Calcium and Phosphate in the Human Body

Study Notes

Average adult body contains approximately 25,000 mmol (1 kg) of calcium, with 99% bound in the skeleton.
Total calcium content of extracellular fluid (ECF) is 22.5 mmol, with 9 mmol in the plasma.
Calcium in bone is stable, with the kidneys filtering 240 mmol/24h of ionized calcium.

Structural: bone, teeth
Neuromuscular: control of excitability, release of neurotransmitter, initiation of muscle contraction
Enzymic: co-enzyme of coagulation factors
Signaling: intracellular second messenger

Two hormones regulate calcium: parathyroid hormone (PTH) and calcitriol (1,25-dihydroxycholecalciferol)
Parathyroid hormone (PTH):
- Secreted by parathyroid glands in response to low plasma calcium concentration
- Inhibited by hypercalcemia
- Acts on bone and kidneys to increase plasma calcium concentration and reduce phosphate concentration
Calcitriol:
- Derived from vitamin D
- Stimulates absorption of dietary calcium and phosphate in the gut
- Promotes mineralization in bone
- Inhibits its own synthesis in the kidneys

Hypocalcaemia stimulates PTH secretion, increasing calcitriol production and calcium and phosphate uptake from the gut and release from bone
PTH is phosphaturic, increasing phosphate excretion, but retaining some mobilized calcium
In hypophosphatemia, calcitriol secretion increases, but PTH secretion is not stimulated

Two conditions account for up to 90% of cases: primary hyperparathyroidism and malignancy

Plasma phosphate concentration has limited diagnostic value
Plasma alkaline phosphatase activity can be elevated in primary hyperparathyroidism and malignancy
Radiographic examination may reveal subperiosteal bone reabsorption and bone cysts of hyperparathyroidism
Measurement of PTH is essential using an assay for the intact hormone

Causes:
- Artefactual (collection of blood in EDTA tube)
- Associated with low PTH: hypoparathyroidism, hypomagnesaemia, hungry bone syndrome, and neonatal hypocalcaemia
- Associated with high PTH: vitamin D deficiency, disorder of vitamin D metabolism, pseudohypoparathyroidism, acute pancreatitis, high phosphate intake, massive blood transfusion with citrate blood, and acute rhabdomyolysis