Endocrine System

Question 1

Define homeostasis

Answer 1

The many mechanisms in the body that seek to maintain the physical parameters within the body

Question 2

Explain the features of a control system in the body

Answer 2

• Stimulus
• Receptor - detects stimuli
  
  • Chemoreceptors, thermoreceptors, proprioceptors (position and movement), nociceptors (pain)
• Sensors communicate through afferent pathway to the brain
  
  • Nervous system uses action potential through nerves
  • Endocrine system uses hormones
• Control center
  
  • Determines set point, analyses afferent input and determines response
  • Controlled in hypothalamus and medulla of brain stem
    
    • Hypothalamus controls endocrine system
    • Medulla controls ventilation and cardiovascular system
• Output produced by control center and communicates via efferent pathway to effectors
• Effectors - elicit change
  
  • Eg. Sweat glands, kidney, muscles
• Negative feedback - output inhibits the function of the control centre and the effector acts to oppose the stimulus
  
  • Response in a way to reverse the direction of change
• Positive feedback - response in a way so as to change the variable even more in the direction of the change
  
  • Blood clotting, ovulation

Question 3

How does a biological clock work

Answer 3

• Circadian (diurnal) rhythm - set points of homeostasis changes at different times
  
  • Eg. Menstrual cycle - women’s core body temperature increase marker for ovulation
  • Eg. Cortisol levels high in the morning and gradually decrease in the day
• Biological clock located in hypothalamus in small group of neurones called suprachiasmatic nucleus
• Crossing time zones can result in mismatch in body clock and cause jet lag
• Pineal gland secretes melatonin - important in setting biological clock

Question 4

How is water in the body distributed in a 70kg male

Answer 4

• 42L (60%) is water
• 28L (66%) is intracellular and 14L (33%) is extracellular
• In the extracellular, 9L is interstitial fluid and 5L is blood plasma

Question 5

Differentiate between osmolality and osmolarity

Answer 5

• Osmolarity is number of osmoles per litre of solution

- Osmolality is number of osmoles per kg of solution

Question 6

What is normal serum osmolality

Answer 6

Serum osmolality = 275-295mOsmol/kg

Question 7

How is body water homeostasis controlled

Answer 7

• Osmoreceptors in hypothalamus monitor changes in osmolality pressure in blood plasma
• In hypertonic environment (high blood osmolality), cells lose water leading to cremation (shrinking)
  
  • Posterior pituitary secretes more ADH which increases reabsorption of water from urine into blood
  • Small volume of concentrated urine
• In hypotonic environment, body needs to excrete water
  
  • Posterior pituitary secretes less ADH
  • Decreased reabsorption of water from urine into blood in collecting ducts
  • Large volume of dilute urine

Question 8

Define hormone

Answer 8

Hormones are chemical signals produced in endocrine glands or tissue travel in the bloodstream to cause an effect on other tissues

Question 9

What are the classes of hormones and gives its structure and example

Answer 9

• Peptide/polypeptide
  
  • Largest group
  • Short or large chains of amino acids - insulin, glucagon, growth hormone
  • Water soluble
• Amino acid derivatives (amines)
  
  • Synthesized from aromatic amino acids
    
    • Adrenaline, noradrenaline, thyroid hormones all synthesized from tyrosine
    • Melatonin synthesized from tryptophan
  • Adrenal medulla hormones water soluble
  • Thyroid hormones lipid soluble
• Glycoproteins
  
  • Large protein molecules often made of subunits
  • Carbohydrate side chain
    
    • LH, FSH, TSH
  • All water soluble
• Steroids
  
  • All derived from cholesterol
  • Steroidogenic tissues convert cholesterol to different hormones
    
    • Cortisol, aldosterone, testosterone
    • All lipid soluble

Question 10

What are mechanisms of communication through hormones

Answer 10

• Autocrine - hormone signal acts back on the cell of origin
• Paracrine - hormone signal carried to adjacent cells over a short distance via interstitial fluid
• Endocrine - hormone signal released into bloodstream and carried to distant target cells
• Neurocrine - hormone originates in neurone and transported down axon and released into bloodstream

Question 11

Through what ways are hormones transported

Answer 11

• Some hormones travel in blood in simple solution
  
  • Peptides, adrenaline
• Some hormones are hydrophobic and require binding proteins for transport
  
  • Increase solubility, increase half-life and make more stable
• Hormone isn’t active when bound to protein

Question 12

What factors determine hormone level in the body

Answer 12

• Rate of production - synthesis and secretion
• Rate of delivery - high blood flow to a particular organ will deliver more hormone
• Rate of degradation - hormones are metabolised and excreted from body

Question 13

What determines the effect of hormones

Answer 13

• Depends on concentration of hormone in blood stream

- For lipophilic hormones, depends on concentration of unbound or free hormones

Question 14

Describe how water soluble hormones have an effect on cells

Answer 14

• Water soluble hormones bind to cell surface receptors
  
  • G-protein coupled receptors and tyrosine kinase
  • Tyrosine kinase undergo autophosphorylation
    
    • Dimerisation (except insulin receptor which is already dimerised)
    • Recruitment of adaptor proteins and signalling complex
  • Both activate protein kinase which causes phosphorylation of target proteins and cellular response
  • Responses for thyroid hormone receptors slower as protein synthesis needed

Question 15

Describe how lipid soluble hormones have an effect on cells

Answer 15

• Lipid soluble hormones bind to intracellular receptors
  
  • Can readily diffuse across plasma membrane
  • Type I - cytoplasmic receptor binds hormone and receptor hormone complex enters nucleus and binds DNA
  • Type II - hormone enters nucleus and binds to pre-bound receptor on DNA
  • Receptor binds to specific DNA sequence called a hormone response element (HRE) in promoter region of specific genes

Question 16

Describe in outline how appetite is controlled

Answer 16

• Control of appetite located in arcuate nucleus of hypothalamus
  
  • Primary neurones sense input
    
    • Stimulatory neurones contain peptides which promote hunger
      
      • Release two peptides - NPY, AgRP
    • Inhibitory neurones yields neurotransmitters which promote satiety (suppress appetite)
      
      • Release POMC which can be enzymatically cleaved to produce peptide hormones
  • Secondary neurones process signal to alter feeding behaviour

Question 17

State the hormones involved in appetite control

Answer 17

Stimulate appetite - ghrelin

Suppress appetite - PYY, leptin, insulin, amylin

Question 18

Outline the role of ghrelin

Answer 18

• Stimulates appetite by stimulating excitatory primary neurones in arcuate nucleus
• Peptide hormone released from stomach
• Filling of stomach inhibits ghrelin release

Question 19

Outline the role of PYY

Answer 19

• PYY (peptide tyrosine tyrosine) - suppresses appetite
• Peptide hormone released by cells in ileum and colon in response to feeding
• Inhibits excitatory primary neurones in arcuate nucleus

Question 20

Outline the role of leptin

Answer 20

• Leptin - suppresses appetite
• Peptide hormone released into blood by adipocytes
• Stimulates inhibitory neurones in arcuate nucleus
• Inhibits excitatory neurones in arcuate nucleus
• Lack of leptin production associated with obesity
  - Causes mitochondria to produce heat rather than ATP

Question 21

Outline the role of insulin in appetite control

Answer 21

• Insulin - suppresses appetite
• Similar mechanism to leptin (inhibit excitatory and stimulate inhibitory neurones in arcuate nucleus)
• Peptide hormone secreted by ß-cells in pancreas

Question 22

Outline the role of amylin

Answer 22

• Amylin - supresses appetite
• Decrease glucagon secretion and slow gastric emptying
• Peptide hormone secreted by ß-cells in pancreas
• Pramlintide used as treatment for type 2 diabetes