The body regulates calcium levels in the serum very carefully. Vitamin D is an essential factor which has different forms depending upon what the brain is reading as far as levels of serum calcium. The level of calcium in the blood determines which of 2 hormones gets released. This causes an increased absorption from the gut and an increase in bone resorption. Decreased HCL, oxalates, phytates, excess magnesium, and malabsorption are all factors which may decrease the absorption of calcium.
Calcium ions add hardness to the bone matrix.
Calcium aids transport across cell membranes. Muscles require calcium to function in their contractility. Without calcium the muscles tend to stay contracted. Calcium also regulates membrane stabilization. In addition, neurotransmitters at synaptic junctions are regulated by calcium.