ENDOCRINE SYSTEM
PREPARED BY MR. ABHIJIT DAS
ENDOCRINE GLAND
An endocrine gland is a special part of the body
that makes and releases hormones. These hormones
are like messengers that travel through the
blood to give instructions to different parts of the body. Unlike some other
glands, endocrine glands don't have tubes (ducts)
to carry their messages. Instead, they release hormones directly into the bloodstream.
HORMONES
Hormones are non-nutrient
chemicals that act as intercellular messengers.
They're produced in small amounts but play vital
roles in regulating various body functions.
TYPES OF HORMONES
1.
Local Hormones:
These hormones work close to where they're
produced and affect nearby cells.
2.
Circulating Hormones:
These hormones travel through the bloodstream to reach distant
parts of the body.
PITUITARY GLAND
Pituitary Gland:
A small gland in the brain that controls other glands and body processes.
Anterior Pituitary:
The front part of the pituitary gland.
Posterior Pituitary:
The back part of the pituitary gland.
HORMONES OF ANTERIOR PITUITARY GLAND
Hypothalamic hormones play a crucial role in
regulating the secretions of the anterior pituitary gland.
I.
GROWTH HORMONE
Originating from the hypothalamus, GHRH (Growth Hormone-Releasing Hormone) prompts the
anterior pituitary gland to release GH (Growth Hormone).
Conversely, GHIH (Growth Hormone-Inhibiting Hormone) from
the same source signals the pituitary gland to cease GH release.
FUNCTIONS OF GROWTH HORMONE
1.
Body Growth:
GH helps children and teenagers grow taller by promoting the growth of bones
and tissues.
2.
Metabolism Support:
GH aids in regulating the body's metabolism, influencing how cells use energy
and helping to maintain a healthy balance of fats, sugars, and proteins.
3.
Cell Repair and Maintenance:
GH supports the repair and maintenance of cells and tissues throughout life,
contributing to overall health and well-being.
II.
PROLACTIN
PRH (Prolactin-Releasing Hormone) from
the hypothalamus sends a message to the anterior pituitary gland to release PRL (Prolactin).
Conversely, PIH
(Prolactin-Inhibiting Hormone), also from the hypothalamus, delivers a
message that tells the pituitary gland to stop releasing PRL (Prolactin).
FUNCTIONS OF PROLACTIN
1.
Milk Production:
Prolactin helps mothers produce milk after giving birth, enabling them to feed
their babies.
2.
Breast Growth:
Prolactin contributes to the growth and development of breast tissue, preparing
the body for milk production.
3.
Reproductive Health:
Prolactin plays a role in regulating menstrual cycles and fertility in women.
III.
THYROTROPIN OR THYROID STIMULATING
HORMONE (TSH)
TRH (Thyrotropin-Releasing Hormone) from
the brain's hypothalamus signals the anterior pituitary to release thyrotropin. Thyrotropin then prompts the thyroid gland to release thyroid
hormones.
IV.
CORTICOTROPIN OR ADRENOCORTICOTROPHIC
HORMONE (ACTH)
The hypothalamus produces CRH
(Corticotropin-Releasing Hormone), which signals the anterior pituitary
gland to release ACTH (Adrenocorticotropic Hormone).
ACTH then prompts the adrenal glands to release cortisol.
FUNCTIONS OF CORTISOL
1.
Stress Response:
Cortisol helps the body deal with stress by providing extra energy and focus.
2.
Inflammation Control:
It regulates inflammation, helping the body heal and reducing swelling.
V.
GONADOTROPINS
The anterior pituitary gland also releases gonadotropins.
There are two kinds: LH for males and FSH for females.
The hypothalamus releases a hormone called GnRH (Gonadotropin-Releasing Hormone) to the anterior
pituitary, prompting it to release LH (Luteinizing
Hormone) and FSH (Follicle-Stimulating Hormone).
LH travels to the testes
in males, signaling the release of testosterone,
while FSH goes to the ovaries
in females, encouraging the release of estrogen.
These hormones play essential roles in reproductive
processes.
HORMONES OF POSTERIOR PITUITARY GLAND
I.
OXYTOCIN
FUNCTIONS
- Uterine
Contraction: Oxytocin stimulates the muscles of
the uterus during labor, helping in childbirth.
- Milk
Ejection: Oxytocin triggers the release of
milk from the mammary glands, facilitating breastfeeding.
II.
VASOPRESSIN
Vasopressin is an Antidiuretic hormone (ADH).
FUNCTIONS
1.
Water Balance Regulation:
Vasopressin helps control the body's water balance by reducing water loss in
the kidneys, which helps maintain proper hydration levels.
2.
Blood Pressure Regulation:
Vasopressin also plays a role in regulating blood pressure by constricting
blood vessels, helping to maintain adequate blood flow and pressure throughout
the body.
ADRENAL GLAND
The adrenal glands are a pair of small,
triangular-shaped endocrine glands located on top of each kidney. Each adrenal
gland is composed of two main parts: the adrenal cortex and the adrenal
medulla, each of which produces different hormones with distinct functions.
1.
Adrenal Cortex:
The adrenal cortex is the outer layer of the adrenal gland and is responsible
for the production of several important hormones. It is divided into three
distinct zones, each of which produces specific hormones:
a. Zona Glomerulosa: This outermost layer
produces mineralocorticoids, primarily aldosterone. Aldosterone helps regulate electrolyte
balance, particularly sodium and potassium, in
the body, which plays a crucial role in controlling blood pressure and
maintaining fluid balance.
b. Zona Fasciculata: The middle layer
produces glucocorticoids, with the primary
hormone being cortisol. Cortisol regulates
metabolism, immune response, and helps the body respond to stress.
c. Zona Reticularis: This innermost layer of
the adrenal cortex produces androgens, including
dehydroepiandrosterone (DHEA) and androstenedione.
These hormones have weak androgenic (male sex hormone) effects and are involved
in the development of secondary sexual characteristics.
2.
Adrenal Medulla:
The adrenal medulla is the innermost part of the adrenal gland and is
responsible for the production of epinephrine (adrenaline).
FUNCTIONS OF ADRENALINE
1.
Boosts Alertness:
Epinephrine helps you stay awake and alert when you're faced with danger or
stress.
2.
Increases Heart Rate:
It makes your heart beat faster, pumping more blood to your muscles and vital
organs to prepare for action.
3.
Dilates Airways:
Epinephrine opens up your airways, making it easier to breathe in more oxygen.
4.
Constricts Blood Vessels:
Epinephrine narrows blood vessels in non-essential areas, directing more blood
to important organs like the muscles and brain, which helps you react quickly
in a crisis.
THYROID GLAND
The thyroid gland is a butterfly-shaped
endocrine gland located in the anterior (front) part of the neck, just below the larynx (voice box) and wrapped around
the trachea (windpipe).
It consists of two lobes,
one on each side of the trachea, and is connected by a narrow band of tissue
called the isthmus.
STRUCTURE OF THYROID GLAND
The thyroid gland is made up of two lobes located in
our neck, and it has a butterfly-like shape.
Thyroid Follicles:
- Inside
the thyroid gland, there are tiny structures called thyroid follicles.
- The
cells around these follicles are cuboidal
epithelial cells.
- These
cells produce the hormones T3 (triiodothyronine)
and T4 (thyroxine), which are crucial for
controlling your body's metabolism.
C Cells:
- In
addition to the follicular cells, there are also C cells in the thyroid
gland.
- C
cells are special cells that make a hormone called calcitonin.
- Calcitonin
helps regulate the levels of calcium in our
blood.
SYNTHESIS OF T3 AND T4
1. Iodine Uptake:
- Iodine
is obtained from the bloodstream by the thyroid gland. It's a crucial
element for making T3 and T4 hormones.
2. Thyroglobulin Protein:
- Inside
the thyroid follicles, there's a protein called thyroglobulin.
- This
protein is made up of building blocks called amino acids, including one
called tyrosine.
3. Iodine Binding:
- Iodine
atoms from the blood are attached (or bound) to tyrosine molecules within
thyroglobulin.
- There
are two ways iodine can bind: "mono-iodination" when one iodine
attaches and "di-iodination" when two iodine atoms attach to a
single tyrosine.
4. Formation of T3 and T4:
- When
thyroglobulin with attached iodine molecules undergoes specific chemical
changes, it results in the production of two thyroid hormones:
- Triiodothyronine
(T3) when three iodine atoms are bound to tyrosine.
- Thyroxine
or Tetraiodothyronine (T4) when four iodine atoms are bound to tyrosine.
These hormones, T3 and T4, are then stored within
the thyroid follicles and later released into the bloodstream as needed to
regulate metabolism and various body functions.
FUNCTIONS OF T3 AND T4
1.
Metabolism Regulation:
T3 and T4 hormones play a central role in regulating metabolism. They control
how quickly the body uses energy from food.
2.
Physical, Mental, and Sexual
Growth: T3 and T4 are essential for growth and development,
especially in children and adolescents. They support the growth of bones and
tissues, including the brain, and are involved in the development of secondary
sexual characteristics during puberty.
3.
Red Blood Cell (RBC) Production:
Thyroid hormones influence the production of red blood cells in the bone
marrow. An optimal level of T3 and T4 is necessary to ensure an adequate number
of RBCs.
DISORDERS OF THYROID GLAND
HYPOTHYROIDISM
Hypothyroidism
is a condition where the thyroid gland doesn't produce enough thyroid hormones,
leading to various health problems.
1.
Myxedema:
Myxedema is a severe form of hypothyroidism characterized
by swelling of the skin and tissues. It can also potentially lead to disruptions in the menstrual cycle in women.
2.
Cretinism:
Cretinism is a type of hypothyroidism that occurs in infants and children,
leading to stunted growth, intellectual disabilities,
and developmental delays.
3.
Goitre:
Goitre is the enlargement of the thyroid gland,
often seen in hypothyroidism, as the body tries to compensate for the lack of
thyroid hormones. It can cause a visible swelling in the neck.
HYPERTHYROIDISM
Hyperthyroidism is a medical condition characterized
by an overactive thyroid gland, leading
to excessive production of thyroid hormones.
1.
Exophthalmic Goitre (Graves'
Disease):
Exophthalmic goitre, also known as Graves' disease, is a thyroid disorder caused by an autoimmune response. In this condition, the immune
system mistakenly produces antibodies called
thyroid-stimulating immunoglobulins (TSIs) that mimic
the action of thyroid-stimulating hormone (TSH).
As a result, the thyroid gland is continually stimulated
to produce excess thyroid hormones (T3 and T4), leading to hyperthyroidism.
Besides the thyroid issue, Graves' disease is often
associated with a noticeable bulging of the eyes (exophthalmos)
and other hyperthyroid symptoms, such as weight loss, rapid heart rate, and
anxiety.
2.
Toxic Multinodular Goiter:
This condition occurs when multiple nodules or lumps
develop within the thyroid gland, and some of these nodules become overactive,
producing excessive thyroid hormones. It is a less common cause of
hyperthyroidism compared to Graves' disease but can lead to similar symptoms,
such as rapid heart rate, weight loss, and nervousness.
PARATHYROID GLAND
The parathyroid glands are small, pea-sized endocrine glands located in the neck,
usually situated behind the thyroid gland. There
are typically four parathyroid glands in the
human body, but the exact number and location can vary from person to person.
The primary hormone produced by the parathyroid
glands is called parathyroid hormone (PTH), also
known as parathormone. PTH plays a crucial role
in the regulation of calcium level in the body
by acting on the bones, kidneys, and intestines.
FUNCTIONS OF PARATHORMONE
1.
Bone:
PTH stimulates the release of calcium from bones into the bloodstream. This
process is known as bone resorption and helps
increase circulating calcium levels when they are too low.
2.
Kidneys:
PTH promotes the reabsorption of calcium in the
kidneys, reducing its excretion in the urine. This helps to conserve calcium in
the body.
3.
Intestines:
PTH indirectly enhances the absorption of calcium from
the intestines by stimulating the production of active vitamin D (calcitriol), which is necessary for calcium
absorption.
PANCREAS
The pancreas is a vital organ with both endocrine and exocrine functions. It is
typically referred to as a mixed gland because
it has both endocrine and exocrine components.
1.
Exocrine Function:
The exocrine portion of the pancreas consists of clusters of cells called acini, which produce and secrete digestive enzymes
into the pancreatic ducts.
2.
Endocrine Function:
The endocrine portion of the pancreas is composed of clusters of
hormone-secreting cells known as the pancreatic islets or islets of Langerhans. There are several types of cells
within the islets, but the two main types are alpha
cells and beta cells, each of which
produces specific hormones:
·
Alpha Cells:
Alpha cells secrete the hormone glucagon.
Glucagon acts to raise blood glucose levels by stimulating the liver to break
down glycogen into glucose (glycogenolysis) and
by promoting the conversion of other molecules into glucose (gluconeogenesis). This helps maintain blood sugar
levels when they are too low, such as during fasting or between meals.
·
Beta Cells:
Beta cells secrete the hormone insulin. Insulin
plays a crucial role in lowering blood glucose levels by promoting the uptake of glucose by cells, especially muscle and fat
cells. It also stimulates the liver to store excess
glucose as glycogen.