Anatomy and Physiology: Endocrine System S – Synthesis of proteins or certain regulatory molecules, such

as enzymes, in the cell

The Endocrine system regulates body processes slower than
the nervous system. If the latter body system uses nerve C – Change in plasma membrane permeability or electrical
impulses to make immediate action, the former body state
regulating system acts by the use of chemical messengers
called HORMONES, which are released into the blood to be A – Activation or inactivation of enzymes
transported leisurely throughout the body that may affect
one or several organs. Glands make up the endocrine system M – Mitosis stimulation
and are responsible for producing and secreting hormones.
The major glands of this body system are the following:
Mechanism of Action

• Hypothalamus Steroid Hormones

• Pituitary gland
• Thyroid gland
Changes in cells are triggered by two mechanisms depending
• Parathyroids
on the type of hormone. Being a lipid-soluble molecule, the
• Adrenals steroid hormones’ mechanism of action is through direct
• Pineal body gene activation. The order of processes involved are the
• Reproductuive organs – ovaries in females and following: (S-T-A-R-T-S)
testes in males
• Pancreas S – Starts to diffuse through the plasma membranes of their
target cells

T – Travels and enters the nucleus, once the hormone is

inside the target cells.

A – Attaches to a specific protein receptor inside the target

cells.

R – Rushes (hormone-receptor complex) to bind to specific

sites on the cell’s DNA.

T – Transcribe messenger RNA or mRNA by activating certain

genes.

S – Synthesis of new proteins is produced by translating the

mRNA in the cytoplasm.

Non-steroidal Hormones

Protein and peptide hormones, otherwise known as non-

steroidal hormones, are unable to enter the target cells. To
Hormones
produce an effect, these amino acid-based chemicals bind to
Hormones are the chemical messengers that transmit
the receptors situated on the plasma membrane of target
information from one cell to another and coordinate bodily
cell. Thus, a second-messenger system is utilized. The
functions. The term hormone comes from a Greek word
following events take place during the process:
which means “to arouse.” This chemical is the key to the
incredible power of the endocrine system! It may be defined
as chemical substances that are secreted by cells into the B – Binding of hormone to the membrane receptor takes
extracellular fluid and regulate metabolic activity of other place.
cells in the body.
A – A series of reactions sets off that activates an enzyme.
A certain hormone affects only a specific tissue cells or organs
called target cells or target organs, although the blood borne S – Second messenger is produced in this case called cyclic
hormone circulates to all the organs of the body. For the AMP or cAMP, also known as cyclic adenine monophosphate,
target cells or organs to respond to a hormone, specific through a catalyzing reaction.
protein receptors must be present on its plasma membrane
or in its interior to which that hormone can attach. The E – Existing cyclic cAMP then oversees additional intracellular
chemical messengers can ONLY influence the workings or changes that promote the typical response of the target cell
function of a cell when the binding would take place. to the hormone.

After hormone binding occurs, cellular activity is altered by Release of Hormones

either increasing or decreasing its normal rate, or maintaining
its usual metabolic process or a new process is stimulated. What prompts the endocrine glands to produce and release
Certain changes that will take place after the binding process or not release their hormones? Let us discuss about that.
depends on the specific hormone and the type of target cells There are stimuli that activate the endocrine organs to
directed. However, either of the following would usually release the chemicals. These stimuli are divided into three
occur: major categories namely:
 • Hormonal stimulus 1. During stress, preaganglionic SNS fiber stimulates
• Humoral stimulus the adrenal medulla cells.
• Neural stimulus 2. Adrenal medulla releases the catecholamines,
epinephrine and norepinephrine, to the blood during
Hormones vary only within a very narrow range or controlled periods of stress.
levels because of the negative feedback mechanisms. This is
the chief means of regulating blood levels of nearly all Major Organs of the Endocrine System
hormones. When secretion of a hormone is triggered by an
internal or external stimulus, the rising levels of hormone Hypothalamus
inhibit the further release of chemicals in the body.
Although part of the nervous system, hypothalamus is also
Hormonal Stimulus recognized as a major organ of the endocrine system. This is
due to the fact that this organ is responsible for producing
This is the most common endocrine stimulus that helps several hormones. Hormones released by the posterior
release the hormones in the body. With this case, the pituitary gland and the release and inhibition of hormones
endocrine organs are prodded to release a hormone by the that regulate the anterior pituitary gland are major functions
stimulation of another hormone. Again, the endocrine organs of this organ via hypothalamic neurons. The releasing
are stimulated to take action by the presence of other hormones are secreted into a capillary network that connects
hormone. For example, during a woman’s menstrual cycle the via portal veins to a second capillary bed in the anterior lobe
following events take place: of the pituitary gland.

1. The hypothalamic hormone, Follicle-Stimulating Pituitary Gland

Hormone Releasing Factor (FSHRF) incites the
anterior pituitary gland, an endocrine gland. The pituitary gland, although the size is approximately the
2. The anterior pituitary gland then releases Follicle size of a grape, has two functional lobes namely, the anterior
Stimulating Hormone (FSH) to prod the ovaries pituitary, the glandular tissue, and the posterior pituitary, the
(female gonads), an endocrine organ to release nervous tissue.
another hormone called estrogen or progesterone.
3. When levels of hormones are rising in the blood, Hormones of Anterior Pituitary Gland (APG)
they feedback mechanism inhibit further chemical
release. The anterior pituitary gland is referred as the “master
endocrine gland,” as it controls the activity of several
When hormones are promoted through this mechanism, the endocrine glands despite its insignificant size. Though this
cycle is rhythmic with hormone levels in the blood rising and glad might appear so powerful it cannot perform its function
falling recurrently. alone. Why? It is because of the fact that the release of each
of its hormones is controlled by releasing and inhibiting
Humoral Stimulus hormones of the hypothalamus.

Humoral stimuli are prodding of hormone release through There are about 6 anterior pituitary gland hormones. Two of
changing blood levels of certain ions and nutrients. An the six namely the growth hormone and the prolactin direct
example of this mechanism is the release of parathyroid their major effects on nonendocrine targets. The other four
hormone (PTH) by cells of the parathyroid glands. The order namely, thyrotropic hormone, adrenocorticotropic hormone
of events occurring with this process is the following: and the two gonadotropic hormones are all tropic hormones.
Tropic hormones trigger target organs which are also
1. Calcium concentration in the capillary blood is endocrine glands to secrete their hormones which in turn
decreased. exert their effects on other body organs and tissues.
2. Low calcium levels in the blood prompts the
parathyroid glands to release parathyroid hormone The characteristics of ALL anterior pituitary gland hormones
(PTH). are as follows:
3. PTH acts by several routes to reverse the decline in
the Calcium level. 1. All anterior pituitary hormones are proteins or
4. Blood calcium levels rise soon. peptides
5. Feedback mechanism inhibits further PTH release. 2. They act through second-messenger systems
3. These hormones are regulated by hormonal stimuli
Aside from the release of parathyroid hormone, other and in most cases negative feedback
hormones released in response to humoral stimuli are the
following: • Growth Hormone (GH)

• Release of calcitonin by the thyroid gland This is a metabolic hormone which is responsible for the
• Insulin release by the pancreas growth of skeletal muscles and long bones of the body. It is a
protein-sparing and an anabolic hormone that causes amino
Neural Stimuli acids to be built into proteins and stimulates most target cells
to grow in size and divide. Hence, GH plays an essential role
When nerve fibers stimulate the release of hormones in the in determining final body size. Aside from that, this hormone
target cells, neural stimuli is used. Sympathetic nervous is also plays a role in maintaining blood sugar homeostasis by
system stimulation is the classic example of this hormone causing fat to break down to be used as an energy while it
release. spares glucose.
 • Prolactin (PRL) The thyroid hormone is made up of two iodine containing
hormones namely the thyroxine otherwise known as T4 and
Structurally similar to growth hormone, this protein hormone triiodothyronine also termed as T3. Thyroid hormones are
is only known to target the breast in humans. It comes from often referred to as the body’s metabolic hormone where
the words “pro” meaning for and “lact” referring to milk. thyroxine is the major hormone secreted by the thyroid
Thus, after birth, this hormone is responsible for stimulating follicles. T3 is formed at the target tissues by the conversion
and maintaining milk production by the mother’s breast. In of T4 to T3.
males, its function is not known.
Functions of thyroid hormones:
• Adrenocorticotropic Hormone (ACTH)
• Controls the rate at which glucose is burned or
This hormone is responsible for regulating the endocrine oxidized and converted to heat and chemical energy.
ability of the cortex portion of the adrenal gland. • Normal tissue growth and development especially in
the reproductive and nervous systems.
• Thyroid stimulating hormone (TSH)
Calcitonin
Also termed as thyrotropic hormone, TSH is responsible for
enhancing the growth and the activity of the thyroid gland. Also called thyrocalcitonin, calcitonin decreases calcium
levels on the blood by causing calcium to be deposited in the
• Gonadotropic hormones bones. The action of this hormone is opposite to the
hormone produced by the parathyroid glands, parathyroid
hormone. Calcitonin is made by the C cells or parafollicular
The hormonal activity of the gonads is regulated by the
cells which are found I the connective tissue between the
gonadotropic hormones. The hormone’s function differs by
follicles. It is directly released to the blood when increasing
gender. In females, the Follicle-stimulating hormone (FSH)
calcium levels in the bloodstream is noted.
stimulates follicular development in the ovaries. As the
follicles mature, estrogen is produced and the eggs are
matured for ovulation. In males, FSH stimulates sperm Parathyroid Glands
development by the testes.
Luteinizing hormone (LH) is responsible for stimulating the The parathyroid glands are glandular tissue masses found on
ovulation of an egg from the ovaries of females and causes the posterior surface of the thyroid gland. The glandular
the rupture of the follicle to release progesterone and some tissues of these glands are responsible for secreting the most
estrogen. It is also refereed as interstitial cell-stimulating important regulator of calcium ion, the parathyroid hormone
hormone (ICSH) in males, as it triggers the production of (PTH) or parathromone. When the levels of calcium in blood
testosterone by the interstitial cells of the testes. drop, the parathyroid glands release PTH, which in turn
stimulates bone destruction cells, osteoclasts, to break down
Hormones of the Posterior Pituitary Gland (PPG) bone matrix and release calcium in the blood. Hence,
parathyroid hormone is a hypercalcemic hormone since it
acts to increase blood levels of calcium.
Strictly speaking, the posterior pituitary gland, is not an
endocrine gland because it doe NOT make the peptide
hormones it releases. Instead, this organ serves as a storage Parathormone and calcitonin operate in negative feedback
area for hormones made by hypothalamic neurons. The mechanism control system that influences each other.
following hormones are released by the PPG: Calcitonin, a hypocalcemic hormone, works oppositely with
PTH depending on the body’s demand. The main target of
PTH is the skeleton. However, it also stimulates the kidneys
1. Oxytocin. This hormone is only release in significant
and the intestines to release more calcium to increase its
amounts during childbirth and in nursing women.
level in the blood.
Presence of this hormone, stimulate powerful
contractions of the uterine muscle during labor,
during sexual relations and when a woman breast- Adrenal Glands
feds her baby. What makes this helpful during
breastfeeding is the fact that it is responsible for Curved over the top of the kidneys, adrenal glands are
milk ejection or let-down reflex in a nursing woman. structurally and functionally two endocrine glands in one.
During the postpartum period, oxytocics are used to Like the pituitary gland, the adrenals have glandular (cortex)
stop uterine bleeding by causing constriction of the and neural tissue (medulla) parts. The central medulla region
ruptured blood vessels at the placental site. is enclosed by the adrenal cortex which contains three
2. Antiduiretic hormone (ADH). This hormone is separate layers of cells.
responsible for retaining more water from forming
urine thus, resulting to decrease in urine volume and Hormones of the Adrenal cortex
an increase in blood volume. When ADH is present in
large amounts, blood pressure rises by causing Major groups of steroid hormones such as
constriction of the small arteries. Hence, this mineralocorticoids, glucocorticoids and sex hormones are
hormone has also been called vasopressin. produced by the adrenal cortex. These hormones are
collectively termed as corticosteroids.
Thyroid Gland
• Mineralocorticoids
The thyroid glands produces two hormones namely the
thyroid hormone and calcitonin. 1. Produced by the outermost layer of the adrenal
cortex, the mineralocorticoids, are mainly
Thyroid hormone aldosterone. These hormones play an essential part
 in regulating the mineral or salt content of the blood 1. Physical or emotional stress and threat would bring
particularly the concentrations of sodium and about the fight-or-flight response. The response is
potassium ions. The target organ of the hormone is necessary to help a person cope up with the stressful
the kidneys that selectively reabsorbs the minerals situation. One of the organs stimulated in these
or allow them to be flushed out in the body in the situations is the adrenal medulla which is
form of urine. responsible for pumping catecholamines or
2. Increased aldosterone levels in the body would hormones into the bloodstream to enhance and
result to rising amounts of reclaimed sodium ions by prolong the effects of neurotransmitters of the
the kidney tubules and secreting more potassium sympathetic nervous system.
ions into the urine. When sodium is reabsorbed 2. Presence of catecholamines has the following
water follows. Hence, the mineralocorticoids aids in effects: increased heart rate, elevated blood
water and electrolyte regulation. pressure and rising blood glucose levels. Also, small
3. Aldosterone is also released when the kidneys passageways of the lungs are dilated with presence
produce the enzyme, renin, when blood pressure of these hormones to cater more oxygen in the
drops. Production of renin triggers a series of blood and a faster circulation to the organs most
reactions that form angiotensin II, which is a potent importantly to the brain, heart and muscles. Increase
stimulator of aldosterone release. glucose and oxygen would make the body fit to fight
4. To prevent aldosterone release, atrial natriuretic or deal with short-term stressors.
peptide or ANP is released by the heart. The main
goal of releasing ANP is to reduce blood volume and Pancreatic Islets
blood pressure.
The best hidden endocrine glands in the body are the
• Glucocorticoids pancreatic islets. These are tiny masses of hormone-
producing tissue scattered among the enzyme-producing
Glucocorticoids are produced by the middle layer of adrenal acinar tissue of the pancreas. Although these million clumps
cortex which includes contisone and cortisol. The functions of of cells are scattered they manufacture hormones and works
this hormone are: like an organ. Two essential hormones produced by the islets
are the glucagon and insulin.
1. Promotion of normal cell metabolism
2. Helping the body resist long-term stressors, by • Insulin
increasing blood glucose levels.
Insulin is released from the beta cells of islets. The secretion
Important information about glucocorticoids: of this hormone is triggered by the increasing levels of
glucose in the blood. Insulin has the unique ability of
1. Fats and even proteins are broken down by body transporting glucose across their plasma membranes and acts
cells and converted to glucose when blood levels of on just about all body cells. Once inside the cells, glucose is
glucocorticoids are elevated in the blood. Hence, oxidized for the energy to be converted to glycogen or fat for
these hormones are said to be hyperglycemic storage. Presence of insulin speeds up activities like this and
hormones. glucose is swept out of blood. Hence, insulin is a
2. Unpleasant effects of inflammation are also hypoglycemic hormone. The release of insulin ends when
controlled by glucocorticoids as they reduce the blood glucose levels decrease.
effects of edema and they reduce the pain by
inhibiting some pain-causing molecules called What makes insulin such an important hormone is the fact
prostaglandins. Thus, because of their anti- that it is the ONLY hormone that decreases blood glucose
inflammatory properties, glucocorticoids are often levels! Without it, no glucose would be used by the body
prescribed as drugs to suppress inflammation for since no other chemical can oxidize them.
patients with arthritis.
3. Glucocorticoids are released from the adrenal cortex • Glucagon
in response to the rising blood levels of ACTH.
If insulin decreases blood levels of glucose, glucagon does the
• Sex Hormones opposite. Glucagon is released by the alpha cells of the islets
and its secretion is triggered by a lowered level of glucose.
The adrenal cortex produces the sex hormones regardless of Hence, its action is hyperglycemic. The primary target organ
one’s gender. Production of sex hormones takes place of glucagon is the liver. It is in this organ where the glucagon
throughout a person’s life but the amount formed is relatively stimulates the process of breaking down stored glycogen to
small. It is the innermost layer of the cortex that produces a glucose and to prompt its release to the circulation.
large amount of androgens, male sex hormones and some
estrogen, female sex hormone. Pineal Gland

Hormones of the Adrenal Medulla Also called the pineal body, the pineal gland is found in the
roof of the brain’s third ventricle. This organ secretes
Like the knot of nervous tissue where posterior pituitary substantial amounts of melatonin.
gland develops, the adrenal medulla has the same
development. When this structure is stimulated by the • Melatonin plays an important role in the day-night
sympathetic nervous system neurons, epinephrine, also cycle of the body. Believed to be a “sleep trigger,”
called adrenaline, and norepinephrine, otherwise known as melatonin levels fluctuate throughout day and night.
noradrenaline, are released into the bloodstream. It reaches its peak level at night, thus, making a
Collectively, these hormones are called catecholamines. person drowsy. The lowest level occurs during
daylight around noon.
 • Another role of melatonin in the body is how it
coordinates the hormones of fertility and inhibits the
reproductive system so that sexual maturation is
prevented before adult body size has been reached.

Thymus Glands

The thymus glands produce the hormone thymosin. Aside

from that, the thymus glands also acts as an incubator for the
maturation of special groups of white blood cells namely the
T cells or T lymphocytes which are important in the immune
response.

Gonads

Hormones of Ovaries

The female gonads or ovaries produces female sex cells

known as the ova or eggs and at the same time produce two
steroid hormones which are:

• Estrogen – produced by the Graafian follicles of the

ovaries. Triggers the development of the secondary
sex characteristics in females. In addition, it works
with progesterone to prepare the uterus to receive
the fertilized egg.
• Progesterone – promotes growth of the uterine
lining. It acts with the estrogen to bring about the
menstrual cycle.

Hormones of Testes

The testes produce the male hormones, androgens, of which

testosterone is the most important. Androgens support
sperm formation and is an important hormone in the
development and maintenance of male secondary sex
characteristics.

References:

1. Anatomy and Physiology by Marieb

2. Brunner and Suddarth’s Textbook on Medical-
Surgical Nursing