tumor)

hyperpigmentation Cushing’s Disease - edema

Anterior Lobe / AdenohypophysisFollicle Stimulating Hormone (FSH)

Release stimulated by gonadotropin releasing hormone

(GnRH) from hypothalamus inhibited by negative feedback

• estrogen and inhibin in females • testosterone and inhibin in males

Actions targets ovaries and testes

• female– stimulates ovarian follicle to mature– stimulates production of estrogen

• male - stimulates sperm production

Anterior Lobe / AdenohypophysisLuteinizing Hormone (LH) [Interstitial Cell

Stimulating Hormone (ICSH) in males]Release

stimulated by GnRH inhibited by negative feedback

• estrogen and progesterone in females (except during LH surge)

• testosterone in males

Actions targets ovaries and testes stimulates

• females - ovulation and production of estrogen and especially progesterone

• males – production of androgens, e.g., testosterone

Anterior Lobe / AdenohypophysisProlactin

Release stimulated by an unidentified Prolactin Releasing

Hormone (PRH) from the hypothalamus enhanced by estrogens, birth control pills and

breast feeding inhibited by:

• dopamine = Prolactin Inhibiting Hormone (PIH)• lack of neural stimulation (no suckling)

Actions targets breast secretory tissue stimulates milk production for lactation

[Note: The seventh anterior pituitary hormone, Melanocyte Stimulating Hormone = MSH is of limited importance in humans.]

Posterior Lobe / NeurohypophysisOxytocin

Release positive feedback

• uterine stimulation (stretch) and suckling stimulate the hypothalamus to release oxytocin from the posterior pituitary

• stimulates uterine contractions (labor) and milk letdown• increases feedback for more oxytocin release

inhibited by lack of these stimuli

Actions targets smooth muscle of the uterus and the

breast stimulates uterine contractions and milk

ejection/letdown

Posterior Lobe / NeurohypophysisAntidiuretic Hormone (ADH) or Vasopressin

Release stimulated by impulses from hypothalamus in

response to:• increased osmolarity (dehydration)• decreased blood volume or blood pressure • stress

inhibited by adequate hydration or ethanol ingestion

Actions (1) targets kidney (ADH effect)

• stimulates kidney tubule cells to reabsorb water• NaCl (salt) will be conserved passively to some degree

(2) targets vascular smooth muscle to constrict• elevates blood pressure (vasopressin effect)

Thyroid Gland Located in the anterior

neck inferior to the larynx (“Adam’s apple”)

Two lateral lobes connected by isthmus

The largest pure endocrine gland in the body

Has a rich blood supply

Thyroid gland (continued) Structure Spherical follicles

lined with cuboidal follicular cells

site of production of thyroid hormones

thyroxine (T4) (tetraiodo- thyronine)

triiodothyronine (T3) amine hormones

Parafollicular (C cells) between follicles produce calcitonin

(thyrocalcitonin) a protein hormone

The interior of the follicle contains the thyroid “colloidcolloid” which is the inactive storage form of thyroid hormones, called thyroglobulinthyroglobulin.

Thyroid Gland (continued)Thyroid Hormones

thyroxine (T4) and triiodothyronine (T3)

amine hormones – unusual in penetrating its target cells to bind with cytoplasmic receptors

formed from an amino acid (AA) – tyrosine

two linked tyrosines with iodine atoms covalently bound

4 iodine atoms - thyroxine (T4) = tetraiodothyronine 3 iodine atoms - triiodothyronine (T3)

Thyroid Hormones (continued) Actions

targets all tissues except adult brain, spleen, testes, uterus and thyroid gland

carried in blood attached to a transport protein, only active when freed from the transport protein to diffuse into the tissues

stimulates glucose metabolism increases basal metabolic rate increases body heat = thermogenesis

important regulator of growth and development in conjunction with hGH

Regulation decreased levels of thyroid hormones stimulate TRH and TSH hypothalamic TRH stimulates the anterior pituitary to release

TSH which stimulates the thyroid to release thyroid hormones

Thyroid Gland Pathologies Hypothyroidism*

adults – myxedema lethargic, low

metabolism, puffy eyes, easily chilled, mental impairment

if due to lack of iodine, then a goiter - increased thyroid size

infants – cretinism short, thick body,

mental retardation improper development

* Note: the defect may be in the pituitary gland or in the thyroid gland itself

Thyroid Gland Pathologies Hyperthyroidism: Graves

disease among others body produces

autoantibodies which bind and stimulate the TSH receptor inappropriately

stimulates excess thyroid hormone production

causes elevated metabolic rate, sweating, rapid heartbeat, high blood pressure, nervousness, bulging eyes (exophthalmia)

* Note: the defect may be in the pituitary gland or in the thyroid gland itself

Thyroid Hormones (continued)(Thyro)Calcitonin

A protein hormoneRelease

from parafollicular (C) cells in thyroid tissue (between the follicles)

triggered by elevated blood calcium levels

Actions targets bones, primarily in childhood inhibits osteoclast activity (stops bone resorption) stimulates osteoblasts for calcium uptake and

incorporation into hydroxyapatite in the bone matrix

Net effect: decreases blood Ca2+ levels

Parathyroid Glands Typically four small

glandson the posterior surface of the thyroid gland

Filled with chief (principal) cells which secrete parathyroid hormone (PTH or parathormone)

Oxyphil cells – larger cells, function unknown

PTH is a protein hormone

Parathyroid Hormone (PTH)Release - negative feedback

stimulated by low blood Ca2+ levels

inhibited by high blood Ca2+ levels

Targets: Bone: osteoclasts dissolve

matrix liberating Ca2+ and PO4- ions

Intestine: absorb Ca2+ and PO4- ions

Kidney: reabsorb Ca2+ and eliminate PO4- ions

activates vitamin D to active vitamin D3 (calcitriol), enhances Ca2+ absorption at the intestine

Net effect: elevates blood Ca2+

levels

The Adrenal Glands Paired glands near the

tops of the kidneys Two separate parts:

adrenal medulla interior of the gland derived from nervous

tissue – works with the sympathetic division of the ANS

adrenal cortex exterior region of

gland made up of three

layers• zona glomerulosa• zona fasciculata• zona reticularis

glandular epithelial tissue

The Adrenal CortexMulti-enzyme pathways convert cholesterol into

the various steroid hormonesSynthetic enzymes are organized in the layers of

the cortexzona glomerulosa (outer)

produces mineralocorticoids (aldosterone) controls homeostasis of electrolytes (ions) and water

zona fasciculata (middle) produce glucocorticoids (cortisol) involved in glucose metabolism and overall

metabolismzona reticularis (inner)

produce male and female gonadocorticoids in small quantities

insignificant contribution to reproductive functions

Mineralocorticoids Regulate electrolyte (ion) levels, particularly Na+ and K+

movement of other ions (K+, H+, Cl-, HCO3- ,etc.) is linked to

Na+ movement an electrostatic equilibrium must be maintained; therefore if

certain positive ions are returned to the plasma, other positive ions must move into the urine or negative ions must move to the plasma to maintain the body fluid electrostatic (charge) equilibrium

water follows Na+ and Cl- by osmosis play an important role in blood pressure regulation and

regulation of acid-base balance Aldosterone

the primary mineralocorticoid in humans causes Na+ and Cl- reabsorption into the blood plasma, by

targeting the kidney, and causes K+ excretion into the urine water is conserved passively because it follows NaCl

movement

Control of Aldosterone Release Aldosterone release from the

zona glomerulosa is regulated by: decreasing plasma levels of

Na+ and increasing levels of K+ which trigger aldosterone release

increasing plasma levels of Na+ and decreasing levels of K+ inhibit aldosterone release

ACTH usually does not stimulate

much mineralocorticoid release

but at high levels, ACTH will stimulate aldosterone production

The Renin-Angiotensin System The kidneys monitor Na+

levels If Na+ is low, special kidney

cells release renin (enzyme) Renin catalyzes the formation

of angiotensin I from angiotensinogen

ACE (angiotensin converting enzyme) catalyzes formation of angiotensin II (hormone)

AII has many functions stimulates aldosterone

release from adrenal cortex increases Na+ reabsorption

at the kidney potent vasoconstrictor stimulates thirst

of lungs

Atrial Natriuretic Peptide (ANP) Aldosterone is inhibited by

Atrial Natriuretic Peptide (ANP)

ANP is released from the heart’s atrial walls in response to:

increase in blood pressure increased stretch of the atrial

walls

ANP actions increases Na+ excretion and

K+ retention at the kidney inhibits aldosterone release

and the renin-angiotensin system

decreases blood pressure

Glucocorticoids Influence cellular metabolism and respond to stress and

inflammation Cortisol (hydrocortisone), cortisone, corticosterone Release (from the zona fasciculata)

regulated by negative feedback stimulated by ACTH from the anterior pituitary negative feedback inhibition by increasing levels of

glucocorticoids Actions

targets most tissues promotes hyperglycemia (insulin antagonist) mobilizes fats for catabolism (energy production) mobilizes protein for catabolism (energy production) resistance to stress by providing nutrient building blocks depresses inflammatory response and immune system as

a normal part of immune system regulation

GonadocorticoidsProduction by the adrenal cortex is relatively

unimportant

Produced in small amounts at the zona reticularis

Both males and females produce small quantities of both androgens and estrogens, even before puberty

androgens = male sex hormones primarily androstenedione - a precursor to

testosterone

estrogens

The Adrenal MedullaA modified sympathetic ganglion in which the

postganglionic neurons have become specialized neurosecretory cells

Produces two very chemically similar amine hormones

Stimulated by the sympathetic nervous system to release epiniphrine and norepinephrine (NE) into the bloodstream, targeting cells with NE receptors

Causes brief excitatory responses the same responses as elicited by the sympathetic

nervous system stimulation these circulating hormones bind to the same

adrenergic receptors in target organs that are stimulated by the ANS

Major Endocrine Glands The Adrenal Gland and Stress

shortterm

longterm

The Pancreas

a soft, fragile organ in abdomen beneath the stomach

a mixed gland with both exocrine and endocrine functions acinar cells (exocrine)

secrete various digestive enzymes pancreatic islets [of Langerhans]

(endocrine) produces protein hormones alpha cellsalpha cells secrete glucagonglucagon beta cellsbeta cells secrete insulininsulin other endocrine cell types present

in small numbers

Glucagon from Alpha Cells Release – direct assesment of the

blood glucose (humoral influence) triggered by hypoglycemia

(decreased blood glucose levels)

also stimulated by increased plasma levels of amino acids

Actions primarily targets the liver increase release of glucose

into blood (insulin antagonist) stimulates glycogenolysisglycogenolysis

(breakdown of glycogen to glucose)

stimulates gluconeogenesisgluconeogenesis (synthesis of “new”glucose from amino acids, lipids and lactic acid)

Insulin from Beta Cells Release - direct assesment of the

blood glucose (humoral influence) triggered by

hyperglycemia (increased blood glucose levels)

triggered by increased levels of amino acids and fatty acids

Actions targets most cells in the body

(except nervous tissue) to increase glucose uptake

increases glucose metabolism increases glycogen synthesis increases conversion of glucose

to fat inhibits breakdown of glycogen

and gluconeogenesis

Insulin Pathologies - DiabetesDiabetes mellitus

insulin problems result in sustained increased blood glucose levels

physiological changes: polyuriapolyuria - excessive urination and resulting

dehydration polydypsiapolydypsia - excessive thirst polyphagiapolyphagia - excessive hunger despite

hyperglycemia often, weight loss over time increased susceptibility to injuries and infections ketoacidosisketoacidosis - fat metabolism yields ketone

bodies including acetone which can be smelled cardiovascular and neurological problems

Types of Diabetes Mellitus Type I - insulin-dependent diabetes mellitus (IDDM)

rapid onset of symptoms prior to age 15 [old name – “juvenile onset”] lack of insulin activity - insulin production problems beta cells destroyed by the immune system daily, frequent dosages of insulin

Type II - non-insulin-dependent diabetes mellitus (NIDDM) [old name – “adult onset”] usually in overweight individuals some insulin is produced by islets but body cells do not

respond adequately to the insulin – a lack of sensitivity insulin receptors do not respond to insulin management by diet and exercise or by oral

antihyperglycemic drugs

The Gonads

Male – Testes

Female – Ovaries

A Preview of Chapters 27 & 28

The Ovarian Cycle Controlled by FSH and LH from the adenohypophysis The target organ is the ovary, which becomes responsive at

puberty The ovary releases estrogens and progesterone in varying

proportions depending on the mix of FSH and LH during the ~28 day cycle

A midcycle pulse of LH triggers ovulation

ovulation

The Menstrual Cycle Is controlled by estrogens and progesterone from the

ovary The target organ is the uterus, which becomes

responsive at puberty The uterine lining increases in anticipation of the arrival

of a developing embryo, if fertilization occurred at the right time during the ~28 day cycle

If there is no pregnancy, the uterine lining will be sloughed producing a discharge of tissue and blood, the “menses”

Pregnancy Placental human chorionic

gonadotropin (hCG) provides the positive feedback loop between placenta and ovaries and the anterior pituitary during pregnancy

Continued growth of the placenta in support of the developing embryo is controlled by estrogens and progesterone supplied by both the ovaries and the placenta

Endocrine Control of Female Cycles

The TestesStructure

seminiferous tubules with interstitial cells between the tubules

seminiferous tubules are the site of sperm production

interstitial cells between the tubules secrete male hormones

Brain-Testicular Axis in Males Anterior pituitary activity

changes during puberty for males (and females) begins to secrete FSH, LH controlled by GnRH from

hypothalamus

LH stimulates the interstitial endocrinocytes results in testosterone

production negative feedback regulates the

levels

FSH stimulates sustentacular cells to produce: androgen-binding protein (ABP) inhibin

Testosterone and Other AndrogensSecondary sex characteristics

muscular and skeletal growth heavier, thicker muscle and bones in men than in

women contributes to epiphyseal closure

pubic, axillary, facial and chest hair patterns oil gland secretion laryngeal enlargement deepens the tone of voice

Sexual functions male sexual behavior and aggression spermatogenesis sex drive in both male and female

Metabolism - stimulates (“anabolic”) protein synthesis

Other Endocrine TissuesHeart

the atria walls have special endocrine cells that secrete Atrial Natriuretic Peptide (ANP)

ANP increases urine output and inhibits Aldosterone release in response to increased blood volume

GI tractenteroendocrine cells scattered through digestive

tractseveral amine and protein hormones which function

to increase or decrease GI secretions and motility

Kidney secretes protein hormone Erythropoietin to target

bone marrow for red blood cell (RBC) productionsecreted in response to low RBC numbers

End Chapter 16