Calcium Metabolism Flashcards
Outline the metabolism of calcium
In response to hypocalcaemia
The parathyroid glands release PTH which increases bone resorption and stimulates 1a hydroxylase in the kidneys (DCT) to convert calcidiol to calcitriol.
Calcitriol stimulates the GIT to increase calcium and phosphate resorption, and calcitriol stimulates the PCT for calcium resorption
Outline vitamin D metabolism
UV rays turn 6-dehydrocholesterol to cholecalciferol in the skin, cholecalciferol goes to the liver and turns to calcidiol, calcidiol turns to calcitriol in the kidneys
What two substances increase calcium levels in the blood?
Vitamin D
PTH
How does PTH affect osteoblasts and osteoclasts?
PTH binds to osteoblasts stimulating their proliferation, and inhibits OPG. By inhibiting OPG, they allow the RANK-RANKL complex to thrive thus stimulating osteoclast proliferation. By stimulating osteoclast proliferation, this leads to increased bone resorption.
What is false hypocalcaemia?
When there is less albumin in the body so the calcium concentration is actually the same but there is more free calcium so the value looks higher
How would alkalosis in the body affect calcium?
Alkalosis means there are fewer H+ ions in the body, thus most albumin will have COO- and not COOH, therefore Ca2+ ions are more likely to bind to albumin, resulting in less free calcium = hypocalcaemia.
The causes of true hypocalcaemia can be subdivided into too little calcium entering the blood, or too much leaving the blood. What causes too little calcium to enter the blood?
Hypoparathyroidism (either through surgical removal, autoimmune destruction, magnesium deficiency or DiGeorge syndrome)
Low levels of vitamin D
The causes of true hypocalcaemia can be subdivided into too little calcium entering the blood, or too much leaving the blood. What causes too much calcium to leave the blood?
Kidney failure (failure to prevent calcium ions being reabsorbed)
Tissue injury (burns, tumour lysis syndrome, rhabdomyolysis = when cells die, phosphate is released and the phosphate uses up and binds to the free calcium)
Acute pancreatitis (FFA bind to calcium, using it up)
Too many blood transfusions (additives in the transfusion bind to the calcium too)
What are the symptoms and signs of hypocalcaemia?
Tetany (involuntary muscle contraction)
Troussaeu’s sign (spasm of hand while BP cuff is on)
Chvostek’s sign (facial nerve spasm after cheek tapped)
Abdominal pain
Perioral tingling
Extreme cases: seizures
How do you diagnose hypocalcaemia?
Low calcium in the blood (<8.5)
Electrocardiogram (may show arrythmias like torsades de pointes, prolonged QT interval or prolonged ST interval)
Tests for PTH, magnesium, phosphorus, vitamin D, albumin
What is the treatment of hypocalcaemia?
IV calcium gluconate, 10ml of 10% solution over 10minutes
Vitamin D supplementation
How does acidosis affect calcium levels?
Acidosis leads to excess H+ ions in the blood causing albumin to be have more -COOH groups. Less free Ca2+ will therefore bind to the albumin = hypercalcaemia
What happens in false hypercalcaemia?
Hyperalbuminaemia, caused by things such as dehydration, causes more calcium to be bound to the albumin molecules. There is still an adequate amount of free calcium due to hormonal regulation, however. So it will be a false picture of hypercalcaemia.
What is the most common cause of hypercalcaemia?
Osteoclastic bone resorption (can either occur through increased PTH or malignant tumours secreting PTH related protein)
= results in lytic bone lesions
Name some causes of hypercalcaemia?
Osteoclastic bone resorption
Excess vitamin D (causing increased calcium resorption in the gut)
Medications (such as thiazide diuretics which increase calcium resorption in the PCT)
