15 Questions
What percentage of calcium in the human body is bound in the skeleton?
99%
What is the total calcium content of the extracellular fluid (ECF)?
22.5 mmol
What is the primary function of calcitriol in the gut?
Stimulating the absorption of dietary calcium
What is the effect of parathyroid hormone (PTH) on bone?
It tends to increase the plasma concentration of calcium
What is the primary action of calcitriol on bone?
Promoting mineralization
What is the effect of calcitriol on kidney function?
It inhibits its own synthesis
What is the role of the calcium-sensing receptor (CaSR)?
Mediating the effects of PTH on the plasma concentration of calcium
What is the primary effect of hypocalcaemia on PTH secretion?
It stimulates PTH secretion
What happens to the fractional reabsorption of calcium by the kidney in response to PTH?
It increases
What is the primary difference between hypocalcaemia and hypophosphatemia in terms of PTH secretion?
PTH secretion is increased in hypocalcaemia and decreased in hypophosphatemia
What is the most common cause of hypercalcemia?
Primary hyperparathyroidism
What is the significance of measuring plasma alkaline phosphatase activity in the investigation of hypercalcemia?
It can be elevated in either hyperparathyroidism or malignancy
What is the primary purpose of measuring PTH in the investigation of hypercalcemia?
To differentiate between hyperparathyroidism and malignancy
What is the effect of high phosphate intake on plasma calcium concentration?
It decreases plasma calcium concentration
What is the characteristic radiographic finding in hyperparathyroidism?
Subperiosteal bone reabsorption
Study Notes
Calcium
- 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.
Calcium Functions
- Structural: bone, teeth
- Neuromuscular: control of excitability, release of neurotransmitter, initiation of muscle contraction
- Enzymic: co-enzyme of coagulation factors
- Signaling: intracellular second messenger
Calcium-Regulating Hormones
- 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
Calcium and Phosphate Homoeostasis
- 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
Causes of Hypercalcemia
- Two conditions account for up to 90% of cases: primary hyperparathyroidism and malignancy
Investigation and Treatment
- 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
Hypocalcaemia
- 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
Learn about the functions and structure of calcium in the human body, including its role in bone and teeth, neuromuscular functions, and enzymic activities.
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