calcium

Calcium is a chemical element with the symbol Ca and atomic number 20. It is an alkaline earth metal and the fifth most abundant element by mass in the Earth's crust. In biology, calcium ions play a central role in many cellular processes.

In humans and other vertebrates, most calcium is stored in the skeleton and teeth, where it forms hydroxyapatite and provides structure and strength. About 1% circulates in the blood and soft tissues, where it participates in muscle contraction, nerve signaling, enzyme activity, and blood clotting.

Calcium homeostasis is tightly regulated by hormones, chiefly parathyroid hormone, vitamin D, and calcitonin. Absorption occurs mainly in the small intestine and is enhanced by vitamin D status and adequate stomach acid; absorption efficiency is lower with age and varies with dietary composition.

Dietary calcium is found in dairy products, leafy green vegetables (though oxalates can reduce absorption), fortified foods, and certain fish. Common supplements include calcium carbonate and calcium citrate, often taken with meals and vitamin D to improve absorption.

Deficiency can lead to rickets in children and osteoporosis or osteomalacia in adults. Hypocalcemia may cause neuromuscular symptoms. Excessive calcium intake is uncommon from diet alone but can cause hypercalcemia, kidney stones, or impaired absorption of other minerals, especially with certain medications or kidney disease.

Industrially, calcium is produced and used in the manufacture of cement, metals, and as a reducing agent in chemistry; its compounds include calcium carbonate, calcium phosphate, and calcium sulfate.