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ZA - Endocrine System > Calcium Homeostasis > Flashcards

Calcium Homeostasis Flashcards

1
Q

Role of Ca in body?

A 
Signalling
Blood clotting
Apoptsis
Skeletal strength
Membrane excitability
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2
Q

How is Ca involved in signalling?

A

Causes exocytosis of synaptic vesicles - neurotransmitters/hormones

Causes contractions of muscle fibres

Alters enzyme function

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3
Q

How is Ca involved in blood clotting?

A

Essential componant of clotting cascade

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4
Q

What is apoptosis?

A

Programmed cell death

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5
Q

How is Ca involved in skeletal strength?

A

99% of calcium in the body is wrapped up in bone where it gives strength to the skeleton.

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6
Q

How is Ca involved in membrane excitability??

A

Ca2+ decreases Na+ permeability

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7
Q

Which of these functions is most critical in short term homeostasis?

A

Membrane excitabilty

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8
Q

How does hypocalcaemia affect membrane excitability?

A

Increases neuronal Na+ permeability causing hyperexcitation - in extreme cases leads to tetany and if it spreads to larynx and resp. muscles = asphyxiation

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9
Q

How does hypercalcaemia affect membrane excitabilty?

A

Decreases neuronal Na+ permeability = reduced excitability which depresses neuromuscular activity and in extreme cases triggers cardiac arrhythmias.

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10
Q

How is Ca distrubuted in the body?

A

Bones = 99%

Intracellular = 0.9% - mostly stored in mitochondria and in sarcoplasmic reticulum

Extracellular fluid = 0.1% - nearly half Ca2+ is bound to protein so only 0.05% of Ca in body in free in solution and physiologically active

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11
Q

How is calcium stored in bones and why is this significant? How much is in bones?

A

In form of hydroxyapatite meaning phosphate homeostasis is important in determing calcium balance

1 kg

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12
Q

Conc of calcium in cells?

A

24mM with 0.001mM in solution in the cytosol and rest in mitochondria and SR

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13
Q

Conc of calcium in plasma?

A

2.2-2.6mM

Conc of calc in ECF is tightly maintained

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14
Q

Since calcium has a very high affinity for proteins, what actual conc of calcium is free ionised and physiologically active in the ECF?

A

1.2mM

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15
Q

If 40% of calcium is bound to proteins in the ECF and 50% is free, what is the other 10% doing?

A

Bound to plasma anions

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16
Q

Name some proteins Ca binds to and the % that binds to them.

A

Albumin - 80%

Globulin - 20%

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17
Q

What type of Ca is physiologically active?

A

Free calcium

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18
Q

What changes the binding capacity for plasma proteins with Calcium?

A

pH

Binding is increased under alkalotic conditions and opposite happens with acidosis

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19
Q

What is total body calcium determined by?

A

Calcium in (diet) - calcium out (exctretion at kidneys and faeces)

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20
Q

The distribution of Ca2+ between BLANk and BLANK is crucial. Explain?

A

Bone and ECF

99% of total body Ca2+ is stored in bone and this can be added to or released as required

Bone functions to provide mechanical support for the body but its role in maintaining Ca2+ balance takes precedence over this.

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21
Q

What are osteoblasts?

A

Bone building cells

Highly active - lay down a collagen extracellular matrix which they then calcify

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22
Q

What are esteocytes?

A

Differentiated osteoblasts in established bone - much less active but regulate activity of osetoblasts and osteoclasts

23
Q

What is an osteoclast?

A

Cells responsible for mobilizing bone

They secrete H+ ions (pH ~4) to dissolve the calcium salts and also provide proteolytic enzymes to digest the extracellular matrix.

24
Q

What hormones act to increase Calcium in the plasma?

A

Parathyroid hormone

Calcitriol

25
Q

What type of hormone is parathyroid hormone and where is it made?

A

Polypeptide hormone

Parathyroid glands

26
Q

What is calcitriol?

A

The active form of Vit. D3

A steroid hormone produced from vit. D by liver and kidneys

27
Q

Are parathyroid glands essential for life? What considerations should be taken clinically about them?

A

Yes - should be thought about if an overactive thyroid gland needs removed

28
Q

What is parathyroid hormone (PTH) released in response to?

A

Decrease in Ca2+ conc in plasma

29
Q

How does PTH increase free Ca2+ conc?

A

Stimulate osteoclasts to increase release/resorption of Ca2+ and phosphate in bone

Inhibits osteoblasts to reduce Ca deposition in bone

Increasing re-absorption of Ca from kidney tubules

Increasing renal excretion of phosphate. This elevates free [Ca2+ ] by preventing it from being deposited back into bone, a process that requires phosphate.

Stimulates the kidney to synthesise calcitriol from vitamin D which promotes calcium absorption at the gut and kidney.

30
Q

What hormone helps form calcitriol in lactating women?

A

Prolactin

31
Q

How is calcitriol made?

A

Produced in two steps (1. liver, 2. kidneys) from dietary vitamin D or from precursors activated by sunlight on skin.

Active vitamin D3 is formed from cholesterol derivatives by the action of UV light on the skin, but Vit D can also be gained from the diet.

32
Q

Where does calcitriol bind?

A

Receptors in intestine, bone and kidney

33
Q

What effect does calcitrol have?

A

Increases absorption of calcium from the gut

The active transport system which moves Ca2+ from the intestinal lumen to the blood is under the control of calcitriol

Also facilitates renal absorption of calcium

Mobilises calcium stores in bone by stimulating osteoclast activity

34
Q

Summaries what happens when there is low plasma calcium.

A

Increase in PTH leads to its effects which include an increase in calcitrol which leads to increase in intestinal absorption of Ca2+

35
Q

What happens when there is high levels of calcium in blood?

A

PTH release is inhibited causing a shift towards osteoblast deposition and less osteoclast resoprtion/release

36
Q

In a healthy individual how much dietary calcium should be absorbed?

A

30%

37
Q

In a Vit. D deficient individual how much dietary calcium is be absorbed?

A

10-15%

38
Q

In a pregnant or lactating individual or someone going through growth spurts. how much dietary calcium should be absorbed? Why is this possible?

A

45-55%

Higher circulating Vit D3

39
Q

When is a person vit. d deficient?

A

When levels fall below 20ng/ml

40
Q

What does vit D def cause in children and why?

A

Rickets

In a vit D def PTH works harder to maintain plasma Ca levels by removing calcium from bone = soft bones that are easily fractures and bent

41
Q

If Vit. D’s direct effect is to release calcium from bone - how does lack of it result in rickets?

A

Because Vit. D also affects gut absorption and absorption of Ca from kidneys and without it the effects are not balanced and you lack Ca

42
Q

What is rickets called in adults?

A

Osteomalacia

43
Q

Why are asians more prone to Vit D def?

A

Chapatti flour has phytate which binds dietary Ca
May be dietary def of Ca
Pigmented skin = harded to make D3 from UV light

44
Q

What diseases is Vit d def implicated in?

A

MS
Cancer
Arthritis
CV disease

45
Q

What is the one hormone which acts to decrease blood ca conc?

A

Calcitonin

46
Q

What type of hormone is calcitonin, where is it secreted from and how does it work?

A

Peptide
Thyroid gland

Binds to osteoclasts and inhibits bone resoption/release of Ca2+ as well as increasing renal excretion

47
Q

Is calcitonin crucial in humans?

A

Not really - if high its affects are over-ridden by PTH

Patients with high or low levels have no adverse effects

48
Q

What can calcitonin be used to treat?

A

Paget’s disease - overactive osteoclasts

49
Q

Name some other endocrine hormones that alter Ca2+ blanace

A 
Cortisol
Insulin
Oestrogen
GH
Prolactin
50
Q

How does cortisol affect calcium?

A

Inhibits osteoblasts
Increases renal excretion of Ca2+ and phosphate
Reduced intestinal absorption of Ca2+

This decreases plasma Ca2+ and combined with inhibited osteoblasts can lead to osteoporosis

51
Q

How does insulin affect calcium?

A

Increases bone formation - diabetics may have bone loss

52
Q

What does oestrogen do for calcium?

A

Promotes bone formation

53
Q

What does GH do?

A

A constant stim for bone formation

54
Q

What does prolactin do?

A

Stimulating systhesis of calcitriol

ZA - Endocrine System (21 decks)

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