What is endocrinology?

Endocrinology = Intercellular Chemical Communication

Endocrinology is communication systems &

information transfer.
 What is the role of the endocrine system?

• Maintain Internal Homeostasis

• Support Cell Growth
• Coordinate Development
• Coordinate Reproduction
• Facilitate Responses to External Stimuli
• Coordination of physiological processes as metabolism, respiration,
excretion, movement and reproduction.
 What is the endocrine system?
Endocrine system = Endocrine gland + Hormone

What is the endocrine gland?

The endocrine gland is a ductless gland that releases its
secretion directly into the blood stream.
• Hormones are synthesized and secreted by living cells.
• Hormones are released directly into the blood stream that
transports it to a target organ.
• Hormone relays information regulating a function or body process.
• The hormone is either stimulated or inhibits certain metabolic
activity without taking part in any reaction.
 Endocrine System Characteristics
◼ Access to every cell.

◼ Each hormone acts only on specific cells (target cells).

◼ Only specific cells have receptors for specific

hormones.
◼ There are 7 major endocrine glands in the adult human
body: hypothalamus, pituitary, thyroid, parathyroid,
adrenal, pancreatic islets of Langerhans and gonads.
Islets of Langerhans
 Types of Hormones

According to the relation

According to the chemical structure of
between the secreting cell and
the hormone
the cell acted upon (target cell)
 According to the relation between the secreting cell and
the cell acted upon (target cell)

1. Endocrine hormone
It is a hormone secreted from a gland or cell into the blood
stream that transport it to its target organ or cell.
2. Paracrine hormone
It is a hormone secreted from a cell into the extracellular space
and affects the neighboring cell.
3. Autocrine hormone
It is a hormone secreted from a cell into the extracellular space
and affects the same cell that secrete it.
4. Neuroendocrine
It is a hormone secreted from a nerve cell into the blood stream
which transports it to a target organ or cell to relay metabolic
information.
 Types of Hormones

According to the relation

According to the chemical structure of
between the secreting cell and
the hormone
the cell acted upon (target cell)
 According to the chemical structure of the hormone
1. Protein or peptide hormones: Water soluble. Bind to
receptors on target cell membranes.
2. Steroid hormones: Lipid soluble chemically derived
from cholesterol. Enter target cells.
3. Tyrosine-derived hormones: Hormones synthesized from the
tyrosine (amino acid).

4. Eicosanoids: These are made up from fatty acids derived from the
arachidonic acid within the plasma membranes of most cells of the
body (prostaglandins, prostacycline, thromboxanes and lekotrienes).
Non-steroid Hormone Action on a Target Cell
 Non-steroid hormones
◼ Water soluble.
◼ Bind to receptors on target cell membranes.
◼ Work through intermediate mechanisms to activate existing
enzymes.
◼ Faster action than steroid hormones, seconds to minutes.
Steroid Hormone Action on a Target Cell
 Steroid Hormones

◼ Lipid soluble, chemically derived from cholesterol.

◼ Enter target cells.
◼ Activate specific genes to produce specific proteins.
◼ Slower acting than non-steroid hormones (Takes minutes
to hours).
 Hypothalamo-Pituitary Axis
• There is a coordination between the endocrine and nervous
systems.
• Pituitary gland is the maestro gland because of its dynamic role in
regulating the body needs and its control of important glands.
• The hormones secreted by the pituitary gland regulate either other
major glands or affect most, if not all, cells of the body i.e. they
have a general effect.
• Hypothalamus (part of the brain) controls the pituitary gland and
now it is considered as an endocrine gland.
• Pituitary gland is composed of two lobes: anterior pituitary
(adenohypophysis) and posterior pituitary (neurohypophysis).
i.e. the two hormones, vasopressin
(antidiuretic hormone, ADH)
and oxytocin are synthesized in
the hypothalamus, but are stored
and secreted by the posterior
pituitary
 Regulation of hormone secretion
Hypothalamus control anterior pituitary hormones in
a three hormone sequence
(1) Hypothalamic hormone (releasing hormone)
that control the secretion of (2).
(2) An anterior pituitary hormone (tropic hormone)
that control the secretion of (3).
(3) A hormone (effector hormone)
from some other endocrine gland. 1

This system is controlled by a negative

feedback mechanism 2

Regulation of the 3-hormone sequence

system keeps the hormones at levels that
3
maintain homeostasis.
 Thyroid Gland
✓ The thyroid gland is located at the base of the neck in front of the
trachea.
✓ The thyroid is the largest gland in the human body.
Thyroid gland secretes:
◼ Thyroxine (tetraiodotheyonine,T4) and Triiodothyronine

(T3).
◼ Calcitonin.
The thyroid hormone synthesis and secretion are regulated by the
anterior pituitary thyrotrophin, thyroid-stimulating hormone (TSH).
The output of TSH is influenced by:

1. Thyroid-releasing hormone (TRH) from the hypothalamus.

2. The inhibitory hormone, Somatostatin, from the
hypothalamus.
3. The blood level of free thyroid hormones (T3 and T4).

• Somatostatin is a hormone that inhibits the secretion of TSH, growth

hormone (GH). “Write”
◼ Negative feedback mechanism:
1. According to the concentrations of free T3 and T4 a negative
feedback mechanism takes place at the level of both the
hypothalamus and the pituitary.

2. They inhibit TRH secretion from hypothalamus which

diminishes TSH secretion from the pituitary.

3. The decreased TSH will decrease the synthesis and release of

thyroid hormones.
 Actions of thyroid hormones
• Regulate metabolic effects.
• Promote normal growth and development.
• Regulate homeostatic functions (energy, heat production).
• Maintain normal blood pressure, heart rate, muscle tone, digestion
and reproductive functions.
• Required for the actions of other hormones.

They are unique in that they exert effects within almost every tissue of
the body throughout the life of the animal.
Dysfunctions of thyroid glans
Example of a Negative
Feedback Loop:
Homeostasis
➢ Calcium homeostasis

Calcitonin

Thyroid
gland Reduces
Stimulates Ca2+ uptake
releases
Ca2+ deposition in kidneys
calcitonin
in bones

STIMULUS:
Rising
blood Ca2+
level Homeostasis: Normal blood STIMULUS:
(imbalance) calcium level (about 10 mg/100 mL) Falling
blood Ca2+
level
(imbalance)

Active
vitamin D
Parathyroid
glands
release parathyroid
hormone (PTH) Parathyroid
gland
Stimulates Increases Increases
Ca2+ release Ca2+ uptake Ca2+ uptake PTH
from bones in kidneys in intestines
Calcium-regulating hormones:
• Calcitonin is the only calcium-lowering hormone in the
body. Calcitonin is lowering the calcium blood level by
three effects:
I) It causes more calcium to be deposited in the bones.
II) It makes the intestine absorb less calcium from the
diet.
III) It makes the kidneys reabsorb less calcium from the
filtrate.
• Bone serves as the main source of calcium in the body.
• The three main organs that involved in maintaining
calcium levels are bone, kidney and gut.
◼ Role of Vitamin D in raising the calcium blood level:
1. Vitamin D (Cholecalciferol) is produced in the humans
by the action of ultraviolet light.
2. Vitamin D is transported to the liver then to the kidney
where it is converted to the active form.
3. The active form of vitamin D enables the parathyroid
hormones to:
i. Increase calcium uptake by the intestine.
ii. Stimulate release of calcium from bones.
iii. Increase calcium uptake by the kidneys.
 Parathyroid Glands

◼ Secretion: response to lowered blood calcium levels

◼ Hormone: Parathyroid hormone (PTH), non-steroidal
◼ Removes calcium from bone
◼ Increases absorption of calcium by the digestive tract
◼ Increases renal retention of calcium
 Adrenal gland
Adrenal gland is a triangular in shape found above the kidney. It
composed of: cortex to the outside and medulla in the center.
 Adrenal cortex
The cortex is divided into 3 distinct zonae: zona glomerulosa, zona
fasciculata and zona reticularis.
Each zona is specialized in synthesizing a particular type of steroid
hormone. Glomerulosa secretes mineralocorticoids, fasciculata
secretes glucocorticoids, while reticularis secretes androgens.
 Adrenal medulla
Adrenal medulla secretes 2 of tyrosine-derived hormones;
adrenaline (epinephrine) and noradrenaline (norepinephrine). These
hormones are called the 3f’s (fear-fight-flight) hormones because they
contribute to the response of stress.
 Pancreas
◼ The pancreas is made up of two functionally different parts: the
exocrine pancreas, the major digestive gland of the body; and the
endocrine pancreas.
Exocrine pancreas
There are many types of cells in the endocrine pancreas:
Alpha-cells: synthesize, store and secrete glucagon
Beta-cells: synthesize, store and secrete insulin.
D-cells: synthesize, store and secrete somatostatin.
F-cells: synthesize, store and secrete pancreatic polypeptide.
 Pancreas
The endocrine pancreas

Islets of Langerhans

Alpha-cells Beta-cells

Glucagon Insulin

Raises blood sugar Lowers blood sugar

➢ Glucose homeostasis

Body
cells
Insulin take up more
glucose

Beta cells
of pancreas stimulated
to release insulin into
the blood Liver takes Blood glucose level
up glucose declines to a set point;
High blood and stores it as stimulus for insulin
glucose level glycogen release diminishes

STIMULUS:
Rising blood glucose
level (e.g., after eating
a carbohydrate-rich
meal) Homeostasis: Normal blood glucose level
(about 90 mg/100 mL) STIMULUS:
Declining blood
glucose level
(e.g., after
skipping a meal)

Blood glucose level

rises to set point; Alpha
stimulus for glucagon cells of
release diminishes pancreas stimulated
to release glucagon
into the blood
Liver
breaks down
glycogen and Glucagon
releases glucose
to the blood
 Insulin
◼ Insulin: is the only hormone that directly lowers blood
glucose levels.
◼ The principle stimulus for insulin secretion is the
increased blood glucose concentration.
◼ The main target organs of insulin are the liver and
almost all cells of the body.
◼ Insulin stimulates glucose uptake and glycolysis for
energy.
 Glucagon

◼ Glucagon: It raises blood glucose concentration.

◼ Glucagon is very important in maintaining blood
glucose and energy substrates in the circulation during
fasting.
◼ The main target organs of glucagon are the liver and
adipose tissue.
◼ Glucagon Stimulates:
➢ Breakdown of stored glycogen (glycogenolysis).
➢ Synthesis of glucose from non-carbohydrate compounds
(Gluconeogenesis).
➢ Ketogenesis
➢ Lipolysis in adipose tissue.
Diabetes Mellitus
Insulin-Dependent Diabetes Mellitus (IDDM) (type 1):
➢ It is characterized by progressive marked decrease in the
number of insulin secreting beta-cells.
➢ It is autoimmune disease, where destruction of beta–
cells by islet-cell antibodies occurs.
➢ The pancreas does not produce insulin enough, so the
glucose, amino acids, and fatty acids increased in the
bloodstream from their respective storage depots.
➢ The continued fat breakdown leads to the production and
accumulation of ketones.
Non Insulin-Dependent Diabetes Mellitus (NIDDM) (type
2):
➢ It is the more prevalent type and occurs predominantly in
adults.
➢ It may be due to either the insulin secreted is not sufficient
(insufficient insulin secretion) or the tissues do not
respond to the hormone (insulin resistance).
➢ The glucose in the blood becomes extremely high and
glucose excreted in the urine.