Calcium is a major constituent of bone and hence an important structural element in the body.
The total plasma calcium concentration is around 2.15-2.55 mmol/L and consists of three fractions:-
1. Protein bound: 40-45% mainly to albumin
2. Ionised: 45-50%
3. Complexed: 5-10% (as citrate, phosphates and bicarbonate)
Vitamin D occurs in two forms:
1. Cholecalciferol (vitamin D) which is formed in the skin by the action of ultraviolet light on the epidermal steroid compound 7-dehydrocholesterol
2. Ergocalciferol (vitamin D2) derived from plant products and irradiated yeast and bread (usually added to foods)