Calcium Homeostasis

Describe the function, distribution, regulation and physiological importance of sodium, chloride, potassium, magnesium, calcium and phosphate ions

Describe the control of plasma calcium.

Calcium is a bivalent cation. Almost all (99%) of calcium is located in bone, with the remainder in plasma and soft tissues. Normal plasma levels are 2.2-2.55 mmol.L^-1, which (in plasma) may be:

• Ionised (free) calcium (50%)
  Normal range 1.1 to 1.3mmol^-1.
• Bound to albumin (40%)
• As calcium compounds (10%)

Functions of Calcium

• Cell Signaling
  Calcium has a number of roles in cell signaling:
  • Affects cell sodium permeability and therefore the RMP of excitable cells
  • Calcium triggers exocytosis of neurotransmitter vesicles
  • Calcium is an important second messenger for some G proteins

• Bone
  Calcium has two functions in bone:
  • Physical structure
  • Alkali reserve
    Calcium phosphate can be mobilised to buffer acidosis.

• Enzymatic cofactor
  Calcium is an important cofactor in enzymatic pathways, including the coagulation cascade. Clinical hypocalcaemia does not cause coagulopathy however, as calcium levels low enough to prevent coagulation are not compatible with life.

Regulation of Calcium

Calcium is regulated to maintain a stable ionised calcium level. Three hormones are involved in the regulation of calcium:

• Parathyroid Hormone
  Protein hormone secreted by the four parathyroid glands, located on the posterior surface of the thyroid, in response to a fall in iCa²⁺ levels, and acts to increase plasma calcium:
  • Increase calcium reabsorption in the PCT and late DCT
  • Increase osteoclastic activity in bone
  • Increase vitamin D activation in the intestine, which in turn increases intestinal absorption of dietary calcium

• Vitamin D/Calcitriol
  Once converted to calcitriol in the kidney (via stimulation from PTH), vitamin D acts to:
  • Increase calcium reabsorption from kidney and gut
  • Increase bone calcification

• Calcitonin
  Peptide hormone secreted by the C cells of the thyroid gland, in response to a rise in iCa²⁺ greater than 2.4mmol.L^-1. Calcitonin acts to:
  • Decrease absorption of calcium from gut and kidney
  • Decrease osteoclastic activity of bone

References

1. Brandis K. The Physiology Viva: Questions & Answers. 2003.
2. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.