Calcium, magnesium and phosphate homeostasis Flashcards
Physiological importance of calcium
muscle contraction, neuronal excitation, enzyme activity (Na/K ATPase, hexokinase, etc), blood clotting
Total body calcium
- Around 1kg
- 99% bon
- 1% intracellular, 0.1% extracellular
Total Ca =
ionised Ca + bound Ca + complexed Ca
If low albumin, what happpns t calcium levels?
reduction in plasma protein to bind = lower bound calcium and total calcium; same ionised calcium
Adjusted Ca
- adjusted for if patient had normal albumin
- Total Ca + [ (40 – Alb) x 0.025]
Phosphate
- Mainly intracellular
- P in ATP – body fuel, intracellular signalling, cellular metabolic processes
- Backbone of DNA + in lipids
Distribution of phosphate
- Around 700g
- 85% in bone
- 14 % extra, 1% intra
- 70% organic form – covalently bound e.g. phospholipids
- 30% inorganic as phosphate e.g. HPO42-
Homeostasis of calcium: Two key controlling factors
- PTH
- Vitamin D and metabolites
PTH
- PTH secreted in response to low calcium/high phosphate
- Act on bone (resorption) and act on kidney (increase calcium reuptake from filtrate + increase excretion of phosphate)
- Increase serum calcium
- Decrease serum phosphate
Vitamin D
- 5-Dihydroxyvitamin D which is active form
- increase the activity of 1-alpha hydroxylase enzyme to increase the activation of Vit D
- Active Vit D acts on intestine to increase absorption of calcium and phosphate
- Precursor of Vit D has higher conc in blood
Hypocalcaemia
- Symptoms include tetany and paraesthesia in extremities
- Most common cause is Vit D deficiency
- Low levels of calcium resorption due to insufficient PTH and thyroid issue
Hypercalcaemia
- Stones, bones, moans. groans
- Caused by hyper parathyroid - too much PTH
- Most common cause = Malignant tumours that secrete chemicals increasing resorption rate
How is ionised calcium measured
blood gas analyser
Phosphate deficiency symptoms
- Haematological issues - haemolysis
- Severe muscle weakness
- Degradation of muscle cells
- Rickets/osteomalacia
- Convulsions, coma, death
Refeeding syndrome
- Cause of phosphate deficiency
- upon refeeding patient calories kickstarts insulin secretion starting glycolysis, protein synthesis, glycogenesis - all require phosphate = severe depletion of circulation concentration of Phosphate
Hyperphosphatemia
- Not as serious as hypo
- Symptoms usually due to hypocalcaemia
- Most common cause is renal failure - AKI or CKD
- Spurious (miscalculation) due to haemolysis or assay interference
Magnesium
- Cofactor for ATP, Enzymatic function – cofactor, Regulates ion channels
- Around 1.1 mol
- 54 % bone
- 45 % intracellular, 0.1 % extracellular
- 60% is ionised MG, 25% bound to albumin, 15% complexed to anions
Homeostasis of magnesium
How does PTH interact with magnesium
- PTH release is stimulated by decreased magnesium and inhibited by increased magnesium
- PTH release is magnesium-dependent so severe hypomagnesaemia will inhibit PTH release and cause hypocalcaemia which won’t resolve with calcium supplementation – need Mg
Hypomagnesaemia signs and symptoms
- CNS – neuromuscular hyperexcitability (tremor, tetany, convulsion),
- Cardiovascular – ECG changes, reduced contractility, arrhythmia
- GI – nausea and anorexia
- hypokalaemia, hypocalcaemia, with associated symptoms
Hypomagnesaemia causes
- Inadequate intake (diet)
- Renal loss due to drugs, Hypercalcaemic states
- Redistribution into cells (refeeding syndrome)
Hypermagnesemia
- Very rare as kidneys have large capacity to excrete excess
- there is a high level of magnesium in the blood. - Symptoms include weakness, confusion, decreased breathing rate, and decreased reflexes.
Hypermagnesemia causes
- Renal Osteodystrophy
- Post-op recent blood transfusion
- Likely vitamin D deficiency
- Primary hypoparathyroidism
