Calcium Calcium Calcium is a chemical element with symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to its heavier homologues strontium and barium. It is the fifth most abundant element in Earth's crust and the third most abundant metal, after iron and aluminium. The most common calcium compound on Earth is calcium carbonate, found in limestone and the fossilised remnants of early sea life; gypsum, anhydrite, fluorite, and apatite are also sources of calcium. The
Calcium metabolism This process is known as the sliding filament model of muscle contraction. The plasma "total calcium" concentration is in the range of 2.2-2.6 mmol/L (9-10.5 mg/dL), and the normal "ionized calcium" is 1.3-1.5 mmol/L (4.5-5.6 mg/dL). The amount of total calcium in the blood varies with the level of plasma albumin, the most abundant protein in plasma, and therefore the main carrier of protein-bound calcium in the blood. The biologic effect of calcium is, however, determined by the amount of "ionized calcium", rather than the total calcium. It is therefore the plasma ionized calcium level which is tightly regulated to