10. Calcium Metabolism and Disorders of Calcium Balance Flashcards
Why is calcium absorption in the GIT considered incomplete?
Requires activated vitamin D
Calcium containing salts aren’t absorbed
What happens to the calcium that is ingested?
1/2 is absorbed
Some lost in stool
In a steady state, remaining calcium is excreted in urine
What form is calcium stored as in bone?
Hydroxyapatite
How is calcium reabsorbed in the kidneys?
Only ionised calcium is filtered
Some reabsorbed passively
Active under endocrine control
What are the forms of calcium in circulation?
40% bound to albumin
15% bound to citrate, nitrate and phosphate
45% ionised/free
How does hypoalbuminaemia affect calcium balance?
Less bound to albumin
How does alkalosis affect calcium balance?
More bound calcium
Which hormone reacts first to a dip in calcium?
PTH: peptide
What effect does PTH have on the GIT?
Increases absorption of calcium and phosphate
What is the overall effect of PTH?
Increase calcium but reduce phosphate
‘Phosphate Trashing Hormone’
What effect does PTH have on bone?
Increases number and activity of osteoclasts
Increased bone resorption
What effect does PTH have on the kidneys?
Increase calcium absorption in the distal nephron
Increase 1a hydroxylase enzyme activity > increased calcitriol
Increased excretion of phosphate into urine
What effect does calcitriol have on the GIT?
Increased TRPV6 channels to increase calcium absorption
What effect does calcitriol have on the bone?
Bind to osteoblasts and osteocytes
Increase pyrophosphate levels which inhibits calcium uptake into bone
What effect does calcitriol have on the kidneys?
Increase TRPV5 channels and calbindin D28K
-increases resorption in distal tubule
No loss of phosphate
What are the clinical effects of hypercalcaemia?
Stones Bones Groans Psychiatric overtones Short QT interval Dehydration (diuretic effects) Chronic kidney disease due to deposition Nephrogenic diabetes insipidus (reduced response to ADH) Type 1 renal tubular acidosis
What are the causes of hypercalcaemia?
PTH excess
Vitamin D excess
Bone turnover
Thiazide diuretics
What is the most common cause of hypercalcaemia in asymptomatic outpatients?
Hyperparathyroidism