Essentials of Human Anatomy & Physiology

The Endocrine System

Dr. Narpat R. Gadhveer

The Endocrine System
• Second messenger system of the body
• Uses chemical messages (hormones)
that are released into the blood
• Hormones control several major
processes

• Reproduction
• Growth and development
• Mobilisation of body defences
• Maintenance of homeostasis
• Regulation of metabolism
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Hormone Overview

• Hormones are produced by specialised

cells
• Cells secrete hormones into
extracellular fluids
• Blood transfers hormones to target sites
• These hormones regulate the activity of
other cells

3
The Chemistry of Hormones
• Amino acid-based hormones
• Proteins
• Peptides
• Amines
• Steroids – made from cholesterol
• Prostaglandins – made from highly
active lipids

4
Mechanisms of Hormone Action

• Hormones affect only certain tissues or

organs (target cells or organs)
• Target cells must have specific protein
receptors
• Hormone binding influences the working
of the cells

5
Effects Caused by Hormones

• Changes in plasma membrane

permeability or electrical state
• Synthesis of proteins, such as enzymes
• Activation or inactivation of enzymes
• Stimulation of mitosis

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Steroid Hormone Action
• Diffuse through the plasma membrane
of target cells
• Enter the nucleus
• Bind to a specific protein within the
nucleus
• Bind to specific sites on the cell’s DNA
• Activate genes that result in synthesis of
new proteins
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Steroid Hormone Action

Figure 9.1a

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 Non steroid Hormone Action
• Hormone binds to a membrane receptor
• Hormone does not enter the cell
• Sets off a series of reactions that
activates an enzyme
• Catalyses a reaction that produces a
second messenger molecule
• Oversees additional intracellular
changes to promote a specific response
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Non steroid Hormone Action

Figure 9.1b

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Control of Hormone Release

• Hormone levels in the blood are

maintained by negative feedback
• A stimulus or low hormone levels in the
blood triggers the release of more
hormone
• Hormone release stops once an
appropriate level in the blood is reached

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Hormonal Stimuli of Endocrine
Glands

• Endocrine glands
are activated by
other hormones

Figure 9.2a

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Humoral Stimuli of Endocrine
Glands

• Changing blood
levels of certain
ions stimulate
hormone release

Figure 9.2b

Slide 9.12
Neural Stimuli of Endocrine Glands

• Nerve impulses
stimulate hormone
release
• Most are under
control of the
sympathetic nervous
system
Figure 9.2c

Slide 9.13
Location of Major Endocrine Organs

Figure 9.3
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Pituitary Gland

• Size of a grape
• Hangs by a stalk from the hypothalamus
• Protected by the sphenoid bone
• Has two functional lobes
• Anterior pituitary – glandular tissue
• Posterior pituitary – nervous tissue

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Hormones of the Anterior Pituitary
• Six anterior pituitary hormones
• Two affect non-endocrine targets
• Four stimulate other endocrine glands
(tropic hormones)
• Characteristics of all anterior pituitary
hormones
• Proteins (or peptides)
• Act through second-messenger systems
• Regulated by hormonal stimuli, mostly
negative feedback
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Hormones of the Anterior Pituitary

Figure 9.4

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Growth Hormone (GH)

• General metabolic hormone

• Major effects are directed to growth of
skeletal muscles and long bones
• Causes amino acids to be built into
proteins
• Causes fats to be broken down for a
source of energy

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Functions of Other Anterior Pituitary
Hormones
• Prolactin (PRL)
• Stimulates and maintains milk production
following childbirth
• Function in males is unknown
• Adrenocorticotropic hormone (ACTH)
• Regulates endocrine activity of the adrenal
cortex
• Thyroid-stimulating hormone (TSH)
• Influences growth and activity of the thyroid
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Functions of Other Anterior Pituitary
Hormones
• Gonadotropic hormones
• Regulate hormonal activity of the gonads
• Follicle-stimulating hormone (FSH)
• Stimulates follicle development in
ovaries
• Stimulates sperm development in
testes

Slide 9.20a
Functions of Other Anterior Pituitary
Hormones
• Gonadotropic hormones (continued)
• Luteinizing hormone (LH)
• Triggers ovulation
• Causes ruptured follicle to become the
corpus luteum
• Stimulates testosterone production in
males
• Referred to as interstitial cell-stimulating
hormone (ICSH)

Slide 9.20b
Pituitary - Hypothalamus
Relationship
• Release of hormones is controlled by
releasing and inhibiting hormones
produced by the hypothalamus
• Hypothalamus produces two
hormones that are transported to
Neuro secretory cells of the posterior
pituitary
• The posterior pituitary is not strictly
an endocrine gland, but does
release hormones
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Hormones of the Posterior Pituitary
• Oxytocin
• Stimulates contractions of the uterus during
labor
• Causes milk ejection
• Antidiuretic hormone (ADH)
• Can inhibit urine production
• In large amounts, causes vasoconstriction
leading to increased blood pressure
(vasopressin)

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Hormones of the Posterior Pituitary

Figure 9.5

Slide 9.22b
Thyroid Gland

• Found at the base of the throat

• Consists of two lobes and a connecting
isthmus
• Produces two hormones
• Thyroid hormone
• Calcitonin

Slide 9.23a
Thyroid Gland

Figure 9.6

Slide 9.23b
Thyroid Hormone

• Major metabolic hormone

• Composed of two active Iodine-
containing hormones
• Thyroxine (T4) secreted by thyroid
follicles
• Triiodothyronine (T3) conversion of T4 at
target tissues

Slide 9.24
Calcitonin
• Decreases blood
calcium levels by
causing its
deposition on
bone
• Antagonistic to
parathyroid
hormone
• Produced by C
( Para
follicular) cells. Figure 9.9
Slide 9.25
Parathyroid Glands

• Tiny masses on the posterior of the

thyroid
• Secrete parathyroid hormone
• Stimulate osteoclasts to remove
calcium from bone
• Stimulate the kidneys and intestine to
absorb more calcium
• Raise calcium levels in the blood

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Adrenal Glands

• Two glands
• Cortex – outer glandular region in three
layers
• Medulla – inner neural tissue region
• Sits on top of the kidneys

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Hormones of the Adrenal Cortex
• Mineralocorticoids (mainly aldosterone)
• Produced in outer adrenal cortex
• Regulate mineral content in blood, water,
and electrolyte balance
• Target organ is the kidney

Slide 9.28a
Hormones of the Adrenal Cortex

Figure 9.10

Slide 9.28b
Hormones of the Adrenal Cortex

• Glucocorticoids (including cortisone and

cortisol)
• Produced in the middle layer of the adrenal
cortex
• Promote normal cell metabolism
• Help resist long-term stressors
• Released in response to increased blood
levels of ACTH
Slide 9.29a
Hormones of the Adrenal Cortex

• Sex hormones
• Produced in the inner layer of the adrenal
cortex
• Androgens (male) and some estrogen
(female)

Slide 9.29b
Hormones of the Adrenal Medulla

• Produces two similar hormones

(catecholamines)
• Epinephrine
• Norepinephrine
• These hormones prepare the body to
deal with short-term stress

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Roles of the Hypothalamus and Adrenal
Glands in the Stress Response

Figure 9.12

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Pancreatic Islets
• The pancreas is a mixed gland
• The islets of the pancreas produce
hormones
• Insulin – allows glucose to cross plasma
membranes into cells from beta cells
• Glucagon – allows glucose to enter the
blood from alpha cells
• These hormones are antagonists that
maintain blood sugar homeostasis
Slide 9.32a
Pancreatic Islets

Figure 9.13

Slide 9.32b
Pancreatic Hormones and Blood Sugar

Figure 9.14
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Pineal Gland

• Found on the third ventricle of the brain

• Secretes melatonin
• Helps establish the body’s wake and sleep
cycles
• May have other as-yet-unsubstantiated
functions

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Thymus

• Located posterior to the sternum

• Largest in infants and children
• Produces thymosin
• Matures some types of white blood cells
• Important in developing the immune system

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Hormones of the Ovaries
• Estrogens
• Produced by Graafian follicles or the placenta
• Stimulates the development of secondary
female characteristics
• Matures female reproductive organs
• Helps prepare the uterus to receive a fertilized
egg
• Helps maintain pregnancy
• Prepares the breasts to produce milk

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Hormones of the Ovaries

• Progesterone
• Produced by the corpus luteum
• Acts with estrogen to bring about the
menstrual cycle
• Helps in the implantation of an embryo in
the uterus

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Hormones of the Testes
• Interstitial cells of testes are hormone-
producing
• Produce several androgens
• Testosterone is the most important
androgen
• Responsible for adult male secondary sex
characteristics
• Promotes growth and maturation of male
reproductive system
• Required for sperm cell production
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