A2AW1 endocrine homeo · Homeostasis: body temperature Now read: •Ross and Wilson, Anatomy &...

Homeostasis

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

Homeostasis

• Constantly changing, dynamic

• Maintain equilibrium within narrow limits

• Biochemical regulation

• Temperature, pH, BP, electrolytes

• Serious risk if balance is lost

Homeostasis

• Core temperature

• Acidity and alkalinity (pH)

• Blood pressure

• Blood glucose levels

• Oxygen and carbon dioxide

• Water and electrolyte balance

Physiological variables:

Homeostasis

• 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

Homeostasis

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

Negative feedback mechanism:Domestic boiler

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)

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

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

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)

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

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’

Cellular respiration products

Now read:•Ross and Wilson, Anatomy &

Physiology in Health & Illness (2014) 12th edition, pages 315 to 316

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!

Homeostasis: pH

Now read:

•Ross and Wilson, Anatomy & Physiology in Health & Illness (2014) 12th edition, pages 24 to 25

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

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

Homeostasis: acid base balance

Kidneys’ role: HCO3: increase or decrease excretion of hydrogen and bicarbonate

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

•CO2is reabsorbed, maintaining buffering capacity

•pH of urine varies: 4.5 - 8: diet, time of day

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

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

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

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

Now read:

•Ross & Wilson, Anatomy & Physiology in Health & Illness (2014) 12th edition, pages 96 to 99: 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

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

• Raises BP to increase tissue perfusion

• Renin – Angiotensin – Aldosterone System (RAAS)

• Antidiuretic hormone (ADH) affects blood volume

• Atrial Natriuretic Peptide: Na+, H

loss: lowers BP

• Opposes RAAS and ADH

Now watch:•YouTube video on blood pressure (4 minutes)

• 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

Now read:

•Ross & Wilson, Anatomy & Physiology in Health & Illness (2014) 12th edition, page 26: Carbohydrates

• 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

Insulin:

• Acts on cell membranes, stimulating uptake

• Increases glycogenesis

• Decreases glycogenolysis

• Prevents gluconeogenesis

Now read:

•Ross & Wilson, Anatomy & Physiology in Health & Illness (2014) 12th edition, pages 226-228: Pancreatic islets

• 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!

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

Now read:

•Ross & Wilson, Anatomy & Physiology in Health & Illness (2014) 12th edition, page 315: Functions of the liver; Carbohydrate metabolism

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

Now watch:•YouTube video on blood sugar regulation (3 minutes)

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

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

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

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: Water and SaltsFluid balance: water in

• Food and drink

• Some produced by metabolic processes

Fluid balance: water out

• Urine

• Faeces

• Sweat

• Breath

Homeostasis: Water and Salts

Now watch:•Human Homeostasis: regulation of water and salts (10 minutes)

Negative feedback mechanism: water regulation

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+

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

Homeostasis: water and saltsNow read:

•Ross & Wilson, Anatomy & Physiology in Health & Illness (2014) 12th editionPage 344 Water balance and urine outputPages 345 Electrolyte balance

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?

The endocrine system: an overview

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?

The endocrine system: function

• Alters metabolic activities

• Regulates growth and development

• Guides reproductive activity

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

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.

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

The endocrine system: definitions

Hormone: • A chemical messenger that

acts on a specific cell (or cells’) membrane

Enzyme:• A biological catalyst

It’s me ‘ormones!

How does it work?

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

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.

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.

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.

The endocrine system: an overviewWhat does it do for us?

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

• 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

• Regulates the activities of many of the other endocrine glands

• Composed of three main areas:

o Anterior pituitary

o Posterior pituitary

o Intermediate lobe

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

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

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

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

Thyroid gland

The endocrine system: an overviewThyroid gland: location

• In the neck

• Anterior to the larynxand trachea

• Level with 5th-7th cervicaland 1st thoracic vertebrae

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)

• 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

Thyroxine

Tri-iodothyronine

Thyroid glandThe endocrine system: an overview

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

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

Thyroid hormone production

Now watch:•Thyroid hormone production (7 minutes)

Parathyroid gland

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

What are the parathyroid glands?

Now watch:•The parathyroid glands (5 minutes)

The pancreas

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

The endocrine system: an overviewThe pancreas

Exocrine function

• Digestive enzymes

Endocrine function

• Blood sugar regulation

• Growth hormone regulation

The pancreas

Glycogenolysis

•Breakdown of glycogen

Gluconeogenesis

•Generation of glucose from non-carbohydrate sources

The pancreas: actions of glucagon

Now watch:•YouTube video: I love my Pancreas (4 minutes)

The adrenal gland

• Situated on the upper pole of each kidney

• Enclosed by the renal fascia

Kidney

Adrenal gland

The adrenal glands

The endocrine system: an overviewThe adrenal glands

The endocrine system: an overviewShort-term response to stress

The endocrine system: an overviewLong-term response to stress

Adrenal gland function

Now watch:•YouTube video: Adrenal gland function (4.5 minutes)

Major organ endocrine functions

Heart: Atrial Natriuretic Hormone

Kidneys: Erythropoietin

GI Tract: Gastrin from stomach wall – cholecystokinin

Placenta: hCG, progesterone, oestrogen

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

• 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

• 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

• 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

• 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

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?

Now read:

•Ross & Wilson, Anatomy & Physiology in Health & Illness (2014) 12th editionChapter 9: The endocrine system from page 215

A2AW1 endocrine homeo · Homeostasis: body temperature Now read: •Ross and Wilson, Anatomy &...

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