15 Questions
What is the approximate amount of calcium in the average adult body?
25 000 mmol
What is the primary function of calcium in bones?
Structural function
What is stimulated in response to hypocalcaemia?
PTH secretion
Which hormone is secreted by the parathyroid glands in response to a fall in plasma calcium concentration?
Parathyroid hormone (PTH)
What is the effect of PTH on phosphate levels?
Decreases phosphate reabsorption
What is the primary action of calcitriol on the gut?
Stimulates absorption of dietary calcium and phosphate
What is the effect of hypercalcemia on parathyroid hormone secretion?
Inhibits secretion
What is the response to hypophosphatemia?
Increased phosphate absorption
What is the role of calcium-sensing receptor (CaSR) in PTH secretion?
Mediates the effects of PTH on plasma calcium concentration
What is a common cause of hypercalcemia?
Primary hyperparathyroidism
What is the effect of calcitriol on PTH synthesis?
Inhibits PTH synthesis
What is measured in the investigation of hypercalcemia?
PTH levels using an assay for the intact hormone
What is a cause of artefactual hypocalcaemia?
Collection of blood in EDTA tube
What is associated with high PTH levels?
Pseudohypoparathyrodism
What is a cause of hypocalcaemia?
Hypomagnesaemia
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 role and distribution of calcium in the human body, including its abundance, skeletal binding, and kidney function.
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