Physiology 4 - Calcium Balance Flashcards

1
Q

What are the roles of calcium in the body?

A
Signalling
Blood clotting
Apoptosis
Bone strength
Membrane excitability (most obviously in muscle contraction)
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2
Q

What happens to your muscle if your hypo or hypercalcaemic?

A

Hypocalcaemia

  • > Increased neuronal Na permeability
  • > Hyperexcitation
  • > Tetany & Asphyxiation in extreme cases

Hypercalcaemia

  • > Depressed excitation
  • > Cardiac arrythmias in extreme cases
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3
Q

Describe the distribution of calcium in the body?

A

99% stored in bones as hydroxyapatite

  1. 9% is in the ICF
  2. 1% is found in the ECF/plasma

Of the 0.1% in the plasma 40% is bound to plasma proteins, 10% is bound to anions and only 50% (~1.2mM) is free and physiologically active

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4
Q

What is the relevance of the protein bound Ca2+ in the blood?

A

The degree of protein binding is pH determined.
Therefore in alkalosis (E.g. hyperventilating), binding increases and hypocalcaemia can occur.

Vice versa acidosis can cause hypercalcaemia

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5
Q

What is the relevance of phosphate to calcium homeostasis>

A

Calcium is bound with phosphate into Hydroxyapatite making up bone.

This means phosphate is essential for calcium storage in bone

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6
Q

List the hormones involved in calcium homeostasis

A

Raises Ca2+:
PARATHYROID HORMONE (PTH)
CALCITRIOL

Lowers Ca2+
CALCITONIN

Other:
Cortisol
Insulin
Oestrogen
Prolactin
Growth Hormone
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7
Q

Describe PTH?

A

A peptide hormone produced in 4 parathyroid glands posterior to the thyroid and released in response to low free plasma Ca2+

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8
Q

What are the functions of PTH?

A

IT serves to raise serum free Ca2+ by:

1) Stimulating osteoclasts
2) Inhibits osteoblasts
3) Increases renal reabsorption of Ca2+
4) Increases renal excretion of phosphate (preventing bone deposition)
5) Stimulates renal synthesis of Calcitriol

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9
Q

Describe calcitriol:

A

A steroid hormone produced from Vit D in the liver and kidneys in response to PTH (and in lactating women - Prolactin)
Also known as Active Vit D3 or 1,25-dihydroxycholecaliferol

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10
Q

How does calcitriol work?

A

Serves to raise free plasma Ca2+ by binding to nuclear receptors in the intestine, bone & kidney to:

  • Stimulates osteoclasts
  • Increases renal reabsorption of Ca2+
  • Increases Ca2+ absorption in the gut
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11
Q

How much dietary Ca2+ is absorped in the gut?

A

30% in a healthy person
10-15% in a vit d deficient person
50% in pregnancy, lactation of growth spurts.

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12
Q

Describe Calcitonin

A

A peptide hormone produced by the thyroid and released when theres a high free plasma Ca2+

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13
Q

How does Calcitonin work?

A

IT lowers plasma Ca2+ by:

  • Inhibiting Osteoclasts
  • Increasing Ca2+ renal excretion
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14
Q

Why isnt calcitonin that relevant clinically?

A

Patients with raised or absent calcitonin don’t get abnormal calcium levels.
PTH seemingly overrides the effects of calcitonin anyway

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15
Q

How is cortisol involved in calcium homeostasis?

A
  • Inhibits osteoblasts
  • Increases renal excretion of Ca2+
  • Decreases Intestinal absorption of Ca2+

Decreasde absorption means more PTH is secreted which means more bone is broken down, combined with the inhibition of osteoblasts this can lead to osteoporosis

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16
Q

How is Insulin involved in calcium homeostasis?

A

Insulin antagonizes cortisol -> Increased bone formation

Hence diabetics have significant bone loss

17
Q

How is Oestrogen involved in calcium homeostasis?

A

Promotes osteoblasts

Thus post-menopausal osteoporosis is possible

18
Q

How is Growth Hormone involved in calcium homeostasis?

A

Stimulates osteoblasts

19
Q

How is prolactin involved in calcium homeostasis?

A

Stimulates calcitriol synthesis -> Increased Ca2+ absorption from gut

20
Q

Which hormones raise/lower Calcium in the blood and bone?

A

PTH & Calcitriol raise Serum Ca2+ at the expense of bone

Calcitonin inhibits bone resorption and excretes Ca2+

Cortisol lowers bone and plasma Ca2+

Insulin, oestrogen & GH all raise Bone Ca2+

Prolactin increases plasma & decreases bone Ca2+ by stimulating calcitriol

21
Q

Who is most likely to suffer from a Vit D deficiency?

A

Over 65s due to reduced gut absorption
Darker skinned people and/or living in low light countries
Asian populations due to dietary Vit D3 deficiency and consumption of chapatti flour which contains phytate that binds dietary Ca2+ preventing absorption

22
Q

How does a vitamin d deficiency present/how does it cause these presentations?

A

Soft, easily fractured bones:
Osteomalacia in adults
Rickets in kids

Calcitriol isn’t absorbing extra Ca2+ so PTH has to drain more from bone.
The high phosphate excretion due to high PTH exacerbates bone loss