homeostasis & hormones
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HOMEOSTASIS & HORMONES
“…it is all about a balancing act…”
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OVERVIEW
A) HomeostasisB) Hormones: chemical nature and mode of
actionC) Hypothalamus and Pituitary glandD) Adrenal glandsE) Hormonal regulation of blood sugar levels
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Homeostasis: Greek: ‘homoios’ = similar
‘stasis’ = standing still
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Intracellular fluid (cytoplasm)
Homeostasis is the maintenance of a relatively stable internal environment
What is the ‘internal’
environment?
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Are variables absolutely constant?
STEADY STATE
Low
Variable
High
NO
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Examples of Physiological conditions requiring homeostasis:
O2 and CO2 levels in the bodyenergy requirementsglucose level in bloodwater / ion balancepHtemperature
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fluid surrounding cells
where organism
lives
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Living systems are seen to be open systems. What does this mean?
Require a continuous exchange of matter between the environment and themselves
Oxygen
Food
Carbon dioxide
Heat
UreaWater
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An organism may be thought of as a complex system of chemical processes
these processes are: self-regulating
tend to maintain a steady state even though the external environment changes
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Why is homeostasis is important?An organism functions more efficiently if
conditions are maintained within narrow limits close to optimum conditions
To prevent large fluctuations from the
optimum
What causes these fluctuations? Changes in external and internal environments
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Internal conditions may be maintained constant within the body by:
developing a variety of mechanisms:
StructuralPhysiologicalBehavioural
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Fat
Fur
Feathers
Structural
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Which structure is missing here?
May 22, 2002
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Sweat
Increase in heart rate Cardiac arrest
in frozen frog
Physiological
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HOT: Seek shade
HOT: Seek shade
COLD: Bask in the sun
Behavioural
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Three basic components of a control system:
1. Detector / Receptor / Sensor
2. Regulator / Control centre / Co-ordinator / Integrating centre
3. Effector
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Stimulus(Input)
Response(Output)
What is the role of each component?
Detects deviation from set point
Produces response
Compares input with set point & determines response
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Integrating Centre in mammals is:
An endocrine glandBrain or spinal
cord
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What is Feedback?Feedback refers to responses made
after a change has been detected
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Time
Positive feedbackTwo forms of feedback:
Negative feedback
Time
EquilibriumDivergence
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Negative Feedback:refers to the mechanism by which a system
responds to a fluctuation in the opposite direction
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Corrective mechanism: DECREASES body temperature
Body temperature: RISES
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Corrective mechanism: INCREASES body temperature
Body temperature: DECREASES
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Sensor + Regulator:
Effector:
Blood sugar regulation: Negative feedback
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In negative feedback (feedback inhibition):
the initial stimulus provokes a response which tends to reduce the magnitude of the stimulus
as conditions return to their optimum, the corrective processes can be switched off
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Negative feedback also applies to the regulation of a population size:
Death rate increases
Birth rate increases
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Why is negative feedback very common in the body?
increases the stability of systems
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Examples of Negative Feedback Control:
O2 and CO2 levels in the bodyhormone levels, e.g. thyroxinesex hormonesmetabolic levels e.g. glucosewater balancethe regulation of pHbody temperature
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Control of thyroxine release as an example of negative feedback
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TRH (thyroid releasing hormone / thyrotrophin releasing hormone)
TSH (thyroid stimulating hormone)
TRH
TSH
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Positive feedback: a disturbance leads to events which increase
the disturbance even further
rare in biological systems WHY rare?
lead to: an unstable situation extreme states
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Examples of Positive Feedback Control:
1. Blood clotting Activated platelet releases chemicals
More platelets are activated
A blood clot forms
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Examples of Positive Feedback Control:
2. Child birth
Oxytocin stimulates muscular contractions of the uterus
More oxytocin is released
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3. The control of ovulationOestrogen stimulates the secretion of LH (luteinising hormone)
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4. Stimulation of a nerve fibresodium gates open and sodium ions rush into
the axon.
Cytoplasm
Extracellular fluid
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Na+Na+
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K+ K+
Na+Na+
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Na+
Na+
Na+
Na+
K+
K+ K+
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Na+
Na+
K+
K+
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Does the disturbance ever stop? in most cases, once the purpose of the
feedback loop is completed, counter-signals are released which:
suppress or break the loop
Oxytocin level drops once baby
is born
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Essay Title
Write an essay on feedback mechanisms and their role in control of biological systems.
[SEP, 2001]
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OVERVIEW
A) HomeostasisB) Hormones: chemical
nature and mode of action
C) Hypothalamus and Pituitary gland
D) Adrenal glandsE) Hormonal regulation of
blood sugar levels
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A chemical communication system is made up of at least two types of cell:one cell:produces & releases the hormone message released
second cell with receptor:receives message target cell activated
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Receptors are SPECIFIC
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Explain why insulin affects all body cells but ADH only the kidney.
Receptors for insulin occur on all body cells but those for ADH only in the kidney.
ADHTarget:Kidney cell
Receptor
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Hormones are chemicals secreted by:
Endocrine cells
Nerve signals (arrows) cause release of neurohormones
Neurosecretory cells
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Hormones are classified over distance they operate:
Histamine
[endocrine only is in syllabus]
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Endocrine hormones : are released into the bloodstream and interact with distant target cells e.g. adrenaline
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Where are the receptors located?
Position of receptor depends on the solubility
of the hormone
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Lipid-soluble (lipophilic) hormones
intracellular receptors
Water-soluble hormones:
receptors on the cell surface
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Water-soluble & lipid-soluble hormones act in different ways:
Water-soluble hormones:Activate enzymes within the cell
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Lipid-soluble Hormones:
Induce transcription
and translation to form
enzymes
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Explain why lipid-soluble hormones are slow acting.
Enzymes need to be formed
before a reaction occurs.
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What can you say about the solubility of ‘growth factor’? Explain.
Water soluble. Receptor is in plasma membrane as it cannot pass through it.
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Steroid Model vs. Protein Model
• Diffuse through cell membrane
• Receptors in cytoplasm• Response has slow onset• Long-lasting• Typically derived from
cholesterol• Examples: Testosterone,
Oestrogens, Progesterone
• Do not diffuse through cell membrane
• Receptors on surface of cell• Response has quick onset• Short-lasting• Typically derived from
amino acids• Examples: Insulin,
Glucagon
Steroid Model Protein Model
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Question: End-of Year SEP, 2010
Use your knowledge of biology to describe the significance of the following.
Steroid hormones act by activating genes.(5 marks)
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Enzymes within the cell are activated when a hormone binds to a receptor
in the membrane by the help of a:
second messenger e.g. cyclic AMP (cAMP)
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cAMP forms from ATPThe enzyme that catalyzes the formation of cAMP from ATP is:located on cytoplasmic side of plasma membranecalled adenylyl cyclase or adenylate cyclase
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cAMP forms from ATP
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Formation of cAMP
Adenylyl cyclase
AB
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Formation of cAMP
G protein receptor
signal
CEnzymes activated
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cAMP is a common second messenger
What is a second messenger?
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A second messenger is a substance released in the cytoplasm after the first messenger – the signal –
binds its receptor
GTP: guanosine triphosphate
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What is the “first messenger”?
Hormone
Why is a “second messenger”
needed? Hormone cannot
enter the cell.
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Cyclic AMP (cAMP) has many effects on the cell :
including glucose mobilisation for: energy muscle contraction
Let us study the role of cAMP in the release of glucose from the liver
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A Cascade of Reactions (part 1) an enzyme cascade is a system for amplifying
a biological signal
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A Cascade of Reactions (part 2)
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cyclic adenosine monophosphatecAMP
bgaGTP
adrenaline
Gs
Inactive proteinkinase A
Active proteinkinase A
adenosine triphosphateATP
aGTPGDP
G-protein linked receptor adenylate cyclase
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Why are only a few molecules of the
hormone needed to produce many molecules of
product?Each enzyme in the cascade can act on many molecules of the next molecule.
Amplification occurs through a cascade of
reactions.
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End-of-Year 2012
Adrenaline affects a range of target tissues in the body. Suggest how the adrenaline molecule can cause different effects in different target tissues. Different tissues have different (types of adrenaline) receptors;(causing) cAMP concentration/ second messenger to increase or decrease;Second messenger (may be) different;activates different / other enzymes / enzyme reactions (in different target cells)
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cAMP is not the only second messenger
e.g. nitric oxide (NO)
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cAMP is not the only second messenger
nitric oxide (NO) causes relaxation of
blood vessels
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Calcium can be a second messenger
calcium is widely occurring
an increase in cytoplasmic Ca2+ concentration will cause:
NOTE: the same second messenger is involved in two very different biological actions
A muscle cell to contract
A salivary gland cell to fluid secrete
12
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Remember: The nature of the biological activity triggered by second messenger is dependent on the cell type
The same second messenger is involved in more than one biological action.
True of all second messenger pathways:
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Protein hormones cannot enter the cell. Say why.
Question: End-of Year SEP, 2010Use your knowledge of biology to describe the significance of the following. (5 marks)
Protein hormones act by means of a second-messenger system.
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A Gland is: a structure which secretes a specific chemical
substance or substances
Sweat gland
Salivary gland
Gastric gland
Thyroid gland
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A Gland secretes chemicals into:
Some other part of the body
The bloodstream
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Glands in the body are:
secrete a product into a duct
secrete hormones have no duct (ductless) have a rich blood supply
Exocrinegland
Endocrine gland
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Why is the pancreas described as both an endocrine and an exocrine gland?
Produces both enzymes and hormones.
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Why is the pancreas richly supplied with blood?
Blood is needed to carry hormones towards the target organs.
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Four different chemical categories of hormones:
1. POLYPEPTIDES hormones composed of less than 100 amino
acids e.g.
Insulin Glucagon Oxytocin Growth hormone ADH
Growth hormone
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2. GLYCOPROTEINS hormones composed of a:
polypeptide larger than 100 amino acids attached to a carbohydrate e.g. FSH, LH
Carbohydrate
Polypeptide
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3. STEROIDS these hormones are lipids derived from
cholesterol e.g. testosterone, oestrogen,
progesterone, cortisol
Oestrogen
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4. AMINES derived from the amino acids tyrosine
and tryptophan
include all hormones produced by the: adrenal medulla thyroid gland
Adrenal glands
Kidneys
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SummaryA hormone is a chemical messenger with the following properties:
it travels in the blood;
it has an effect at a site different from the site where it is made, called the target;
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Summaryit fits precisely into receptor molecules in the
target like a key in a lock – it is therefore specific for a particular target;
it is a small soluble organic molecule;
it is effective in low concentrations.
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OVERVIEW
A) HomeostasisB) Hormones: chemical nature and mode of
actionC) Hypothalamus and Pituitary glandD) Adrenal glandsE) Hormonal regulation of blood sugar levels
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The Hypothalamus & Pituitary Gland
1. Nervous 2. Endocrine systems
major centres in the body for the coordination of the:
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Position of Hypothalamus & Pituitary Gland
Pituitary gland
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The Pituitary Glandis a small red-grey glandweighs about 0.5g [size of a pea]hangs from the base of the brain
by a short stalk
is a link between the nervous system and many endocrine glands
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Neurosecretory cells end at the Posterior Pituitary
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The Posterior Pituitary Gland:
does not synthesise any hormones but stores and releases two hormones:
ADH (antidiuretic hormone or vasopressin)
Oxytocin
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Mammary glands,uterine muscles
Hypothalamus
Kidney tubules
OxytocinHORMONE
TARGET
ADH
Posteriorpituitary
Neurosecretorycells of thehypothalamus Axon
Anteriorpituitary
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Function of ADH: Increased permeability to water by:
Distal convoluted tubule
Collecting tubule
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ADH causes formation of aquaporins in tubule walls
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Functions of Oxytocin: contraction of the uterus during birth ejection of milk from the nipple
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The Anterior Pituitary is connected to the hypothalamus by a:
Portal system:connects two
organs, neither of which is the heart
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Relationship between secretory cells & blood vessels in hypothalamus & pituitary
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The hypothalamus collects information from:
1. other regions of the brain 2. blood vessels passing
through it
What does the hypothalamus do with this information?
passes it to the pituitary gland
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How does the pituitary gland respond?
By releasing hormones,
which directly or indirectly
regulate the activity of all other endocrine
glands
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What parts of the body does the pituitary gland affect?
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How does the hypothalamus control the anterior pituitary gland?
Hypothalamus produces:1. Releasing hormones2. Release-inhibiting hormones
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The pituitary gland is under the influence of the hypothalamus in two ways. How?
1. The anterior pituitary produces hormones or stops producing them when the hypothalamus releases its own hormones.
2. The hormones released from the posterior pituitary are actually secreted by neurosecretory cells in the hypothalamus.
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Hypothalamus hormone
Anterior pituitary hormones
Site of action
Prolactin-releasing hormone Prolactin release-inhibiting hormone
Prolactin
Inhibition of prolactin secretion
Ovary and mammary gland
Gonadotrophin-releasing hormone (GnRH)
FSHLH
Ovary and testis
Thyrotrophin-releasing hormone (TRH)
TSH Thyroid gland
Adrenocorticotrophin-releasing hormone (CRH)
ACTH Adrenal cortex
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Hormones Released by the Anterior Pituitary:
TSH- Thyroid Stimulating Hormone ACTH - Adrenocorticotrophic hormone LH - Luteinising hormone FSH – Follicle Stimulating Hormone Prolactin
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Functions of Anterior Pituitary Hormones:
Prolactin – stimulates milk production and secretion
FSH - stimulates growth and maturation of eggs in
females- stimulates sperm production in males
LH – stimulates secretion of sex hormones from ovaries and testes
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Functions of Anterior Pituitary Hormones:
TSH – stimulates secretion of thyroxine from the thyroid gland
ACTH – stimulates release of glucocorticoids such as cortisol from adrenal cortex
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Hormones Released by the Anterior Pituitary:
TSH ACTH LH FSH Prolactin
Tropins or Trophic hormones
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What is a trophic
hormone?
A trophic hormone
stimulates other endocrine glands to release their
hormones
Anterior pituitary secretes
hormones that stimulate:
Gonad(testis)
Thyroid gland
Adrenalcortex
Other glands to secrete hormones
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The release of the trophic hormones is
regulated by negative feedback of hormones from the target glandsacting on receptors in the: hypothalamus anterior
pituitary
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The Pituitary Gland has often been called “the master gland” of the body
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The Pituitary Gland has often been called “the master gland” of the body
such a term is justified since the anterior and posterior pituitary glands release a battery of hormones that collectively influence all cells and affect virtually all physiologic processes
the pituitary gland may be the king, but the
power behind the throne is clearly the hypothalamus
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HOWEVER: the pituitary gland secretes some
hormones that are actually produced by neurones in the hypothalamus
under the influence of still other
brain hormones, it produces a number of its own hormones, which control the activities of various endocrine glands throughout the body
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Essay Titles:
Describe the role of the anterior pituitary gland in the co ordination of hormonal activity. ‑
[SEP, 1996]The pituitary gland is referred to as a “master
gland” in the human body. Discuss. [MAY, 2004]
Give an account of the role of the hypothalamus and pituitary gland in hormonal control in the human body. [SEP, 2011]
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OVERVIEW
A) HomeostasisB) Hormones: chemical nature and mode of
actionC) Hypothalamus and Pituitary glandD) Adrenal glandsE) Hormonal regulation of blood sugar levels
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Where are the adrenal glands positioned?
Adrenal glands
Kidneys
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Two regions in the adrenal glands:Cortex
Medulla
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Two regions in the adrenal glands:
- richly supplied with nerves and blood vessels
- cells of the medulla are modified neurones of the sympathetic NS
Cortex- forms 80% of the gland- produces steroid hormones
Medulla
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Steroid hormones of the Adrenal CortexMineralocorticoids mainly aldosterone
(stimulates kidney to conserve sodium and excrete potassium)
Glucocorticoids mainly cortisol: (under stress, raises blood glucose concentrations )
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The Cortex is able to:
i) synthesise cholesterolii) take up cholesterol from the circulation
following absorption from the diet
Why does the cortex need a supply of cholesterol ?
To synthesise steroid hormones.
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Hormones of the Adrenal Medulla
Adrenaline (epinephrine)Noradrenaline (norepinephrine) [not in syllabus]
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Adrenaline
– Blood glucose increases– Blood glycerol and fatty
acids increase– Heart rate increases– Blood pressure rises– Breathing rate increases– Air passages dilate– Pupils dilate– Blood flow redistributes,
leading to increased alertness and decreased digestive and kidney activity
• Causes short term “fight or flight” or alarm stage:
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Adrenal medulla is closely linked to the nervous system
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OVERVIEW
A) HomeostasisB) Hormones: chemical nature and mode of
actionC) Hypothalamus and Pituitary glandD) Adrenal glandsE) Hormonal regulation of blood sugar levels
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The Pancreas
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The Pancreas produces:
Pancreatic duct
1. HORMONES2. PANCREATIC JUICE
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Is it correct to state that the pancreas is both an endocrine & an exocrine gland?
YES. Endocrine: produces hormones
Exocrine: produces enzymes.
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Hormones and Pancreatic juice do not mix
Produce hormones
Produce enzymes
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Why is the islet of Langerhans highly vascularised?
Hormones produced here
must be transported to
their target organ via the
bloodstream.
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Beta cells produce insulin
Which cell produces which hormone?
REMEMBER: BIN
sugar
Alpha cells produce glucagon
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When are insulin and glucagon secreted?
Insulin
Glucagon
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Islet of Langerhans
Beta cell
INSULIN
Cells become permeable to GLUCOSE
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How is it possible for insulin to bring an effect inside body cells?
Insulin binds to its
receptors.
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How does insulin make cells more permeable to glucose?
Glucose transporters are inserted within the plasma membrane.
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Insulin is:a small protein composed of 51 amino acidsreleased: blood glucose level rises above
90 mg per 100 cm3 bloodcarried in the plasma bound to globulin
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Insulin has an effect on:
every organ of the bodybut main effect is on the:
Liver Muscles
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Glucagon is a protein composed of 29 amino acids
role:- increases blood glucose level target: liver effects:
1. Glycogenolysis2. Gluconeogenesis
glucagon has no effect on muscle glycogen
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Glycogenolysis:Conversion of glycogen to glucose
Glycogenesis:Conversion of glucose to glycogen
Gluconeogenesis:Conversion of non carbohydrate sources (proteins, fats and lactic acid) to glucose
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Question: MAY 2013
The following statements are linked to human physiology. Use your knowledge of biology to write short notes on each of the following.
Gluconeogenesis aids in maintaining adequate sugar levels in the blood during strenuous physical exercise. (5 marks)
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By what type of feedback mechanism is blood sugar controlled?
Negative feedback
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In blood sugar what is the DETECTOR, REGULATOR, EFFECTOR?
Muscles & Liver:
Effector
Pancreas:Detector ; Regulator
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Maintaining the set-point
90 mg per 100 cm3 of blood
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Diabetes Mellitus is a metabolic disease:
glucose is excreted in urine as it exceeds the maximum which the kidneys can reabsorb
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Hypoglycaemia:blood glucose level
falls
Hyperglycaemia:blood glucose level
rises
Normal blood glucose level [70-100 mg/100 cm3 of blood]
Both lead to COMA
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Two Types of diabetes:Type I Diabetes [Juvenile Onset]
Type II Diabetes [Maturity Onset]
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Type I Diabetes – Juvenile Onset Decrease in the number
of beta cells insulin deficiency
Requires daily injections of insulin hormone
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Type II Diabetes – Maturity OnsetInsulin is secreted
BUTBody cells lose ability to
use insulinWHY?
Membrane receptors in the target cells are insensitive to insulin
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Type II Diabetes
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Diabetes mellitus makes a person feel thirsty
Glucose in urine causes water to be
withdrawn from blood
This results in diuresis.
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Diuresis is the condition when:
large volumes of dilute urine are produced
Diuresis dangerous as it leads to:
dehydration of the body
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Why does a diabetic person lose weight ?
The breakdown of glycogen is not inhibited
The stores of glycogen are quickly used up
Body fats and proteins are then used as respiratory substrates
A rapid loss of body mass
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Time
Glu
cose
C
on
cen
trati
on
Meal eaten
Insulin is produced and glucose levels fall to normal again.
Glucose levels rise after a meal.
Normal
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Time
Glu
cose
C
on
cen
trati
on
Meal eaten
Insulin is not produced so glucose levels stay high
Glucose levels rise after a meal.
Diabetic
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Cortisol is an anti-stress hormone: secreted by the adrenal cortex:
in a circadian rhythm [any biological process that displays an endogenous oscillation of about 24 hours]
1
2
Under stress
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Some effects of cortisol :1. stimulates cells to:
decrease their use of blood glucose
shift instead to utilise fats and proteins for energy
2. raises blood sugar level
3. blocks the immune system reaction
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Keep your balance!!