The human endocrine system

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1. Terms:
● Endocrine glands: Ductless glands secreting hormones directly into bloodstream
● Exocrine glands: secrete substances into ducts that lead into cavities in the body or
lead directly to the external environment. (Examples: sweat glands, mammary glands, the
liver, salivary glands and the pancreas.)
● Hormones: organic chemical messengers secreted directly into the blood by an
endocrine gland.
● Homeostasis: is the process of maintaining a constant internal environment within
narrow limits, despite changes that take place internally and externally.
● Negative feedback mechanism: when there is an increase from normal, a corrective
mechanism causes a decrease and vice versa to maintain a balanced system.

2. What is the endocrine system?

• The Endocrine system works in conjunction with the Nervous system. The endocrine
system is responsible for chemical coordination, regulating the functioning of all the
organs in the body
• Consists of glands situated throughout the body
• These endocrine glands secrete organic chemical messengers called hormones in
the bloodstream
• Hormones are organic substances and are mostly proteins, but a few are lipids(fats)
(usually the sex hormones)
• Hormones are produced in small quantities
• They are carried in blood stream to target organ/tissue where they control the
activities of a target organ to perform a specific function
• Hormones work together as an integrated system where they either stimulate or
inhibit organs

3. The differences between the endocrine system and the nervous system
• The nervous system and endocrine system controls different types of activities in the
body.
• They are jointly responsible for the functioning of all the different organs and
systems, this is known as coordination
• The nervous system co-ordinates very quick responses to external stimuli
• The endocrine system controls responses that are not that fast but are long-lasting
and reflect the body’s internal environments

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 Endocrine system Nervous system
1. Made up of glands 1. Made up of nerves
2. Produces hormones 2. Produces nerve impulses
3. Hormones transported by the blood 3. Impulses transmitted along the nerves
4. Effects are slower and more general 4. Effects are very quick and very specific
5. Hormones control long-term changes 5. Nerve impulses control short-term
(e.g. growth) changes (e.g. sneezing, lifting your arm)

4. The differences between endocrine glands and the exocrine glands

Endocrine glands Exocrine glands

Glands are ductless, Secrete their substances into ducts and not the bloodstream (think
secrete directly into of the salivary glands secreting saliva in ducts that transports it to
bloodstream the mouth)

Secrete hormones Does not secrete hormones

The pancreas is the only gland that is both exocrine (pancreatic juices for digestion) and
endocrine (insulin and glucagon).

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 5. Position and function of the endocrine glands

Hormone Gland Location Function

Antidiuretic Neurosecretory In the centre of the Regulates

hormone (ADH) cells of the brain osmoregulation in the
hypothalamus kidneys (in the distil
convoluted tubules and
ADH is stored in the collecting tubules)
the Pituitary
gland

Thyroxin Thyroid gland Below the larynx in ● Regulates the basal

the neck region metabolic rate of the
cells in the body
● Affects growth and
functioning of the
heart and the
nervous system

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 Hormone Gland Location Function

● Stimulates growth
and differentiation of
tissue in a foetus and
in children
● Regulates the body
temperature when
stimulated by the
hypothalamus
Adrenalin Adrenal gland Above the kidney Prepares the body to
(fight-and-flight deal with stress:
hormone) ● Increase in heartbeat
rate
● Increase in breathing
rate
● Increase in blood
pressure
● Increase in muscle
tone
● Increase in blood
sugar levels
● Decrease in blood
supply to the skin
and digestive system
● Causes pupils to
dilate
Aldosterone Adrenal gland Above the kidney Helps the uptake of
sodium ions in the loop
of Henle in the kidneys
Prolactin Pituitary gland: Base of the brain and ● Stimulates the
attached to the mammary glands to
hypothalamus produce milk
● Counteracts the
effect of dopamine
which is responsible
for sexual arousal
Oestrogen Gonads: ovaries Located in the lower ● Oestrogen promotes
abdominal region the development of
with each ovary the secondary sexual
located within the characteristics in
pelvic bones (in line females like breasts,
with the ball-and- the thickening of the
socket joints of the endometrium
femurs) (uterus) and the
female body shape
● Necessary for the

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 Hormone Gland Location Function

process of ovulation
● Oestrogen inhibits
the secretion of FSH
by the anterior
pituitary gland so
that only one follicle
is produced during
ovulation
● High oestrogen levels
will trigger the
secretion of
luteinising hormone
(LH)
Progesterone Gonads: ovaries Located in the lower ● Progesterone
abdominal region prepares the
with each ovary endometrium of the
located within the uterus for
pelvic bones (in line implantation once
with the ball-and- fertilisation of the
socket joints of the ovum has occurred
femurs) ● Necessary for the
production of the
mucus plug to
prevent sperm or
other substances
from entering the
uterus during
pregnancy
● Decrease in
progesterone levels
causes menstruation
● Progesterone
improves memory
and cognitive ability
Testosterone Gonads: testes Leydig cells in the ● Testosterone is
testes of males responsible for the
located in the secondary sexual
scrotum at the characteristics in
bottom of the pelvis males like a deeper
voice, pubic hair, hair
on face
● Necessary for the
normal development
of sperm
● Activates genes in

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 Hormone Gland Location Function

the cells of Sertoli to

promote the
differentiation of the
spermatogonia
Glucagon Pancreas: Islets Endocrine cells of the Controls the increase in
of Langerhans pancreas the blood sugar level by
causing the conversion
of glycogen to glucose
Insulin Pancreas: Endocrine cells of the ● Controls blood sugar
Islets of pancreas by causing the
Langerhans conversion of glucose
into glycogen
● Inhibits the
functioning of
glucagon
Growth hormone Pituitary gland: Base of the brain and For growth, repair and
(somatotrophin) attached to the replacement of cells
hypothalamus
Follicle Pituitary gland: Base of the brain and In males: stimulates
stimulating attached to the spermatogenesis
hormone (FSH) hypothalamus In females: stimulates
the development of the
follicle for process of
ovulation
Luteinising Pituitary gland: Base of the brain and In males: stimulates the
hormone (LH) attached to the synthesis of the
hypothalamus hormone testosterone
by the Leydig cells in the
testes
In females: LH stimulates
the release of the
secondary oocyte from
the Graafian follicle and
then the development
into the corpus luteum
Thyroid- Pituitary gland: Base of the brain and Stimulates the
stimulating attached to the production of thyroxin
hormone (TSH) hypothalamus by the thyroid gland

Prolactin Pituitary gland Base of the brain and Stimulates mammary

attached to the glands to secrete milk
hypothalamus

6. Homeostasis

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It is a process of maintaining a constant internal environment (blood and tissue fluid) within
the body. This enables the body to function efficiently, despite changes in the external or
internal environment
The following changes of the internal environment in the tissue fluid and blood, will affect
the homeostatic balance of the body:
● temperature
● glucose levels
● carbon dioxide levels
● water levels and
● salt levels

7. NEGATIVE FEEDBACK MECHANISMS

They operate in the human body to detect changes or imbalances in the internal
environment and to restore the balance.

General sequence of events in a negative feedback mechanism:

• STEP 1- An imbalance is detected

• STEP 2 - A control centre is stimulated
• STEP 3- Control centre responds
• STEP 4- Message sent to target organ/s
• STEP 5- The target organ responds
• STEP 6- It opposes/reverses the imbalance
• STEP 7- Balance is restored

7. Regulation of thyroxin levels in body

• Thyroxin levels are too high:

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 • Thyroxin levels are too low

8. Regulation of Glucose levels through negative feedback mechanism

When the glucose level in the blood INCREASES above normal levels:
Step 1 Glucose levels in the blood increase above normal levels
Step 2 The beta cells of the pancreas are stimulated
Step 3 to secrete insulin into the blood
Step 4 Insulin travels in the blood to the liver and muscle cells (target organ/s)
Step 5 where it stimulates the conversion of excess glucose to glycogen which is
then stored
Step 6 The glucose level in the blood now decreases
Step 7 and returns to normal

When the glucose level in the blood DECREASE below normal levels:
Step 1 Glucose levels in the blood decrease below normal levels
Step 2 The alpha cells of the pancreas are stimulated
Step 3 to secrete glucagon into the blood
Step 4 Glucagon travels in the blood to the liver and muscle cells (target organ/s)
Step 5 where it stimulates the conversion of stored glycogen to glucose
Step 6 The glucose level in the blood now increases
Step 7 and returns to normal
9. A flowchart depicting the negative feedback mechanism of glucose levels in the blood

Pancreas secretes insulin Glucose is converted to

glycogen

Glucose level increases Glucose level decreases

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Normal Glucose Levels in blood
10. The regulation of Carbon dioxide levels in the blood:

11. The regulation of water balance in the blood. Osmoregulation in the blood.

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12. The regulation of Salt levels in the bloodstream

Flow chart depicting the regulation of salt levels in the bloodstream

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