Describe the function, distribution, regulation and physiological importance of
sodium, chloride, potassium, magnesium,calcium and phosphateions
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
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:
- 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
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
- Brandis K. The Physiology Viva: Questions & Answers. 2003.
- Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.