Calcium

Normal serum ~ 2.1-2.6 mmol / l
Normal urinary ~ 2.5-7.5 mmol /24 hours

  • Most abundant mineral in the body

    • about 1 kg (25mol) in average adult

  • tightly controlled
  • relationship between calcium and phosphate levels
    • If PTH appropriate (and no GFR impairment) then calcium and phosphate move in same direction

Correction

  • 45% protein bound so must do correction for albumin levels
    • = [Ca++] + 0.02(40-[albumin])

Functions

  • Structural
    • bone
    • teeth
  • Neuromuscular
    • control of excitability
    • neurotransmitter release
    • muscle contraction
  • Enzymic
    • Coenzyme for coagulation factors
  • Hormonal
    • intracellular second messenger

Tissues Involved

  • bone
    • 99% calcium as hydroxyapatite
    • re-absorbed and deposited in bone reshaping
  • blood
    • 2.1-2.6 mmol/l = 9 mmol (300mg) in the blood
    • 45% protein bound
    • 10% complexed with citrate or phosphate
    • 45% ionised
    •  
  • intestine
    • Diet 25mmol (1g)
    • about 0.25-0.5g absorbed
    • Vitamin D required for absorption
    • some is lost in faces due to enteric secretion
  • kidney
    • excreted in urine
  • skin
    • excreted in sweat
  • skeletal, cardiac muscle
    • Calcium released from sarcoplasmic reticulum
  • secretory cells
    • calcium released from mitochondria

Control Mechanisms

  • Intake
    • Diet 25mmol (1g)
      • about 0.25-0.5g absorbed
    • Vit. D required for absorption
  • Losses
    • urine, faeces, sweat
  • principally controlled by:
  • decreased calcium leads to increased PTH., increased 1,25-(OH)2-D
    • increased uptake calcium and phosphate from gut
    • release of calcium from bone
    • excess phosphate excreted
  • Other factors that have an effect on calcium levels in blood
    • calcitonin
    • glucocorticoids
      • in excess may lead to osteoporosis
    • androgens
      • low levels may lead to osteoporosis
    • oestrogen
      • low levels may lead to osteoporosis
    • thyroid hormones
      • high T4 may lead to osteoporosis
    • growth hormone
      • increases 1,25-(OH)2-D

Measuring

  • serum
    • can be done in serum tube (brown)
    • venous stasis (e.g. if prolonged tourniquet) can cause increased protein and calcium levels
  • urinary calcium
    • needs acidified collection to prevent precipitation

Disorders

  • Normal in
    • osteoporosis
    • Paget's disease
    • Renal tubular disorders of phosphate reabsorption
 

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