calcium (Ca) /kal″sē·əm/ [L, calx, lime] , 1. calcium is the fifth most abundant element in the human body and is mainly present in the bone. The body requires calcium ions for the transmission of nerve impulses, muscle contraction, blood coagulation, cardiac functions, and other processes. It is a component of extracellular fluid and of soft tissue cells. The average daily human intake of calcium varies from 200 to 2500 mg. In North America, dairy products are the major dietary sources of this element. The daily dietary allowances recommended by the Food and Nutrition Board vary from 360 mg for infants to 1200 mg for women 15 to 18 years of age. More than 90% of the calcium in the body is stored in the skeleton, which constantly exchanges its supplies with the calcium of the interstitial fluids. The endocrine system controls the concentration of ionized calcium in the plasma. Only a fraction of this amount is ionized and diffusible; the rest is bound to proteins, especially albumin. It is the ionized, diffusible portion of calcium that participates in the physiological changes associated with hypocalcemia. About one third of the calcium ingested by humans is absorbed, primarily in the small bowel. Vitamin D, calcitonin, and parathyroid hormone are essential in the metabolism of calcium. The degree of cell permeability varies inversely with calcium ion concentration. Abnormally high levels of ionized calcium in the extracellular fluid can produce muscle weakness, lethargy, and coma. A relatively small decrease from the normal level of this element can produce seizures. Normal adult blood levels of calcium are 9 to 10.5 mg/dL or 2.25 to 2.75 nmol/L. 2. an alkaline earth metal element. Its atomic number is 20; its atomic mass is 40.08. Its metallic form is a white flammable solid, brittle and somewhat harder than lead. Calcium is commonly produced by the electrolysis or thermal dissociation of calcium chloride. Calcium carbonate is the most common calcium compound. Calcium also occurs as a component of the natural compound gypsum, which forms plaster of paris when heated. It is also a component of calcium cyanamid, a fertilizer and progenitor of other nitrogen compounds.
