calcium homeostasis Flashcards
why is calcium homeostasis important?
Ca2+ ion plays an important role in many biological processes.
what do Ca2+ ions play an important role in?
- Excitation-contraction coupling
- Excitation-secretion coupling
- Excitability of nerves and muscle
- Clotting of blood
what do muscle contractions require in terms of Ca2+ concentration?
an increase in intracellular Ca2+ ion concentration
what happens if there is an increase in Ca2+ ion concentration?
an increase in extracellular Ca2+ ion concentration decreases neuromuscular contractility and hence reduces likelihood of muscle contraction occurring.
what is normal plasma calcium level?
9-10 mg/dl (2.4±0.2 mmol/L)
what makes up total body calcium?
- Bone - 99%
- Intracellular in soft tissues - 0.9%
- Extracellular fluid - 0.1%
which hormones control calcium homeostasis directly?
Parathyroid hormone or Parathormone (PTH)
Vitamin D metabolite (1,25-dihydroxyvitamin D3 or Calcitriol
Calcitonin
where is parathyroid hormone secreted from?
Chief cells of parathyroid glands (4 glands embedded in the back surface of thyroid gland)
where is PTH metabolised?
by the liver and kidney
what is the half-life of PTH?
2-5 minutes
what controls PTH secretion?
The concentration of Ca2+ in plasma regulates parathyroid gland secretion
what happens to PTH if there is low plasma Ca2+?
Low plasma Ca2+ cause increased secretion and high Ca2+ has an opposite effect
what does sustained hypocalcaemia result in?
hyperplasia and hypertrophy of the gland.
what is the primary function of PTH?
Maintain a normal level of Ca2+ in ECF
what organs does PTH target?
- bone
- kidney
- intestine
what are the effects of PTH on the kidney?
- Decrease Ca2+ excretion
- Increase PO43- excretion
what are the effects of PTH on the intestine?
- Increases absorption of both Ca2+ & PO43- by activation of Vitamin D
what are the effects of PTH on the bone?
- Fast Ca2+ efflux into the plasma from the bone fluid [Normal action]
- Slow transfer of Ca2+ and PO43- by dissolution of bone itself [during chronic hypocalcaemia]
what is the cause of PTH deficiency?
Most common cause: operative procedure on either thyroid or parathyroid gland
what are the effects of PTH deficiency?
Hypocalcaemia
what are the signs and symptoms of PTH deficiency?
- Tetany of hands and feet with mechanical stimulation of peripheral nerves
- With complete removal of the glands death occur due to asphyxiation due to tetany of respiratory muscle
- Occasionally seizers can occur due to increased excitability in the brain
what is the treatment for PTH deficiency?
Vitamin D and Calcium supplementation
what causes excess PTH?
A tumour in one of the parathyroid glands
what are the effects of excess PTH?
Hypercalcaemia & hypophosphataemia
what are the effects of excess PTH on the bone?
Mild: no effect
Severe: extensive decalcification with large punched out areas of bone filled with osteoclasts
what are the neuromuscular effects of excess PTH?
muscle weakness decreased mentation poor memory constipation, abdominal pain (groan) peptic ulcer depressed relaxation of heart during diastole
what are the effects excess PTH on the kidney?
Increased tendency for formation of kidney stones
what is the treatment for excess PTH?
surgical removal of tumour if possible
what doe metabolite vitamin D enhance?
intestinal absorption of calcium and phosphate
the responsiveness of bone to PTH
what does reduced calcium absorption lead to?
Reduced Calcium absorption leads to demineralisation of bone resulting from PTH action to maintain normal plasma Ca2+ level.
what is osteoporosis?
a condition characterised by reduction in bone mass that result from bone resorption exceeding bone formation.
what is calcitonin?
hypocalcaemic hormone, the effects generally opposite to those of PTH
where is calcitonin produced?
by parafollicular cells or C cells of thyroid gland
what stimulates the secretion of calcitonin?
stimulated by hypercalcaemia and inhibited by hypocalcaemia
what is a therapeutic use of calcitonin?
To reduce hypercalcaemia, useful in disorders of increased skeletal remodeling such as Paget’s disease
