The normal steady state regulation of calcium metabolism involves dietary intake, physicochemical factors and hormonal control. Intestinal calcium absorption is dependent upon various digestive mechanisms as well as the amount of calcium available from the diet. Cal-cium excretion is controlled by variation of renal tubular resorption. Ninety -nine percent of total body calcium is stored in bone.
Calcium is present in the blood as an ionized, a protein bound, and a complexed form.
The upper parathyroid glands are derived from 4th pouch and the lower pair from the 3rd pouch. They are small glands measuring about 3 mm in greatest diameter . Though most individuals have four glands some people have two and others six. This aspect has clinical significance because if the glands are removed, the person may develop parathyroid insufficiency. Early in life the parathyroids are composed of sheets of chief cells, but as the individual matures, fat cells replace the tissue.