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Homeostasis
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Homeostasis
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homeo = same or equalstasis = standing still
(Claude Bernard)
Literally means ‘unchanging’
Definition:
•Close control of composition of body’s internal environment
•Maintenance of stable internal environment, despite continuous external changes
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Homeostasis
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• Constantly changing, dynamic
• Maintain equilibrium within narrow limits
• Biochemical regulation
• Temperature, pH, BP, electrolytes
• Serious risk if balance is lost
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Homeostasis
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• Core temperature
• Acidity and alkalinity (pH)
• Blood pressure
• Blood glucose levels
• Oxygen and carbon dioxide
• Water and electrolyte balance
Physiological variables:
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Homeostasis
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• Maintained and controlled by systems that detect and respond to changes
• Control system: 3 basic components
• Control centre: sets limits, receives info
• Detector: measures actual reading, sends info to control centre
• Effector: executes changes required, managed by control centre
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Homeostasis
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Negative feedback mechanisms• Effector response decreases or negates effect of stimulus• Maintaining or restoring homeostasis• Numerous, e.g. body temperature
Positive feedback mechanisms• Cascade or amplification• Stimulus increases response from effector• Fewer• E.g. oxytocin: stimulation of uterine contractions during labour
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Negative feedback mechanism:Domestic boiler
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Homeostasis
Now read:•Ross & Wilson, Anatomy & Physiology in Health & Illness (2014)
12th edition, pages 6 to 7 (Homeostasis)
Now watch:•YouTube video on homeostasis (11 minutes)
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Homeostasis: body temperatureNegative feedback - if temperature falls below set level: • Hypothalamus nerve endings detect change: Control Centre • Activates effector mechanisms to raise temperature• Skeletal muscles: shiver• Vasoconstriction: reduce heat loss
• Behaviour changes: clothes on
• When temperature increases, nerve endings are not stimulated
• Signals to hypothalamus stop
• Effectors cease: stop shivering, stop vasoconstriction
• Return to 36.8 degrees centigrade
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Negative feedbackIf temperature rises above set level:
• Hypothalamus nerve endings detect change: Control Centre
• Activates effector mechanisms to lower temperature
• Sweating
• Vasodilation – increases heat loss
• Behaviour changes – clothes off
• When temperature decreases, nerve endings are not stimulated
• Signals to hypothalamus stop sending messages
• Effectors cease: stop sweating, stop vasodilation
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Homeostasis: body temperature
Now read:•Ross and Wilson, Anatomy & Physiology in Health & Illness (2014)
12th edition, pages 365 to 367 on temperature regulation
Now watch:•YouTube video on body temperature regulation (10 minutes)
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Homeostasis: pH
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Homeostasis: pH
• pH scale: standard measurement of Hydrogen ions in solution: 0-14
• Water = 7 = neutral = midpoint of pH scale
• pH value below 7 = acidic
• pH value above 7 = alkaline
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Homeostasis: Acid base balance: pH
• Controlled by kidneys and lungs
• Cellular reactions produce acids and alkalis
• All biochemical reactions influenced by normal pH maintenance, e.g. saliva
• Critical and closely regulated
• More acid than alkali
• Buffers neutralise fluctuations ‘mop up’
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Cellular respiration products
Now read:•Ross and Wilson, Anatomy &
Physiology in Health & Illness (2014) 12th edition, pages 315 to 316
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Homeostasis: pH
Chemical buffer systems: resist changes in H+ concentrations by: • Binding to H+ ions when pH drops• Releasing H+ ions when pH rises
3 chemical buffer systems:• Bicarbonate• Phosphate• Protein
Working together, buffers buffer each other!
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Homeostasis: pH
Now read:
•Ross and Wilson, Anatomy & Physiology in Health & Illness (2014) 12th edition, pages 24 to 25
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Homeostasis: pH
Lungs’ role: to excrete CO2
CO2increases H
+ions: it combines with water to produce volatile carbonic
acid which dissociates into a bicarbonate ion and a hydrogen ion
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Homeostasis: pH
Kidneys’ role: HCO2
• Buffers: temporary mopping-up of excess acids and bases
• BUT need to eliminate from body
• Phosphoric, Uric, Lactic acids and Ketones
• Kidneys: increase and decrease excretion of hydrogen and bicarbonate ions
• pH drops: H+
excretion increases, and HCO3 is conserved
• pH rises: H+
is conserved and HCO3 is excreted
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Homeostasis: acid base balance
Kidneys’ role: HCO3: increase or decrease excretion of hydrogen and bicarbonate
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Homeostasis: pHProximal convoluted tubules: •secrete hydrogen ions•H
+combine with buffers:oHydrogen + Bicarbonate = Carbonic AcidoHydrogen + Ammonia = AmmoniumoHydrogen + Hydrogen Phosphate = Dihydrogen PhosphateoExcreted in urine
•Carbonic acid is converted to CO2and H
2O
•CO2is reabsorbed, maintaining buffering capacity
•pH of urine varies: 4.5 - 8: diet, time of day
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Homeostasis: blood pressureWhat is it?
•Force exerted by blood on the vessel wall
•Homeostatic control: essential
•Too high? Vessel damage, rupture, haemorrhage
•Too low? Reduced blood flow to tissues, dangerous for essential organs: heart, brain and kidneys
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Homeostasis: blood pressure• Arterial blood pressure: created from systole
or contraction of left ventricle
• Variables: time of day, posture, exertion, gender and age
• Two readings: systolic and diastolic
• Systolic: ventricular contraction: higher
• Diastolic: ventricular resting: lower
• e.g., 120/80 mmHg
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Homeostasis: blood pressure• Very elastic arterial walls
• Allows expansion, distension, propulsion
• Recoil maintains BP during diastole
• Arterioles: smallest arteries
• Tunica Media – smooth muscle
• Stimulated by nerves & chemicals
• Constriction and dilation affect peripheral resistance
• Constriction = BP raise
• Dilation = BP fall
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Homeostasis: blood pressure
• Determined by cardiac output and peripheral resistance
• Blood pressure = cardiac output x peripheral resistance
• Cardiac output - volume of blood pumped per minute
• Determined by stroke volume and heart rate
• Stroke volume: volume pumped with each heartbeat
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Homeostasis: blood pressure
Now read:
•Ross & Wilson, Anatomy & Physiology in Health & Illness (2014) 12th edition, pages 96 to 99: Blood pressure
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Homeostasis: blood pressure
Control of blood pressure
• Autoregulation: organs adjust own flow
• Baro and chemo receptors: short-term
• Renin-Angiotensin-Aldosterone
• Kidneys control in longer term
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Homeostasis: blood pressure
Short-term
• Cardio Vascular Centre (CVC): medulla & pons in brain stem
• Autonomic Nervous System: sympathetic and parasympathetic
• Baroreceptors: pressure sensors in carotid artery and aorta
• Controls BP by affecting heart rate, vasoconstriction and vasodilation chemoreceptors: CO
2, O
2, pH
• Raises BP to increase tissue perfusion
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Homeostasis: blood pressure
• Renin – Angiotensin – Aldosterone System (RAAS)
• Antidiuretic hormone (ADH) affects blood volume
• Atrial Natriuretic Peptide: Na+, H
2O
loss: lowers BP
• Opposes RAAS and ADH
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Now watch:•YouTube video on blood pressure (4 minutes)
Homeostasis: blood pressure
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• Liver and pancreas: key roles maintaining plasma glucose levels
• Lose all willpower and eat 4 doughnuts!
• Glucose levels rise, insulin converts to glycogen for storage
• When doughnuts wear off, glucose levels fall glucagon stimulates conversion of glycogen into glucose again
Homeostasis: blood glucose
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Now read:
•Ross & Wilson, Anatomy & Physiology in Health & Illness (2014) 12th edition, page 26: Carbohydrates
Homeostasis: blood glucose
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Homeostasis: blood glucose
• Pancreas: three main types of cells in the pancreatic islets (Langerhans)
• Alpha cells: Secrete glucagon: increases blood glucose
• Beta cells: Secrete insulin: lowers blood glucose, secretion is stimulated by raised blood glucose. Also lowers amino acids and fatty acids
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Insulin:
• Acts on cell membranes, stimulating uptake
• Increases glycogenesis
• Decreases glycogenolysis
• Prevents gluconeogenesis
Homeostasis: blood glucose
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Now read:
•Ross & Wilson, Anatomy & Physiology in Health & Illness (2014) 12th edition, pages 226-228: Pancreatic islets
Homeostasis: blood glucose
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• Doughnuts are fuel!
• Neurones and erythrocytes can use only glucose for energy
• Maintenance of blood glucose: essential
• Digested doughnuts, mostly glucose absorbed via villi in small intestine, into capillaries to hepatic portal vein to the liver to be dealt with!
Homeostasis: blood glucose
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How does the liver deal with digested doughnuts?
• Oxidised: chemical energy: ATP used for metabolism
• Blood glucose level maintenance: 3.5 – 8 mmol/L
• Excess is stored: glycogen (liver and skeletal muscles) using insulin
• Further excess: converted to fat
Homeostasis: blood glucose
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Now read:
•Ross & Wilson, Anatomy & Physiology in Health & Illness (2014) 12th edition, page 315: Functions of the liver; Carbohydrate metabolism
Homeostasis: blood glucose
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All body cells need energy, not doughnuts!
Metabolic processes:
•Multiplication and replacement
•Contraction of muscle fibres
•Synthesis of secretions
Oxidation of carbohydrates and fat give majority
Glycogen stores low: can make glucose from amino acids, glycerol = gluconeogenesis
Homeostasis: blood glucose
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Now watch:•YouTube video on blood sugar regulation (3 minutes)
Homeostasis: blood glucose
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Homeostasis: Oxygen and Carbon Dioxide (O2 and CO2)
Blood gases regulated despite physiological, environmental and pathological variables•Usually involuntary•Voluntary: speaking, singing, breathing exercises•Overridden if CO2 rises = hypercapnia•Respiratory Rhythmicity Centre (RRC): medulla•Rate and depth of breathing•Under direction of pons: •Motor impulses from RRC to phrenic and intercostal nerves then diaphragm and intercostal muscles
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Homeostasis: O2 and CO2
Chemoreceptors
• respond to changes in PO2 and PCO2
of blood and cerebrospinal fluid (CSF)
• Central = medulla oblongata
• Peripheral = arch of aorta and carotid bodies
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Central chemoreceptors:
• PCO2 rises; hypercapnia, stimulates RRC, raise respiration rate/depth to reduce PCO2
• Key factor in maintaining normal gas levels
Peripheral chemoreceptors:
• Sensitive to small rises in PCO2 than drops in PO2, nerve impulses stimulate RRC = adjusted rate and depth of respiration
• pH: raised H+ ions will also stimulate peripheral chemoreceptors, increasing ventilation, increasing excretion of CO2, increasing blood pH
Homeostasis: O2 and CO2
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Now read:
•Ross & Wilson, Anatomy & Physiology in Health & Illness (2014) 12th edition, pages 260-261: Control of Respiration
Now watch:
•YouTube video on central and peripheral chemoreceptors (4.5 minutes)
Homeostasis: O2 and CO2
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Homeostasis: Water and SaltsFluid balance: water in
• Food and drink
• Some produced by metabolic processes
Fluid balance: water out
• Urine
• Faeces
• Sweat
• Breath
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Homeostasis: Water and Salts
Now watch:•Human Homeostasis: regulation of water and salts (10 minutes)
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Negative feedback mechanism: water regulation
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Homeostasis: water and salts
Electrolytes:
• Ionic compounds, dissolved in H2O
• Conducts electricity!
• Essential for nerve & muscles
• Exert osmotic pressure, maintaining H2O placement
• Act as buffers, resisting pH
• E.g. sodium chloride
• Ions: Na+ Cl- K+ Ca2+
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Homeostasis: water and salts
Electrolyte balance:
• Na+ most common cation: extracellular, usually taken in excess and excreted
• K+ most common cation: intracellular
• High concentrations in gastric juices: these are reabsorbed in health
• RAAS system maintains K+ and Na+ within normal limits
• Calcium: regulated by parathyroid hormone and calcitonin
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Homeostasis: water and saltsNow read:
•Ross & Wilson, Anatomy & Physiology in Health & Illness (2014) 12th editionPage 344 Water balance and urine outputPages 345 Electrolyte balance
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Homeostasis: quick exercise
From memory, write down the five most important things about homeostasis systems: five features that, if you forgot everything else, would capture the essence of this system.
Take 15 minutes
Look back at your notes. How did you do?
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The endocrine system: an overview
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Quick knowledge test
1. Name the two types of gland
2. Give a simple definition of the word hormone
3. What hormone do the parathyroids secrete?
4. Give two functions of the pituitary gland
5. What do our kidneys do for us?
6. What organ produces most of our digestive enzymes?
7. Where is the hypothalamus situated?
8. What are the features of a negative feedback loop?
9. Besides endocrine glands, which major organs secrete hormones?
10. Which hormones are released when we are stressed?
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The endocrine system: function
• Alters metabolic activities
• Regulates growth and development
• Guides reproductive activity
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The endocrine system: an overviewWhat we will cover:
What is the endocrine system?
• What is it constituted of?
• How does it work?
• What does it do for us?
Focus on four areas
• The pituitary gland and hypothalamus
• The thyroid and parathyroid glands
• The pancreas
• The adrenal glands
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What is the endocrine system?
The endocrine system comprises around seven major endocrine glands, and many more minor glands or patches of tissues scattered throughout the human body.
Many of our major organs also have endocrine functions.
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The endocrine system
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The endocrine system
Definitions of glands
Exocrine:
•secrete products into ducts that carry those secretions to body cavities, e.g. sweat, mucus, pancreas, tears
Endocrine:
•secrete products directly into the extracellular fluid which bathes them, then into the blood stream
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The endocrine system: definitions
Hormone: • A chemical messenger that
acts on a specific cell (or cells’) membrane
Enzyme:• A biological catalyst
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The endocrine system: an overview
It’s me ‘ormones!
How does it work?
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The endocrine system: an overview
How does it work? Via the action of hormones
Hormones: a. Synthesised by endocrine glands
b. May be stored in glands
c. Secreted into blood stream: highly vascular
d. Travel to site of action
e. Interact with specific target cells
f. Have powerful effects at low concentrations
g. May work on more than one tissue and exert more than one effect
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The endocrine system: an overview
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The endocrine system: an overviewCells respond to a hormone when they possess a specific receptor for that hormone. The
hormone binds to the specific receptor protein, resulting in the activation of a signal transduction mechanism that ultimately leads to cell type-specific responses.
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The endocrine system: an overview
Two main classes of hormones:
• Water-soluble (Peptides, glycoproteins, amines)
• Lipid-soluble (Steroids, thyroid hormones)
Lipid-soluble hormones are able to diffuse across cell membranes and bind to receptors located within the cell.
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The endocrine system: an overview
Concepts
• Hormone receptors are constantly being synthesised and broken down
• When a hormone is in excess, number of target cell receptors decrease =
Down-Regulation
• When a hormone is deficient, number of receptors increase to maximise binding.
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The endocrine system: an overviewWhat does it do for us?
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The endocrine system: an overviewThe pituitary gland
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The endocrine system: an overviewThe pituitary gland
Location
In the hypophyseal fossa of the sphenoid bone, below the hypothalamus at the base of the brain
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The endocrine system: an overviewThe pituitary gland
• Size of a pea: 0.5 grammes
• Acts as a unit, along with the hypothalamus (thalamus = ‘inner room’ in Greek)
• A protrusion off the hypothalamus at the base of the brain; rests in a small bony cavity covered by a dural fold
• Functionally connected by a stalk
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The endocrine system: an overviewThe pituitary gland
• Regulates the activities of many of the other endocrine glands
• Composed of three main areas:
o Anterior pituitary
o Posterior pituitary
o Intermediate lobe
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The endocrine system: an overview
The pituitary gland
• Supplied by arterial blood
• Directly from hypothalamus: contains releasing or inhibiting hormones secreted by the hypothalamus
• Hypothalamic hormones influence the secretion of hormones by the anterior pituitary
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The endocrine system: an overviewAnterior pituitary
Growth hormoneThyroid stimulating hormoneAndrenocorticotrophic hormoneProlactinFollicle stimulating hormoneLuteinising hormone
Target tissue- Most tissues and cells- Thyroid gland- Andrenal gland- Breast- Ovaries and testes- Ovaries and testes
Posterior pituitaryOxytocin
Target tissue- Uterine smooth muscle; muscle
cells of lactating breast
Anti-diuretic hormone - Kidney
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The endocrine system: an overviewAnterior pituitary
Growth hormone stimulates growth and division of most body cells
Thyroid Stimulating Hormone stimulates growth and activity of thyroid gland and production of thyroxine and tri-iodothyronine.
Adrenocorticotrophic Hormone raises concentration of steroids and cholesterol in the adrenal cortex. Raises output of steroid hormones esp. cortisol.
Prolactin involved in initiating and maintaining lactation
Follicle Stimulating Hormone and Luteinising Hormone regulationof menstrual cycle in females (with oestrogen and progesterone). In males LH stimulates production of testosterone. FSH – production and maturation of gametes – both sexes
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The endocrine system: an overviewPosterior pituitary
Oxytocin increases force of uterine contractions and stimulates greater stretching of cervix during childbirth
Anti-diuretic hormone (ADH) increases water reabsorption from kidney tubules
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The endocrine system: an overview
Thyroid gland
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The endocrine system: an overviewThyroid gland: location
• In the neck
• Anterior to the larynxand trachea
• Level with 5th-7th cervicaland 1st thoracic vertebrae
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The endocrine system: an overviewThyroid gland
• Butterfly-shaped
• Consisting of two lobes joinedby a narrow isthmus
• Two parathyroid glands lieon each posterior side(total = 4)
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The endocrine system: an overviewThyroid gland
• Secrete Thyroxine (T4) and Tri-iodothyronine (T3)
• Iodine is needed for this synthesis
• Production of T3 and T4 is stimulated by Thyroid Stimulating Hormone from the pituitary
• This in turn is regulated by Thyrotropin Releasing Hormone from the hypothalamus
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The endocrine system: an overview
Thyroxine
Tri-iodothyronine
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The endocrine system: an overview
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The endocrine system: an overview
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Thyroid glandThe endocrine system: an overview
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Thyroid glandThe endocrine system: an overview
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The endocrine system: an overviewThyroid gland
Thyroid hormones do the following:
• Combine with receptor sites on surfaces of most cells
• Regulate expression of genes in the nucleus
• Enhance effects of other hormones (i.e. adrenaline)
• Affect BMR and heat production
• Regulate metabolism of all food groups
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The endocrine system: an overview
Thyroid glandCalcitonin
• Regulates calcium metabolism. Lowers calcium levels when they are raised. Does this by:
o Reducing reabsorption of calcium from bones
o Reducing reabsorption of calcium from kidney tubules
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The endocrine system: an overview
Thyroid hormone production
Now watch:•Thyroid hormone production (7 minutes)
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The endocrine system: an overview
Parathyroid gland
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The endocrine system: an overview
Parathyroid glandParathormone
• Regulates calcium metabolism. Raises calcium levels when they are low. Does this by:
o Increasing absorption of calcium from small intestine
o Increasing reabsorption of calcium from kidney tubules
o Increasing reabsorption of calcium from bones
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The endocrine system: an overview
What are the parathyroid glands?
Now watch:•The parathyroid glands (5 minutes)
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The endocrine system: an overview
The pancreas
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The endocrine system: an overviewThe pancreas
Location
• Posterior and inferior to the stomach
Structure
• Head, body and tail
• Approx 1% of tissue comprised of pancreatic islets (islets of Langerhans)
• Approx 99% of tissue composed of clusters of cells called acinar cells
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The endocrine system: an overviewThe pancreas
Exocrine function
• Digestive enzymes
Endocrine function
• Blood sugar regulation
• Growth hormone regulation
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The endocrine system: an overview
The pancreas
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Glycogenolysis
•Breakdown of glycogen
Gluconeogenesis
•Generation of glucose from non-carbohydrate sources
The endocrine system: an overview
The pancreas: actions of glucagon
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The endocrine system: an overview
Now watch:•YouTube video: I love my Pancreas (4 minutes)
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The endocrine system: an overview
The adrenal gland
• Situated on the upper pole of each kidney
• Enclosed by the renal fascia
Kidney
Adrenal gland
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The endocrine system: an overview
The adrenal glands
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The endocrine system: an overviewThe adrenal glands
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The endocrine system: an overviewShort-term response to stress
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The endocrine system: an overviewLong-term response to stress
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The endocrine system: an overview
Adrenal gland function
Now watch:•YouTube video: Adrenal gland function (4.5 minutes)
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The endocrine system: an overview
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The endocrine system: an overview
Major organ endocrine functions
Heart: Atrial Natriuretic Hormone
Kidneys: Erythropoietin
GI Tract: Gastrin from stomach wall – cholecystokinin
Placenta: hCG, progesterone, oestrogen
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The endocrine system: an overview
Atrial Natriuretic Hormone• Powerful vasodilator
• Protein hormone secreted by the heart muscle
• Homeostatic control of body water, sodium, potassium and fat
• Released by muscle cells in atria, in response to high blood pressure
Local hormones
Cholecystokinin• Synthesised in small intestine
• Secreted in duodenum
• Causes release of digestive enzymes and bile from pancreas and gall bladder, hunger suppressant
• Recent evidence suggests a role in opiate tolerance, implicated in experiences of pain in withdrawal
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• Tiny, pine-cone shaped: 10mm long
• Location: hangs from roof of third ventricle in diencephalon
• Tends to atrophy after puberty
• May calcify in later life
• Pinealocytes: secretory cells arranged in chords or clusters
• Between pinealocytes are dense particles of calcium salts (‘brain sand’)
• Radio opaque: landmark
Pineal glandThe endocrine system: an overview
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• Main secretory product: Melatonin hormone controlling circadian rhythms, day / night / seasonal changes
• Peak levels at night = drowsiness
• Lowest level at noon = daylight
• Receives input from visual pathways: light/dark
• Antigonadotrophic effect in children: preventing/inhibiting sexual maturation, timing of puberty
Pineal glandThe endocrine system: an overview
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• Lies retrosternally in the upper mediastynum
• Extends upwards into root of the neck
• Weighs 10-15g at birth, grows to 30-40g at puberty then shrinks back to birth weight
• Two lobes – joined by areolar tissue
• Fibrous outer capsule that divides lobes into lobules
ThymusThe endocrine system: an overview
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• Function
• Secretes thymosin
• Maturation of T-lymphocytes from lymphocytes origination in bone marrow
• Differentiate between foreign bodies and ‘self’
• Enables T-lymphocytes ‘antigen specific’
ThymusThe endocrine system: an overview
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Quick knowledge test
1. Name the two types of gland
2. Give a simple definition of the word hormone
3. What hormone do the parathyroids secrete?
4. Give two functions of the pituitary gland
5. What do our kidneys do for us?
6. What organ produces most of our digestive enzymes?
7. Where is the hypothalamus situated?
8. What are the features of a negative feedback loop?
9. Besides endocrine glands, which major organs secrete hormones?
10. Which hormones are released when we are stressed?
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Now read:
•Ross & Wilson, Anatomy & Physiology in Health & Illness (2014) 12th editionChapter 9: The endocrine system from page 215
The endocrine system: an overview
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