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.