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Endocrine > Calcium Homeostasis > Flashcards

Calcium Homeostasis Flashcards

1
Q

What are the roles of calcium in the body?

A 
Signalling
Blood clotting
Apoptosis
Skeletal strength 
Membrane excitability
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2
Q

How is calcium involved in signalling?

A

E.g. - exocytosis of synaptic vesicles (e.g. neurotransmitters/hormones etc.), contraction of muscle fibres, alters enzyme function

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3
Q

How is calcium involved in blood clotting?

A

It is an essential component of the clotting cascade

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4
Q

What is apoptosis?

A

Programmed cell death

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5
Q

How does calcium contribute to skeletal strength?

A

99% of calcium in the body is wrapped up in bone –> gives strength to skeleton

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6
Q

How does Ca affect membrane excitability?

A

Ca decreases Na permeability

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7
Q

How does hypocalcaemia affect the membrane excitability of neurons? Clinically what does this mean?

A

Increases Na permeability –> hyperexcitation of neurons

Can cause tetany, if spreads to larynx + respiratory muscles –> asphyxiation

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8
Q

How does hypercalcaemia affect the membrane excitability of neurons? Clinically what does this mean?

A

Decreases neuronal Na permeability which will reduce excitability + depress neuromuscular activity

May in extreme cases trigger cardiac arrhythmias

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9
Q

Describe the calcium distribution in the body

A

Bones - 99%
Intracellular - 0.9%
Extracellular - 0.1%

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10
Q

Where is most of the intracellular Ca stored?

A

Mitochondria and sarcoplasmic reticulum

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11
Q

What is most of the extracellular Ca in the plasma bound to?

A

Protein

NB: only 0.05% of Ca in the body is free in solution + physiologically active

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12
Q

How is Ca stored in bones?

A

It is calcified into the ECM of bone mostly in the form of hydroxyapatite (Ca10(PO4)6(OH)2)

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13
Q

What is the normal range for calcium in the plasma?

A

2.2-2.6mM

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14
Q

What does Ca have a very high affinity for?

A

Proteins (small positive charge attracted to large negative charge)

NB: 40% protein bound in plasma

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15
Q

How much Ca in the blood is actually free ionised?

A

1.2mM (50% of plasma Ca)

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16
Q

What accounts for the other 10% of plasma Ca that is not free nor bound to plasma proteins?

A

10% bind to plasma anions

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17
Q

What Ca is physiologically active?

A

Only free calcium

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18
Q

What two plasma proteins does calcium mostly bind to?

A

Albumin (80%)

Globulin (20%)

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19
Q

What anions can Ca bind to?

A 
HCO3
H2PO4
HPO4
Sulphate
Citrate
Lactate
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20
Q

How does plasma pH affect the binding capacity of Ca?

A

Binding capacity is increased under alkalotic conditions

This may precipitate a hypocalcaemic tetany

Opposite occurs in acidosis, where binding capacity reduces and free Ca rises

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21
Q

Why does acidosis lead to more free Ca?

A

Increased [H+] displaces bound Ca2+ so there is more free Ca

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22
Q

Total body calcium =

A

Ca in (diet) - Ca out (excretion at kidneys + faeces)

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23
Q

Bone functions to provide mechanical support for the body but its role in _________________ takes precedence over this.

A

Maintaining Ca balance

Ca stored in bone can be added to or released as req.

24
Q

What are osteoblasts?

A

Bone building cells - lay down a collagen extracellular matrix which they calcify

25
Q

What do osteoblasts differentiate into?

A

Osteocytes in established bone

26
Q

What are the roles of osteocytes?

A

Appear to regulate the activity of osteoblasts + osteoclasts

27
Q

What is the role of osteoclasts?

A

Mobilising bone

28
Q

How do osteoclasts mobilise bone?

A

Secrete H ions to dissolve Ca salts + provide proteolytic enzymes to digests the EC M

29
Q

What are the two main hormones that act to raise plasma Ca levels?

A

Parathyroid hormone

Calcitriol

30
Q

What kind of hormone is PTH and where is it produced?

A

Polypeptide hormone

Produced by the parathyroid glands

31
Q

What is calcitriol?

A

Active form of VitD3, steroid hormone produced from Vit D by the liver + kidneys

32
Q

Where are the parathyroid glands found?

A

Usually 4 on the posterior surface of the thyroid gland

NB: 1 in 10 have aberrant distribution (important for thyroid surgery)

33
Q

True or false:

Parathyroid glands are not essential for life and can be removed during thyroid surgery without complication

A

FALSE

They are essential for life

34
Q

When is PTH secreted?

A

In response to low [Ca]plasma

35
Q

What does PTH do?

A

Increases free [Ca] plasma by:

  1. stimulating osteoclasts to increase resorption of Ca + phosphate in bone
  2. Inhibit osteoblasts
  3. Increasing reabsorption of Ca at kidney tubules
  4. Increasing renal excretion of phosphate
  5. Stimulating kidneys to make calcitriol
36
Q

How does calcitriol increase free Ca in the plasma?

A

Binds to nuclear receptors in target tissues (intestine, bone, kidney) to increase absorption of Ca from the gut, facilitate renal absorption of Ca and mobilise Ca stored in bone by stimulating osteoclasts

37
Q

How does increasing renal excretion of phosphate increase free Ca levels?

A

Prevents it being deposited back into bone, a process that req. phosphate

38
Q

Describe the production of calcitriol

A

Endogenous precursors are acted upon by sunlight on skin to produce vitamin D or acquired from diet

Vit D processed in liver to become vit D3 and then metabolised in the kidneys to become calcitriol

39
Q

What other hormone stimulates the production of calcitriol?

A

Prolactin in lactating women

40
Q

What foods are high in vit D?

A

Fatty fish, egg yolks etc.

41
Q

How much dietary calcium is normally absorbed?

A

30%

42
Q

Why are older individuals at more risk of becoming vit D deficient?

A

Reduced gut absorption and reduced Ca mobility

43
Q

What does vitamin D3 deficiency result in?

A

Intestinal malabsorption of Ca –> reduced Ca in plasma –> increased PTH which promotes phosphate deficiency and aggravates Ca loss from bone

44
Q

Why are Asian populations at most risk of vit D3 deficiency?

A

Chapatti flour contains phytate which binds dietary Ca
Dietary deficiency of Vit D3
Pigmented skin less able to make Vit D3 in response to UV light

45
Q

What conditions has vit D deficiency been implicated in?

A

MS
Cancer
CVD

46
Q

What hormone decreases [Ca] plasma?

A

Calcitonin

47
Q

Where is calcitonin produced?

A

Thyroid gland

48
Q

What stimulates the secretion of calcitonin?

A

Increased plasma Ca level

49
Q

What is the main action of calcitonin?

A

Bind to osteoclasts + inhibit bone resorption + increase renal excretion of Ca

50
Q

Why is it thought that calcitonin is not that important?

A

Where calcitonin levels are really high (e.g. thyroid tumours), plasma Ca levels are normal, so xs calcitonin is overridden by PTH

51
Q

How does cortisol affect Ca balance?

A

Inhibits osteoblasts, increases renal excretion of Ca+ phosphate + reduces intestinal absorption of Ca –> increased PTH –> increased bone resorption which over time can produce osteoporosis

52
Q

How does insulin affect bone formation?

A

Increases bone formation + antagonises action of cortisol

53
Q

How does oestrogen affect bone formation?

A

Promotes bone formation via oestrogen receptor on osteoblasts

54
Q

How does growth hormone affect bone formation?

A

Constant stimulus for bone formation

55
Q

How does prolactin affect calcium balance?

A

Promotes calcium absorption from gut via stimulating synthesis of calcitriol

Endocrine (11 decks)

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