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Functions of calcium |
Calcium deficiency | Disease prevention | Disease treatment |
Sources of calcium |
Safety of calcium |
References | ||
The function of calcium in the human body
Bone structure
Calcium is a major structural element in bones and teeth. The mineral component of bone consists mainly of hydroxyapatite crystals, which contain large amounts of calcium and phosphorus (about 40% calcium and 60% phosphorus) (2). Bone is a dynamic tissue that is remodeled throughout life. Bone cells called osteoclasts begin the process of remodeling by dissolving or resorbing bone. Bone-forming cells called osteoblasts then synthesize new bone to replace the bone that was resorbed. During normal growth, bone formation exceeds bone resorption. Osteoporosis may result when bone resorption exceeds formation (1).
Cell signaling
Calcium plays a role in mediating the constriction and relaxation of blood vessels (vasoconstriction and vasodilation), nerve impulse transmission, muscle contraction, and the secretion of hormones, such as insulin (3). Excitable cells, such as skeletal muscle and nerve cells, contain voltage-dependent calcium channels in their cell membranes that allow for rapid changes in calcium concentrations. For example, when a muscle fiber receives a nerve impulse that stimulates it to contract, calcium channels in the cell membrane open to allow a few calcium ions into the muscle cell. These calcium ions bind to activator proteins within the cell that release a flood of calcium ions from storage vesicles inside the cell. The binding of calcium to the protein, troponin-c, initiates a series of steps that lead to muscle contraction. The binding of calcium to the protein, calmodulin, activates enzymes that breakdown muscle glycogen to provide energy for muscle contraction (1).
Cofactor for enzymes and proteins
Calcium is necessary to stabilize or allow for optimal activity of a number of proteins and enzymes. The binding of calcium ions is required for the activation of the seven "vitamin K-dependent" clotting factors in the coagulation cascade. The term, "coagulation cascade," refers to a series of events, each dependent on the other that stops bleeding through clot formation (4).
Regulation of calcium levels
Calcium concentrations in the blood and fluid that surrounds cells are tightly controlled in order to preserve normal physiological functioning (diagram). When blood calcium decreases (e.g., in the case of inadequate calcium intake), calcium-sensing proteins in the parathyroid glands send signals resulting in the secretion of parathyroid hormone (PTH) (5). PTH stimulates the conversion of vitamin D to its active form, calcitriol, in the kidneys. Calcitriol increases the absorption of calcium from the small intestine. Together with PTH, calcitriol stimulates the release of calcium from bone by activating osteoclasts (bone resorbing cells), and decreases the urinary excretion of calcium by increasing its reabsorption in the kidneys. When blood calcium rises to normal levels, the parathyroid glands stop secreting PTH and the kidneys begin to excrete any excess calcium in the urine. Although this complex system allows for rapid and tight control of blood calcium levels, it does so at the expense of the skeleton (1).
Copyright 2001-2003 The Linus Pauling Institute