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Characteristics of the
Endocrine System
• Composed of glands that secrete hormones into the
circulatory system.
• Hormones are secreted in minute amounts into the
interstitial space.
• Hormones eventually enter the circulatory system
and arrive at specific target tissues.
Characteristics of the
Endocrine System
• Functions are similar to the nervous system.
• Differences;
– Amplitude-modulated vs. Frequency-modulated
– Response of target tissue to hormones is usually slower
and of longer duration than that to neurons.
Chemical Structure
of Hormones
• Peptides & Proteins: Most hormones are either
peptides or proteins and are usually referred to as
peptide hormones.
• Amines: Amine hormones are derivatives of the
amino acid tyrosine.
• Lipids & Steroids: Steroid hormones are produced
by the adrenal cortex and the gonads.
Control of
Secretion Rates
• Hormones control the rates of many activities in the
body.
• The rate at which each hormone is secreted is
controlled by a negative feedback mechanism.
• Three major patterns of regulation:
– Non-hormone substance (e.g. insulin)
Control of
Secretion Rates
• Hormones control the rates of many activities in the
body.
• The rate at which each hormone is secreted is
controlled by a negative feedback mechanism.
• Three major patterns of regulation:
– Non-hormone substance (e.g. insulin)
– Stimulation by the nervous system (e.g. epinephrine)
Control of
Secretion Rates
• Hormones control the rates of many activities in the
body.
• The rate at which each hormone is secreted is
controlled by a negative feedback mechanism.
• Three major patterns of regulation:
– Non-hormone substance (e.g. insulin)
– Stimulation by the nervous system (e.g. epinephrine)
– Hormone from another endocrine tissue (e.g. TRH, TSH)
Transport and Distribution
in the Body
• Hormones are dissolved in the blood plasma and
transported in free form or bound to a protein
carrier.
• As a result, hormones can be distributed throughout
the body relatively quickly.
• Hormones diffuse from the capillary to the
interstitial space.
Transport and Distribution
in the Body
• Lipid-soluble hormones diffuse through the walls of
all capillaries.
• Water-soluble hormones must pass through pores.
Metabolism and Excretion
• Hormones are only active in the body for a certain
time because they are destroyed and eliminated
shortly after they are secreted.
• Half-life = length of time that it takes to eliminate
half of the total amount of hormone that was
secreted.
Metabolism and Excretion
• Water-soluble hormones have relatively short halflives because they are rapidly broken down by
enzymes.
• These hormones normally have concentrations that
increase and decrease rapidly in the blood.
• They generally regulate activities that have a quick
onset and a short duration.
Metabolism and Excretion
• Lipid-soluble hormones are usually bound to protein
carriers.
• The rate at which these hormones are broken down
is greatly reduced.
• Therefore, these hormones have longer half-lives.
Metabolism and Excretion
• Hormones are removed from the blood in four ways.
– Excretion (kidney, liver)
– Metabolism (enzymes)
– Active transport (actively transported into cells and
secreted again)
– Conjugation (attach water-soluble molecules to hormone
and then excreted by kidney or liver).
Interaction of Hormones
with Their Target Tissues.
• Hormones only interact with cells that have binding
sites that are specific for the particular hormone.
Classes of
Hormone Receptors.
• Hormones can be placed into one of two major
categories.
– Hormones that cannot pass through the plasma
membrane.
– Hormones that can pass through the plasma membrane.
• As a result, hormone receptors need to be located in
different locations.
Membrane-Bound
Hormone Receptors.
• Some receptors are located in the membrane of the
target tissue.
• After a hormone binds to the receptor, the receptor
initiates events that lead to a response.
– Some receptors alter membrane permeability.
Membrane-Bound
Hormone Receptors.
• Some receptors are located in the membrane of the
target tissue.
• After a hormone binds to the receptor, the receptor
initiates events that lead to a response.
– Some receptors alter membrane permeability.
– Some receptors activate G proteins.
Membrane-Bound
Hormone Receptors.
• Some receptors are located in the membrane of the
target tissue.
• After a hormone binds to the receptor, the receptor
initiates events that lead to a response.
– Some receptors alter membrane permeability.
– Some receptors activate G proteins.
– Some receptors alter intracellular enzyme activity.
Intracellular
Hormone Receptors.
• Some receptors are located in the target cell.
• They are located either in the cytoplasm or in the
nucleus of the target cell.
• Once the hormone binds to the receptor, the effects
of the hormone take place.
Functions of the
Endocrine System
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Metabolism and tissue maturation.
Ion regulation.
Water balance.
Immune system regulation.
Heart rate and blood pressure regulation.
Control of blood glucose and other nutrients.
Control of reproductive functions.
Uterine contractions and milk release.
Pituitary Gland
and Hypothalamus
• Pituitary gland is responsible for secreting nine
hormones.
• Hypothalamus regulates the pituitary gland’s
secretions.
Pituitary Gland
• Divided into a posterior and anterior portion.
• Posterior pituitary is also known as the
neurohypophysis, because it is continuous with the
brain.
• Anterior pituitary is also known as the
adenohypophysis, because it acts more as a gland.
Relationship of the
Pituitary Gland to the Brain
• The hypothalamus and the anterior pituitary are
connected to each other via blood vessels.
• Hypothalamus produces neurohormones that travel
to the anterior pituitary via the blood.
• Neurohormones then leave the blood and act on the
cells in the anterior pituitary.
Relationship of the
Pituitary Gland to the Brain
• Some neurohormones acts as releasing hormones
and others act as inhibiting hormones.
• This is how the hypothalamus controls the anterior
pituitary gland.
• See table 18.1 for a complete description of the
hormones of the hypothalamus.
Relationship of the
Pituitary Gland to the Brain
• There are no blood vessels that connect the
hypothalamus to the posterior pituitary.
• Instead, neurohormones produced in the
hypothalamus travel to the posterior pituitary via
axons.
Hormones of the
Posterior Pituitary
• Posterior pituitary stores and secretes two
polypeptide hormones:
– Antidiuretic hormone (ADH)
– Oxytocin
Antidiuretic Hormone (ADH)
• ADH is synthesized in the hypothalamus and
transported to the posterior pituitary.
• ADH is then released into the circulatory system and
carried to the primary target tissue in the kidney.
• ADH promotes water retention and reduces urine
volume.
Hormones of the
Anterior Pituitary
• Anterior pituitary hormones are called tropic
hormones.
Hormones of the
Anterior Pituitary
• Anterior pituitary hormones are called tropic
hormones.
• We will only address one of the many tropic
hormones:
– Thyroid-stimulating hormone
Thyroid-stimulating
Hormone (TSH)
• TSH is also known as thyrotropin.
• Stimulates the synthesis and secretion of thyroid
hormones from the thyroid gland.
• TSH secretion is controlled by TRH from the
hypothalamus.
Thyroid Gland & Hormones
• Thyroid hormones exist in two forms:
– Triiodothyronine (T3)
– Tetraiodothyronine (T4) (also called thyroxine)
– Approx. 90% T4 and 10% T3 secreted.
Thyroid Gland & Hormones
• Thyroid hormones are transported in the blood in
combination with plasma proteins.
• Approx. 70-75% of T3 & T4 are bound to thyroxinbinding globulin (TBG).
• 20-30% are bound to other plasma proteins.
• Therefore, high half-life.
Thyroid Gland & Hormones
• Thyroid hormones diffuse through the target cell
membrane.
• Bind to receptors in the nucleus.
• Thyroid hormones affect nearly every tissue in the
body.
• Factors such as metabolism, growth, and maturation
are affected.
Regulation of
Thyroid Hormone Secretion
• TRH & TSH help control thyroid hormone levels.
• Exposure to stress and cold increases TRH.
• Prolonged fasting decreases TRH.
Regulation of
Thyroid Hormone Secretion
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TRH & TSH help control thyroid hormone levels.
Exposure to stress and cold increases TRH.
Prolonged fasting decreases TRH.
Abnormal thyroid conditions are listed in Table 18.5
The Adrenal Glands
• Located atop the kidneys.
• Composed of an inner medulla and an outer cortex.
The Adrenal Medulla
• Two major hormones:
– Epinephrine (80%)
– Norepinephrine (20%)
• See Table 18.7 for structure, target tissue, and
response.
The Adrenal Cortex
• All hormones are steroids.
• Lipid soluble and are carried in the plasma by
protein carriers.
• Three hormone types:
– Mineralocorticoids (e.g. aldosterone)
– Glucocorticoids (e.g. cortisol)
– Androgens
The Adrenal Cortex
• Adrenocorticotropin-releasing hormone (ACTH) is
necessary to maintain the sensory activity of the
adrenal cortex.
• Corticotropin-releasing hormone (CRH) is released
from the hypothalamus and stimulates the anterior
pituitary to secrete ACTH.
The Adrenal Cortex
• Table 18.9 outlines the abnormalities associated with
hypersecretion and hyposecretion of adrenal
hormones.
The Adrenal Cortex
• Table 18.9 outlines the abnormalities associated with
hypersecretion and hyposecretion of adrenal
hormones.
• Predict 7
Predict 7
• Cortisone, a drug similar to cortisol, is sometimes
given to people who have severe allergies or
extensive inflammation or who suffer from
autoimmune diseases. Taking this substance
chronically can damage the adrenal cortex.
• Explain how this damage can occur.
The Adrenal Cortex
• Table 18.9 outlines the abnormalities associated with
hypersecretion and hyposecretion of adrenal
hormones.
• Predict 7
• Clinical Focus – Stress (page 621; 633)
Effects of Aging
• What happens to the endocrine system when we get
old?
• Endocrine glands differ in how they respond to the
aging process.
• Some experience a gradual decrease in secretion
while others are not affected to a great degree.
Effects of Aging
- Growth Hormone • There is a decrease in growth hormone (GH).
• Decrease is greater in people who do not exercise.
• Decreases in GH may explain a gradual decrease in
lean-body mass in these people.
Effects of Aging
- Melatonin • Melatonin decreases in aging people.
• May influence sleeping patterns.
• May also affect the patterns of other hormones like
GH and testosterone.
Effects of Aging
- Thyroid Hormone • Decreases slightly with increasing age.
• Age-related damage to the thyroid gland may also
occur.
Effects of Aging
- Insulin • There does not appear to be a age-related decrease in
the ability to maintain blood glucose levels.
• However, there is an age-related tendency to
develop Type II diabetes.
Hormonelike Substances
• Autocrine chemical signals are released from cells in
a local area and influence activity of the same cell
type.
• Paracrine chemical signals are produced by a wide
variety of tissues and secreted into tissue spaces.
Hormonelike Substances
• Autocrine and paracrine signals are not like
hormone signals.
• They are not secreted by endocrine glands.
• They act locally not systemically.
• They are also not completely understood…
Autocrine Chemical Signals
• Autocrine chemical signals include the chemical
mediators of inflammation.
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Prostaglandins
Thromboxanes
Prostacyclins
Leukotrienes
• Collectively known as eicosanoids.
Autocrine Chemical Signals
• These products are released from injured cells.
• Responsible for initiating some of the symptoms of
inflammation.
• Pain receptors are stimulated directly by
prostaglandins.
Prostaglandins
• Produced in all cells in the body except for red
blood cells.
• Once prostaglandins enter the circulatory system,
they are metabolized rapidly.
• They are synthesized from essential fatty acids
(omega-3, omega-6).
• Ratio of omega-6:omega-3 seems to be important in
the synthesis of prostaglandins.
Prostaglandins
• Anti-inflammatory drugs like asprin inhibit
prostaglandin synthesis.
• Prostaglandins can both intensify or diminish
inflammation and increase or decrease the clotting
tendency of the blood.
• In order for the body to remain healthy, these two
potentials must be properly balanced.
Prostaglandins
• In simple terms; prostaglandins derived from
omega-6 fatty acids promote inflammation and
blood clotting, while those derived from omega-3
fatty acids oppose those effects.
• Remember that both are needed to maintain health.
Prostaglandins
• Evidence suggests that our diet can affect this
balance.
• The Western diet is deficient in omega-3 fatty acids.
• An ideal omega-6:omega-3 ratio is between 1:1 and
4:1.
• A typical American diet is between 20:1 and 40:1!
Prostaglandins
• Essential fatty acid deficiency and omega6:omega-3 imbalance is linked with the following
serious health conditions:
Heart attack
Cancer
Insulin
resistance
Lupus
Schizophrenia Depression
Stroke
Obesity
Diabetes
Alzheimer’s
Disease
Arthritis
Asthma
Prostaglandins
• Omega-3 sources:
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Flaxseed oil
Fish oil
Flax seeds, hemp seeds
Walnuts, walnut oil
Salmon, herring, mackerel, sardines
Soybeans, soybean oil
Prostaglandins
• Omega-6 sources:
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Corn oil
Peanut oil
Meat
Poultry
Safflower oil
Sesame oil
Sunflower oil