Flashcards in Physiology 4 - Calcium Balance Deck (22):
What are the roles of calcium in the body?
Membrane excitability (most obviously in muscle contraction)
What happens to your muscle if your hypo or hypercalcaemic?
-> Increased neuronal Na permeability
-> Tetany & Asphyxiation in extreme cases
-> Depressed excitation
-> Cardiac arrythmias in extreme cases
Describe the distribution of calcium in the body?
99% stored in bones as hydroxyapatite
0.9% is in the ICF
0.1% 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
What is the relevance of the protein bound Ca2+ in the blood?
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
What is the relevance of phosphate to calcium homeostasis>
Calcium is bound with phosphate into Hydroxyapatite making up bone.
This means phosphate is essential for calcium storage in bone
List the hormones involved in calcium homeostasis
PARATHYROID HORMONE (PTH)
A peptide hormone produced in 4 parathyroid glands posterior to the thyroid and released in response to low free plasma Ca2+
What are the functions of PTH?
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
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
How does calcitriol work?
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
How much dietary Ca2+ is absorped in the gut?
30% in a healthy person
10-15% in a vit d deficient person
50% in pregnancy, lactation of growth spurts.
A peptide hormone produced by the thyroid and released when theres a high free plasma Ca2+
How does Calcitonin work?
IT lowers plasma Ca2+ by:
- Inhibiting Osteoclasts
- Increasing Ca2+ renal excretion
Why isnt calcitonin that relevant clinically?
Patients with raised or absent calcitonin don't get abnormal calcium levels.
PTH seemingly overrides the effects of calcitonin anyway
How is cortisol involved in calcium homeostasis?
- 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
How is Insulin involved in calcium homeostasis?
Insulin antagonizes cortisol -> Increased bone formation
Hence diabetics have significant bone loss
How is Oestrogen involved in calcium homeostasis?
Thus post-menopausal osteoporosis is possible
How is Growth Hormone involved in calcium homeostasis?
How is prolactin involved in calcium homeostasis?
Stimulates calcitriol synthesis -> Increased Ca2+ absorption from gut
Which hormones raise/lower Calcium in the blood and bone?
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
Who is most likely to suffer from a Vit D deficiency?
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