CALCIUM CARBONATE IS USED AS AN ANTACID, WHICH NEUTRALISE HYDROCHLORIC ACID IN THE GASTRIC SECRETION. CALCIUM CARBONATE BINDS PHOSPHATE IN THE G.I. TRACT TO FORM INSOLUBLE COMPLEXES AND REDUCES PHOSPHATE ABSORPTION. CALCIUM IS THE FIFTH MOST ABUNDANT ELEMENT IN THE BODY. CALCIUM IS A DIVALENT METAL ESSENTIAL FOR THE MAINTENANCE OF THE NERVOUS, MUSCULAR, AND SKELETAL SYSTEMS, AND FOR CELL MEMBRANE AND CAPILLARY PERMEABILITY. ITS ROLE IN BONE STRUCTURE AND MUSCLE CONTRACTION IS WELL KNOWN, BUT CALCIUM IS ALSO IMPORTANT FOR BLOOD COAGULATION, NERVE CONDUCTION, AND ELECTRICAL CONDUCTION IN THE HEART. CALCIUM STIMULATES GASTRIN SECRETION, WHICH OFFSETS THE UTILITY OF ORAL CALCIUM CARBONATE AS A TREATMENT OF PEPTIC ULCER DISEASE . CALCIUM AND MAGNESIUM ARE, IN SOME WAYS, PHYSIOLOGICAL OPPOSITES, AND CALCIUM CAN BE USED FOR THE TEMPORARY CONTROL OF CARDIAC ARRHYTHMIAS ASSOCIATED WITH HYPOKALEMIA.