LESSON 9: FEEDBACK MECHANISMS
AND THE ENDOCRINE SYSTEM
Hormone Action
Chemical Regulation
Aside from the nervous system, the
What are Hormones?
endocrine system also functions in the
communication and the coordination of body Hormones are chemical messengers
functions. What is the difference between the secreted by endocrine glands. They travel
two systems? The nervous system produces through the bloodstream to target cells and
electrical signals. making it possible for the regulate various body functions such as growth,
body to respond rapidly. The endocrine system metabolism, and reproduction.
on the other hand, uses chemical signals called
hormones. Hormones are synthesized by
endocrine glands. All the endocrine glands Types of Hormones (Based on Solubility):
comprise the endocrine system. 1. Lipid-Soluble Hormones
Chemical regulation, in the context of the – can cross the cell membrane easily.
endocrine system, refers to the intricate control
– act inside the cell (cytoplasm or
mechanisms governing hormone production,
nucleus).
release, and action. These mechanisms ensure
that hormone levels remain within a narrow,
optimal range, maintaining physiological
Examples: Steroid hormones (e.g.,
balance (homeostasis)
estrogen, testosterone), thyroid hormones.
Homeostasis is the fundamental process that
maintains the stable internal environment
necessary for all the cells and organs in your 2. Water-Soluble Hormones
body to function correctly. Without it, even – cannot cross the cell membrane.
small fluctuations in things like blood sugar,
temperature, or pH could quickly become lethal. – bind to receptors on the cell surface.
Examples: Peptide hormones (e.g.,
insulin), amine hormones (e.g., adrenaline).
3 STEPS OF HORMONE ACTION:
1. Reception
• The hormone binds to its specific The target cells of hormones can vary widely.
receptor on or inside the target cell. Sex hormones, for example, target majority of
the tissues in the body, inducing sexual
characteristics that differentiate females and
2. Signal Transduction males. In contrast, there are hormones like
• The hormone-receptor binding starts a glucagon. produced by pancreas, with few
chain of events inside the cell. targets only (liver and fat cells). Aside from
those, other hormones target other endocrine
glands just like the thyroid-stimulating
3. Response hormone, produced by pituitary which targets
the thyroid.
• The target cell responds by:
Table 9.1 Major Hormones in Humans
• Making new proteins (in lipid-soluble GLAND HORMON CHEMI FUNCTI
hormone action), or E CAL ONS
CLASS
• Activating enzymes or cell functions
Hypothal Posterior pituitary and anterior
(in water-soluble hormone action).
amus pituitary hormones
• This leads to changes like growth, Posterior Oxytocin Peptide Uterine
metabolism, or other body activities. gland contracti
on and
Vertebrate Endocrine System milk
productio
Endocrine glands can be classified into three
n in the
major groups based on their roles. A good mammar
number of glands release hormones into the y glands
blood just like the thyroid gland. However, Antidiuretic Peptide Minimiz
there are glands like pancreas which have both hormone es water
endocrine and nonendocrine functions (ADH) loss
Anterior Growth Protein Induces
Moreover, other glands, have primary roles
pituitary hormone growth
which are not related to the endocrine system.
gland (GH) and
Table 9.1 lists the major hormones in humans. metaboli
Take note of the chemical classes of hormones c
functions
and their differences in terms of eliciting a
Prolactin Protein Induces
response Remember that protein-based (PRL) milk
hormones (amines, peptides, and proteins) are productio
water soluble while steroid hormones are lipids; n
hence, water-insoluble. Note that not all Follicle- Protein Induces
hormones and glands are included in the list. stimulating gamete
hormone productio
(FSH) n Pancreas Insulin Protein Decrease
Luteinizing Protein Stimulate s blood
hormone s gonads glucose
(LH) level
Thyroid- Protein Stimulate Glucagon Protein Increases
stimulating s thyroid blood
hormone gland glucose
(TSH) level
Adenocortic Peptide Stimulate Ovaries Estrogens Steroid uterine
otropic s adrenal lining,
hormone glands stimulate
(ACTH) develop
Pineal Melatonin Amine Function ment of
gland s in daily female
and sex
seasonal character
rhythmic istics
actions Progesterone Steroid Induces
Thyroid Thyroxine Amine Regulate uterine
gland (T4) and s lining
triiodothyron metaboli growth
ine (T3) c Testes Androgens Steroid Aids in
processes sperm
Calcitonin Peptide Decrease productio
s blood n;
calcium stimulate
level s
Parathyr Parathyroid Peptide Increases develop
oid gland hormone blood ment of
(PTH) calcium male sex
level character
Thymus Thymosin Peptide Induces istics
develop
ment of
The Connection of Nervous and Endocrine
T cell
Adrenal Epinephrine Amine Short- Systems
gland and term
norepinephri stress
ne response The functions of the nervous and the endocrine
Adrenal Glucocortico Steroid Long- systems are intimately related to each other.
medulla ids term This is apparent in how the hypothalamus
Adrenal Mineralocort Steroid stress works. The hypothalamus is the main controller
cortex icoids response
of the endocrine system. It works by stimulating
the pituitary gland to release hormones which (ACTH), follicle-stimulating hormone (FSH),
will regulate most of the body functions. These and luteinizing hormone (LH). Prolactin (PRL)
hormones are secreted as a response to the induces milk production in mammals but has
messages sent by the nervous system based on different effects in other animals.
the internal and external conditions.
Growth hormone (GH) affects the most number
The pituitary gland is found under the of cells in the body since it stimulates protein
hypothalamus. It has two lobes: the posterior synthesis and fat breakdown as source of
and the anterior pituitary. The posterior pituitary energy. Because of this, GH is necessary for
does not produce its own hormones. Instead, it developing mammals. However, disorders can
stores and releases the hormones from the result from abnormal amount of GH secretion.
hypothalamus. On the other hand, the anterior Gigantism is due to high concentrations of GH,
pituitary synthesizes its own hormones. The while very small quantities can lead to pituitary
hypothalamus regulates the anterior pituitary by dwarfism. However, dwarfism may be
producing stimulating and inhibiting hormones. prevented by giving growth hormones to
Stimulating hormones trigger the pituitary to children with deficiency.
release specific hormones. This action is then
regulated by the inhibiting hormones which
prevent the pituitary from secreting more of its Another hormone produced by the anterior
hormones. pituitary is endorphins. Endorphins act as
natural pain killers because they minimize pain
The posterior pituitary secretes oxytocin
by binding to the pain receptors in the brain.
and antidiuretic hormones (ADH) produced by
Some activities associated with endorphin
the hypothalamus. These hormones are
secretion include running, meditation, and
transported through the bloodstream to reach
eating spicy food.
their targets. Oxytocin stimulates the uterus to
contract when a baby is born, and it also Hormonal Control of Homeostasis
induces milk release when the baby starts Homeostasis - refers to the body’s ability to
suckling. ADH signals the kidney to retain more maintain a stable internal environment despite
water when the body needs it. However, ADH external changes.
release decreases when there is too much water.
Thyroid Hormones
The hormones that the anterior pituitary
secretes in response to the stimulating hormones
from the hypothalamus control various body
functions. Some hormones stimulate other
endocrine glands such as thyroid-stimulating
hormone (TSH). adrenocorticotropic hormone
Normal levels of thyroid hormones have to reabsorption in the kidneys. When the calcium
be maintained because abnormal amounts lead level is too low, the parathyroid glands produce
to disorders. Hyperthyroidism (excessive parathyroid hormone (PTH), which increases
thyroid hormones) can lead to increased body blood C a2+¿ ¿ concentration. Increased calcium
temperatures and severe weight loss. level is achieved with actions opposite to those
Hypothyroidism (too tittle) can cause cold induced by calcitonin, IncreasedC a2+¿ ¿
intolerance and obesity. The latter condition reabsorption in the kidneys and C a2+¿ ¿ uptake
may be due to damage in the thyroid gland or In the intestines, coupled with enhanced C a2+¿ ¿
iodine deficiency in the diet. Children who lack release from the bones, produce the desired
iodine can suffer from problems in bone growth effect of increasing blood C a2+¿ ¿ levels.
and mental development. In adults, this
Pancreatic Hormones in Blood Glucose
deficiency results in goiter, a condition of
Control
oversized thyroid gland. The enlargement is due
to lack of thyroid hormones which are necessary The pancreas also produces hormones
for feedback inhibition. which help keep homeostasis of glucose in the
blood. Glucose can be stored as glycogen in the
Since T3 and T4 are lacking, the
liver or used by body cells to fuel different
hypothalamus and the pituitary gland do not
metabolic reactions. Too much glucose in the
receive the signal to stop; hence, both organs
blood, typically after a carbohydrate-rich meal,
keep on stimulating the thyroid gland, leading
induces one isle in the islets of Langerhans
to its increase in size. Such conditions can be
(endocrine cells) in the pancreas, which are
addressed by including iodine in the diet such as
called beta cell, to produce insulin.
using iodized salt or increasing seafood intake.
Insulin induces the body cells to obtain
Thyroid and Parathyroid Hormones in
more glucose and the liver to stockpile glucose
Calcium Homeostasis
as glycogen (a polysaccharide composed of
Another example of hormones which glucose). These actions lower the blood glucose
maintain homeostasis is the thyroid and level. However, if the blood glucose becomes
parathyroid glands. These glands produce scarce, the pancreas induces a different isle of
hormones which keep the levels of calcium endocrine cells, called the alpha cells, to
ions (Ca) within a set range. Cations are critical produce glucagon. The glucagon hormone has
in several body functions, including muscle opposite effects from that of insulin, stimulating
contraction, nerve signal propagation, blood the liver to break down glycogen and release
clotting, and transport of molecules. The thyroid glucose in the blood. This results in increased
gland secretes calcitonin which lowers the blood glucose level.
blood Ca.
This is made possible by enhancing calcium
absorption in the bones and minimizing calcium
