text: <unk>  plays a vital role in the anatomy, physiology and biochemistry of organisms and of the cell, particularly in signal transduction pathways. The skeleton acts as a major mineral storage site for the element and releases Ca2+ ions into the bloodstream under controlled conditions.  Circulating<unk>  is either in the free, ionized form or bound to blood proteins such as serum albumin.  The hormone secreted by the parathyroid gland, parathyroid hormone, regulates the resorption of Ca2+ from bone.
option_0: Calcium in biology
option_1: Blood flow
option_2: Insulinoma causes
option_3: Cardiac cycle
option_4: Oxygen permeability
option_5: Fluence
option_6: Gastrointestinal physiology
option_7: Magnesium and the cardiovascular system