Chapter 16 – The Endocrine
System
Objectives
1. List the major endocrine organs and
briefly describe their location in the body.
2. List the 5 ways hormones affect target
cells/organs.
3. Describe how hormones are classified
chemically.
4. Define hormone.
5. Describe 5 types of cellular changes
hormones cause and 2 major mechanisms by which hormones exert their effects.
6. Describe how hormone release is
regulated.
7. Describe the 3 types of hormonal
stimulation.
8. Describe the functional relationship
between the hypothalamus and pituitary.
9. Name the 6 anterior pituitary hormones
and describe their effects.
10. Name the 2 posterior pituitary hormones
and describe their effects.
11. Describe the effects of the 2 groups
of hormones produced by thyroid gland.
12. Describe the role of the parathyroid
gland in the regulation of calcium.
13. List the hormones produced by the adrenal
gland and cite their effects.
14. Compare and contrast the effects of
the 2 major pancreatic hormones.
15. Name and gonadal hormones and describe
their effects.
The Endocrine System
The
endocrine system affects bodily activities by releasing chemical messengers,
called hormones, into the bloodstream. The system as a whole sends messages to
cells in virtually any part of the body. It not only helps to regulate the
activity of smooth and cardiac muscle and some glands, it significantly affects
virtually all other tissues as well. Endocrine organs are widely scattered
throughout the body.
The
nervous and endocrine systems coordinate their activities and together are
considered the control systems of the body.
Although
the effects of hormones are many and varied, their actions can be categorized
into 5 broad areas:
1. Help control the internal environment
by regulating its chemical composition and volume.
2. Respond to changes in the
environmental conditions to help the body cope with emergency demands such as
infection, trauma, emotional stress, dehydration, starvation, hemorrhage, and
temperature extremes.
3. Integration of growth and
development.
4. Contribute tot he basic processes of
reproduction.
5. Help regulate organic metabolism and
energy balance.
There
are two kinds of glands in the body:
endocrine and exocrine
1. Exocrine glands have ducts through
which their non-hormonal products are routed to a membrane surface. Examples
are sweat, sebaceous, mucous, and digestive glands.
2. Endocrine glands are ductless glands
which release hormones directly into the blood and lymph. They typically have a
rich vascular and lymphatic drainage. Endocrine glands include the pituitary,
thyroid, parathyroid, adrenal, pineal, and thymus glands. Several organs in the
body, such as the pancreas and gonads, contain discrete areas of endocrine
tissue within their substance and produce hormones as well. The hypothalamus is
both an integral part of the nervous system and also a producer and releaser of
hormones; it is considered a neuroendocrine organ. There are pockets of
hormone-producing cells elsewhere, but these are the major endocrine organs.
A
hormone is defined as a chemical substance, secreted by cells into the
extracellular fluids, that regulates the metabolic function of other cells in
the body. All hormones have in common the function of maintaining homeostasis
by changing the rate of physiological activities of cells.
They are
classified chemically into one of two large groups of biochemical molecules:
1. amino-acid based hormones – Most
hormones belong to this group. They function via the second messenger
mechanism.
2. steroid hormones – These hormones are
synthesized from cholesterol. Only the gonadal hormones and the adrenocortical
hormones are steroids. They function via direct gene activation.
A
hormone influences the activity of only certain tissue cells. The cell must
possess specific protein receptors in order to respond to the hormone. Although
hormone-receptor binding is the crucial first step, the extent of target cell
activation depends equally on three factors:
1. blood levels of hormone
2. relative numbers of receptors on the
target cell
3. affinity of the binding between hormone
and receptor (higher affinity = better activity)
Hormones
alter cell activity by increasing or decreasing the rates of normal cellular
processes. A hormonal stimulus typically results in one or more of the
following changes:
1. Plasma membrane permeability
2. Synthesis of proteins or other
regulatory molecules within the cell
3. Enzyme activation or deactivation
4. Induction of secretory activity
5. Stimulation of mitosis
Hormones
are produced as needed. There are 2 major mechanisms by which hormone binding
is harnessed to the specific intracellular machinery needed for action.
1. Formation of one or more
intracellular second messengers
2. Direct gene activation by the hormone
itself
Second
Messenger – The
hormone is called the first messenger. To give the cell its message, the
hormone attaches to a specific receptor in the cell membrane. Once attachment
occurs there is an increase in the synthesis of cyclic AMP inside the cell.
Cyclic AMP acts as the second messenger to alter cell function by activating
enzymes inside the cell which go on to catalyze specific responses, such as
inducing secretion, activating protein synthesis, and altering membrane
permeability.
Direct
Gene Activation
– Being lipid soluble, steroid hormones can diffuse easily into their target cells.
Once inside, they bind to a receptor located in the nucleus. The activated
hormone-receptor complex then interacts with another receptor on the DNA. The
result of this is to cause the transcription of certain genes, which leads to
synthesis of the proteins coded for by the genes. These proteins may be enzymes
that promote metabolic activity, structural proteins, or proteins exported from
the cell.
The
concentration of a hormone in blood at any time reflects its rate of release
and the speed of its inactivation and removal. Some hormones are rapidly
degraded by enzymes within the cell, but most are removed from the blood by the
kidney and liver enzymes. The time of onset for hormonal activity varies
greatly. Some are immediate (second messenger); others may require hours or
days before effect is seen (direct gene activation).
Various
endocrine glands are stimulated to manufacture and release their hormones by
three main types of stimuli:
1. Hormonal – In some cases hormones
stimulate other endocrine glands to secrete other hormones. For example, the
release of anterior pituitary hormones is regulated by releasing and inhibiting
hormones produced by the hypothalamus. Then, the anterior pituitary hormones in
turn stimulate other endocrine glands to release their hormones. As the
end-result hormones increase in the blood, they inhibit the release of anterior
pituitary hormones and thus shut down their own release.
2. Humoral – Hormone release is
stimulated by substances dissolved in extracellular fluids. Usually changing
blood levels or certain ions or nutrients stimulate hormone release. For
example, the release of parathyroid hormone is prompted by decreasing blood
calcium levels. Other examples are calcitonin, insulin, and aldosterone.
3. Neural – In some cases, nerve fibers
stimulate hormone release. For example, catecholamines (epinephrine and
norepinephrine) are released during times of stress by stimulation of the
adrenal medulla by the sympathetic nervous system. Also, the hypothalamus
stimulates the release of oxytocin and anti-diuretic hormone by neural
stimulation.
Pituitary
Gland (Hypophysis)
About
the size and shape of a pea, located in the sella turcica of the sphenoid bone,
attached to the hypothalamus via a stalk called the infundibulum. It has two
major lobes, one neural and one glandular.
Posterior
Lobe (Neurohypophysis) – composed of nervous tissue; releases neurohormones that it
receives ready-made from the hypothalamus; oxytocin and anti-diuretic hormone
(ADH); released on demand in response to nerve impulses from hypothalamic
neurons.
Anterior
Lobe (Adenohypophysis) – composed of glandular tissue; manufactures and releases its own
hormones as a result of stimulation by releasing hormones from the
hypothalamus; called the master endocrine gland; releases 6 hormone products
which all function via second messenger systems.
Four of
the six are hormones that regulate the functioning of other endocrine glands:
thyroid stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle
stimulating hormone (FSH), and luteinizing hormone (LH)
Remaining
two affect non-endocrine targets:
growth hormone (GH) and prolactin.
1. Thyroid Stimulating Hormone (TSH) –
Stimulates the thyroid gland to secrete thyroid hormone.
2. Adrenocorticotropic Hormone (ACTH) –
Stimulates the adrenal cortex to release corticosteroid hormones
(glucocorticoids, gonadocorticoids, and mineralocorticoids).
3,4. Gonadotropins – Follicle Stimulating
Hormone (FSH) and Luteinizing Hormone (LH)
Regulate
the function of the gonads (ovaries and testes). In both sexes, FSH stimulates
gamete production, while LH promotes the production of gonadal hormones
(estrogen and testosterone).
5. Growth Hormone (GH) – Stimulates
most body cells to increase in size and divide. Its major targets are the bones
and skeletal muscles. Induces protein synthesis and encourages the use of fats
for fuel, thus conserving glucose. GH release is controlled by hormones from
the hypothalamus which stimulates or inhibits its release. Abnormalities result
from hypersecretion (acromegaly) and hyposecretion (dwarfism).
6. Prolactin (PRL) – Stimulates milk
production by the breast.
1. Antidiuretic Hormone (ADH) –
Influences body water balance. Targets kidney tubules, which respond by
reabsorbing more water from the forming urine and returning it to the
bloodstream; less urine is produced and blood volume is increased. Ingestion of
alcohol inhibits ADH secretion and causes copious urine output. Under certain
conditions, such as severe blood loss, exceptionally large amounts of ADH are
released. At high levels, ADH has a vasoconstrictor effect to try to maintain
blood pressure.
2. Oxytocin – Stimulates contraction of
smooth muscle of uterus during childbirth.
The
thyroid is a butterfly-shaped gland located in the anterior neck, overlying the
trachea. Internally, the gland is composed of hollow spherical structures called
follicles with other cells located between the follicles.
1. Thyroid Hormone – referred to as the
body’s major metabolic hormone. It comes from a precursor molecule called
thyroglobulin which is produced by the cells making up the walls of the
follicles. Thyroglubulin is stored inside the follicle and requires iodine
atoms to function properly. When thyroid hormone is needed, it is produced by
breaking down thyroglobulin into smaller pieces. Thyroid hormone is actually
two active iodine-containing hormones, thyroxine (T4) and triiodothyronine
(T3). T4 is secreted from the thyroid in greater amounts than T3. T3 is the
most active and is formed at target tissues by conversion of T4 to T3. Except
for the adult brain, spleen, testes, uterus, and the thyroid gland itself,
thryoid hormone affects virtually every cell in the body. It stimulates enzymes
concerned with glucose oxidation, increases basal metabolic rate and body heat
production, increases adrenergic receptor in blood vessels, and helps in tissue
growth. The thyroid gland is unique in its ability to make, store, and release
its hormones.
2. Calcitonin – produced by
parafollicular cells of the thyroid gland. It is released in response to high
blood levels of calcium. Its function is to lower blood calcium levels by
stimulating calcium uptake by bone. It is generally more important in
childhood.
It is
the most important hormone controlling calcium balance. Calcium is critically
important for nerve impulse conduction, muscle contraction, and clotting. PTH
is released by the parathyroid gland in response to low blood calcium levels.
Its function is to increase blood calcium levels by stimulating three target
organs:
Skeleton
– causes digestion of bone matrix and release of calcium
Kidneys
– causes increased reabsorption of calcium from the forming urine and returns
it to the blood
Small
intestine – causes increased absorption of calcium by the intestinal mucosal
cells.
The
paired adrenal glands are located on top of the kidneys where they are enclosed
in a fibrous capsule and a cushion of fat. Each gland is structurally and
functionally two endocrine glands in one:
1. the inner adrenal medulla is made up of
nervous tissue; part of the sympathetic nervous system
2. the outer adrenal cortex surrounds the
medulla, forms the bulk of the gland, and is organized into three layers
All
these hormones are steroid hormones. The cortex is organized into the following
layers and major products:
1. zona glomerulosa (outermost) – produces
mineralocorticoids which help control the balance of minerals and water in the
blood
2. zona fasciculata (middle) – secretes
the metabolic hormones called glucocorticoids
3. zona reticularis (innermost) – secretes
the glucocorticoids and small amounts of adrenal sex hormones or
gonadocorticoids
Most
important function is regulation of the electrolyte concentration in
extracellular fluids, particularly that of sodium and potassium ions. Aldosterone
is the most potent of these. It decreases the excretion of sodium from the
body. Its target is the kidney tubules where it stimulates reabsorption of
sodium ions. Along with sodium retention, water is also retained (increasing
blood pressure), and potassium is excreted.
Influence
the metabolism of most body cells and help provide resistance to stressors.
Absolutely essential for life. Helps the body adapt to external changes and
intermittent food intake by keeping blood sugar levels fairly constant.
Maintain blood volume by preventing shift of water into tissue cells. Main function is to help the body with long
term response to stress (such as infection and trauma). An example is cortisol.
The bulk
of gonadocorticoids are androgens, or male sex hormones, with testosterone as
the most common. The adrenal cortex also makes small amounts of female
hormones. The testosterone produced by the adrenal cortex is thought to be
responsible for the sex drive.
The
adrenal medulla is stimulated by the sympathetic division of the autonomic
nervous system. Two powerful hormones are secreted: epinephrine and norepinephrine, collectively known as
catecholamines. When the body is activated by a short term stressor or emergency
the sympathetic nervous system is mobilized resulting in blood sugar level
increases, vasoconstriction, and a faster heart rate.
Located
behind the stomach. It is a mixed gland composed of both endocrine and exocrine
gland cells. The bulk of the gland is exocrine, producing digestive enzymes
that are carried by a duct to the small intestine. Scattered among the exocrine
part are minute clusters of cells (islets of Langerhans) that produce
pancreatic hormones. The islets contain 2 major populations of
hormone-producing cells:
1. alpha – make glucagon
2. beta – make insulin
Insulin
and glucagon are both involved in the metabolism and regulation of blood
glucose
Released
in response to low blood sugar levels. Its function is to raise blood sugar to
the proper level. Its major target is the liver where it promotes:
1. glycogenolysis – breakdown of
glycogen to glucose
2. gluconeogenesis – formation of
glucose from non-carbohydrate precursors such as fatty acids and amino acids.
Released
in response to high blood sugar levels. Its function is to lower blood sugar to
the proper level by the following mechanisms:
1. inhibits breakdown of glycogen to
glucose
2. enhances transport of glucose into
cells
3. promotes breakdown of glucose inside
cells for ATP production
4. joins glucose together to form glycogen
5. converts leftover glucose to fat
after the body’s immediate energy needs are met
Produce
gonadal sex hormones, identical to those produced by adrenal cortex. Sex
hormones are steroid hormones. Ovaries produce estrogen and progesterone.
Estrogen is responsible for maturation of female reproductive organs and
secondary sex characteristics of females at puberty. During pregnancy the
levels of both hormones rise. The testes produce testosterone which is
responsible for maturation of male reproductive organs, secondary sex
characteristics of males at puberty, and the sex drive.