copyright © john wiley & sons, inc. all rights reserved. c hapter 18 the endocrine system -...

Copyright © John Wiley & Sons, Inc. All rights reserved.

CHAPTER 18The Endocrine System -Summary Notes


ENDOCRINE SYSTEM GLANDS Glands that secrete endocrine hormones into

the bloodstream are called endocrine

glands

They are one of two major types of glands

in the body, the other being exocrine

glands (which

secrete their products into ducts )


INTRODUCTION The nervous and endocrine systems

coordinate all of the body systems

The nervous system does so through the

action of neurons, and the

neurotransmitters they secrete

The endocrine

system uses

hormones produced

by endocrine structures

to produce their

effects


The nervous system releases

neurotransmitters and is fast acting (millisec),

but its effects tend to be of shorter duration

(millisec)

The endocrine system releases hormones,

and is slower acting (sec/hrs/days), but the

effects are much longer lasting (sec/days)

Most hormones circulate through the blood and

bind to receptors on “target cells.”

COMPARISON OF CONTROL BY THE NERVOUS AND ENDOCRINE

SYSTEMS


Endocrine glands secrete hormones. They

do not have ducts and secrete their

hormones directly into the interstitial fluid

that surrounds them and then they enter the

blood stream.

The hormones diffuse into the blood stream

through capillaries and are carried to target

cells throughout the body.

ENDOCRINE GLANDS


Hormones are simply chemicals called

mediator molecules that have effects on

cells in the local environment, or in a distant

part of the body

Some hormones, called autocrine

hormones are local hormones that are

secreted into the interstitial fluid, and then

bind to the same cell.

TYPES OF HORMONES


TYPES OF HORMONES Paracrine hormones are local hormones

that are secreted into interstitial fluid and

act on nearby cells


Endocrine hormones are secreted into

interstitial fluid and then absorbed into the

bloodstream to be carried systemically

to any cell that

displays the

appropriate type

of receptor

TYPES OF HORMONES


Hormones traveling throughout the body will only

affect target cells that possess specific protein

receptors. When stimulated, an endocrine gland will release

its hormone in frequent bursts, increasing the

concentration of the hormone in the blood,

consequently protein receptors are continually

being synthesized or broken down Receptors may be down-regulated in the

presence of high concentrations of hormone. Receptors may be up-regulated in the presence

of low concentrations of hormone.

HORMONE ACTIVITY


SOLUBILITY OF HORMONES Hormones can be divided into two broad

chemical classes. This chemical

classification is useful because the two

classes exert their effects differently

Lipid soluble hormones bind to receptors

in the cytoplasm or nucleus of the cell

Water soluble hormones bind to

receptors on the surface of the cell


SOLUBILITY OF HORMONES Lipid soluble hormones require a carrier

protein for transport in the watery environment of

the blood

Once they arrive at their

destination, however, they

are able to freely pass

through the plasma

membrane to bind to

receptors located in the cytoplasm

or the nucleus of the target cell


SOLUBILITY OF HORMONES Water soluble hormones are easy to

transport in the watery blood. The plasma

membrane of target cells, however, is

impermeable to them

Water soluble hormones exert their effects by

binding to receptors exposed to the

interstitial fluid on the surface of target cells

• the hormone binding to its receptor acts as

the first messenger in a cascade of

signaling in the cell


The first messenger (the hormone) then

causes production of a second messenger

inside the cell, where specific hormone-

stimulated responses take place

SOLUBILITY OF HORMONES


CONTROL OF HORMONES Hormone secretion is regulated by signals

from the nervous system, chemical

changes in the blood, and other

hormones

• Most hormonal regulatory systems work

via negative feedback, but a few

operate via positive feedback


CONTROL OF HORMONES In a negative feedback system the hormone

output reverses a particular stimulus. For example:

Blood Ca2+ level is controlled by the parathyroid

hormone (PTH). If blood Ca2+ is low, there is a

stimulus for the parathyroid glands to release

more PTH. PTH then exerts its effects in the body

until the Ca2+ level returns to normal. If the level

gets too high the body will cease PTH production

and the parafollicular cells of the thyroid gland

will secrete calcitonin (CT) to lower the Ca2+

levels.


CONTROL OF HORMONES In a positive feedback system

the hormone output reinforces

and encourages the stimulus.

For example, during childbirth,

the hormone oxytocin stimulates

contractions of the uterus, and

uterine contractions in turn

stimulate more oxytocin release,

a positive feedback effect


THE ENDOCRINE SYSTEM The endocrine system consists of the pituitary,

thyroid, parathyroid, adrenal and pineal

glands

Some of the most important glands of the

endocrine system are not exclusively endocrine

glands: The hypothalamus, thymus,

pancreas, ovaries, and testes are paramount;

the kidneys, stomach, liver,

small intestine, skin, heart,

and placenta also contribute


THE ENDOCRINE SYSTEM


THE HYPOTHALAMUS The hypothalamus is the major link

between the nervous and endocrine systems

It receives input from several regions in the

brain


THE PITUITARY GLAND The hypothalamus now known as the master

control gland and mainly controls the pituitary

gland. The pituitary hangs down from the

hypothalamus

on a stalk called the infundibulum

The gland is divided into an anterior

adenohypophysis and a posterior

neurohypophysis - the anterior pituitary

accounts

for about 75% of the total

weight of the gland


Specialized neurosecretory cells in the

hypothalamus secrete releasing hormones

into the hypophyseal portal system

that supplies blood to the

anterior pituitary

gland

THE ADENOHYPOPHYSIS


THE ADENOHYPOPHYSIS The second capillary system of the

hypophyseal portal system delivers the

hypothalamic releasing hormones to the

anterior pituitary

5 types of anterior pituitary cells secrete

seven hormones


ANTERIOR PITUITARY HORMONES

Hypothalamus Hormone

Hormone released from Adenohypoph

ysis

Major Function/ Target

Growth hormone releasing hormone (GHRH)

Human Growth Hormone (hGH)

Stimulates secretion of insulin-like growth factors (IGFs) that promote growth in many tissues

Thyrotropin releasing hormone (TRH)

Thyroid Stimulating Hormone (TSH)

Stimulates synthesis and secretion of thyroid hormones by the thyroid gland

Prolactin releasing hormone (PRH)

Prolactin (PRL)

Stimulates breast growth, and development of the mammary glands




Hormone released

from Adenohypop

hysis


Gonadotropic releasing hormone (GnRH)

Follicle Stimulating hormone (FSH)

Ovaries initiate development of oocytes (immature egg cell); testes initiate development of spermatozoa

Gonadotropic releasing hormone (GnRH)

Luteinizing hormone (LH)

Ovaries stimulate ovulation (release of eggs from ovaries); testes stimulate testosterone production




Hormone released from Adenohypoph

ysis


Corticotropin releasing hormone (CRH)

Adrenocorticotropic Hormone (ACTH)

Stimulates release of steroid hormones, including cortisol from the adrenal cortex

Corticotropin releasing hormone (CRH)

Melanocyte Stimulating hormone (MSH)

Stimulate the production and release of melanin by melanocytes in skin and hair. MSH signals to the brain have effects on appetite and sexual arousal


THE NEUROHYPOPHYSIS The posterior pituitary (neurohypophysis)

releases, but does not synthesize any

hormones

When stimulated, neurosecretory

cells in the hypothalamus

release oxytocin and

ADH from their axon

terminals located in

the posterior pituitary


THE NEUROHYPOPHYSIS Oxytocin targets smooth muscle in the uterus

and breasts. In the uterus, oxytocin stimulates

uterine contractions, and in response to the

sucking from an infant, oxytocin stimulates

“milk letdown” in the breasts

ADH targets the collecting ducts in the kidney

and sweat glands in the skin to minimize

water loss. It also directly causes arterioles to

constrict thereby increasing blood pressure


THE THYROID GLAND The butterfly-shaped thyroid gland is

located inferior to the larynx and anterior to

the trachea. It has two laterally placed lobes

separated by a bridge-like isthmus


The two hormones released are:

thyroxine or T4

(tetraiodothyronine)

and T3

(triiodothyronine) and need

iodine for their synthesis

THYROID HORMONES


THYROID HORMONES Together with hGH and insulin, thyroid

hormones accelerate body growth, particularly

the growth of the nervous and skeletal systems

Thyroid hormones regulate oxygen use and

metabolic rate, cellular metabolism, and growth

and development. Secretion is controlled by

TRH from the hypothalamus and thyroid-

stimulating hormone from the anterior pituitary


A goiter is an enlargement of the thyroid gland

and may be associated with hyperthyroidism,

hypothyroidism, or euthyroidism

In many third-word countries

dietary iodine intake is inadequate;

the resultant low level of thyroid

hormone in the blood stimulates

secretion of TSH, which causes

thyroid gland enlargement

THYROID HORMONES


THE PARATHYROID GLANDS The parathyroid glands are small, round

masses of tissue attached to the posterior

surface of the lateral lobes of the thyroid

gland

There are usually two

parathyroid glands

attached to each

lobe of the thyroid,

one superior and one inferior


PARATHYROID GLANDS The parathyroid glands are embedded on

the posterior surfaces of the thyroid.

Parathyroid hormone (PTH) regulates the

homeostasis of calcium, and PTH secretion is

controlled by the level of calcium in the

blood.


THE ADRENAL GLANDS There are two adrenal glands, one superior

to each kidney (also called the suprarenal

glands). During embryonic development,

the adrenal glands differentiate into two

structurally and functionally distinct regions

• the adrenal cortex

• the adrenal medulla Catecholamines like epinephrine

Steroid hormones like cortisol


THE ADRENAL GLANDS


THE ADRENAL CORTEX The adrenal cortex is peripherally located

and makes up 80-90% of the total weight of

the gland

The cortex is subdivided into three zones,

each of which secretes a different group of

steroid hormones, all formed

from the cholesterol

molecule


THE ADRENAL MEDULLA The inner region of the adrenal gland, the

adrenal medulla.

The catecholamines epinephrine (80%),

and norepinephrine (20%), are secreted

at the adrenal medulla and serve to

prolong the sympathetic response


THE PANCREAS The pancreas is both an endocrine and an

exocrine gland. It is located posterior and

inferior to the stomach. We will discuss its

endocrine functions here and its exocrine

functions

in detail in chapter 24


Most of the exocrine cells of the pancreas

are arranged in clusters called acini and

produce digestive enzymes which flow

through ducts into the GI tract

Distributed among the acini are clusters of

endocrine tissue

called pancreatic

islets (islets of

Langerhans)

THE PANCREAS


Each pancreatic islet contains four types of

hormone-secreting cells: alpha (A), beta (B),

delta (D), and F cells

Alpha cells secrete glucagon which

increases blood glucose levels by acting

on the liver to

convert glycogen

to glucose

Beta cells secrete

insulin

PANCREATIC HORMONES


PANCREATIC HORMONES Insulin is an anabolic hormone - it decreases

blood glucose levels by acting on

the liver to convert glucose

to glycogen and then facilitating

diffusion of glucose into the cells

Insulin and glucagon are counter-

regulatory hormones in that

their actions act to balance one

another in terms of blood glucose


GONADAL HORMONES The ovaries are paired oval bodies located in

the female pelvic cavity. They produce several

steroid hormones including estrogens,

progesterone, relaxin, and inhibin

Estrogens, along with FSH and

LH from the anterior pituitary,

regulate the menstrual cycle,

maintain pregnancy, and prepare

the mammary glands for lactation


GONADAL HORMONES Ovarian hormones also promote

enlargement of the breasts and widening of

the hips at puberty, and help maintain these

female secondary sex characteristics

Progesterone prepares the uterus lining for

implantation of a fertilized ovum


GONADAL HORMONES The male gonads, the testes, are oval

glands that lie in the scrotum. The main

hormone produced and secreted by the

testes is testosterone, an androgen (male

sex hormone)

Testosterone is needed for

production of sperm and

maintenance of male

secondary sex characteristics


THE PINEAL GLAND The pineal gland is a small

endocrine gland

attached to the roof of the

third ventricle – it is part of

the epithalamus

The pineal gland secretes the hormone

melatonin, which contributes to maintaining

the biological clock (seasonal and daily cycles)

• more melatonin is secreted in darkness; the

pineal gland is very developed in nocturnal

animals


THE THYMUS GLAND The thymus gland secretes thymosin, which

promotes the proliferation and maturation of

T cells

T cells are a type of white blood cell

(lymphocyte) that destroys microorganisms

and foreign substances

through direct

cellular contact

copyright © john wiley & sons, inc. all rights reserved. c hapter 18 the endocrine system -...

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