Calcium and phosphorus Distribution and transport
Approximately 2% of the adult human body is calcium, and 1% is phosphorus (about 1200 g and 670 g in a 70-kg man, respectively); 99% of the calcium and 80% of the phosphorus in our bodies are found as constituents of bone and teeth, giving them strength and rigidity. Because of this common role and their many interactions in the body, calcium and phosphorus are considered together in most discussions. Yet it is important to recognize that their roles outside the skeleton are distinct and no less essential than their funclion in the bone.
In an adult, approximately 10 g of calcium are found in the extracellular fluids and soft tissues. In body fluids calcium exists in three forms: as ionized calcium (CaH), which is the physiologically active form; as a complex with organic or inorganic acids, such as calcium citrate, calcium phosphate, and calcium sulfate; and as proteinbound calcium. The first two forms are known as dijjUsible or ultra filterable fractions because they can move from one fluid compartment to another and are filtered by the renal glomeruli, in contrast to the nondiffusible protein-bound fraction.
A relatively large proportion of phosphorus outside the bone is present in the soft tissues as organic phosphates. It is an important component of membrane lipoproteins, cellular nudeic. adds, hig,h-energ,y compounds, and other substances important in cellular metabolism. Soluble phosphate ions also contribute a sizable fraction of the intracellular anions.
About half of plasma phosphorus circulates as filterable phospha te ions (HP04 = and H2P04 -) and about one-third occurs as complexes with such cations as sodium, calcium, and magnesium; the remaining phosphorus is protein-bound.
Functions of calcium and phosphorus
Formation and maintenance of bone and tooth structure. Bone is made up of a flexible, extremely strong organic matrix in addition to bone salts, which are deposited within the lattice-like framework ofthe matrix to make them hard and rigid. The bone matrix formed by the osteoblasts consists of protein collagen embedded in a gelatinous ground substance composed of mucopolysaccharides, such as chondroitin sulfate. The ground substance varies in consistency from a relatively thin fluid to a thick gel, which forms the interconnection with the tissue fluid that permits an exchange of ions and other elements in the blood. The bone salts or inorganic constituents of bone consist of small crystals of calcium phosphate in the form of hydroxyapatite (3CaiP04). Ca(OH)2) and noncrystalline or amorphous calcium phosphate. Small amounts of magnesium, sodium, carbonate, citrate, chloride, and fluoride are also present in bone salts. The hydrooxyapatite crystals, although relatively stable in structure, are extremely small, which gives them a large area, and can rapidly exchange ions at their surface. The intercrystalline fraction is more soluble, and its elements can go back to the blood by the simple process of solution. During early life the amorphous material pre-dominates, but it is replaced by the crystalline form in more mature bone. The size of the crystals increases with maturity as does the calcium to phosphorus ratio. Bones blood and lymph vessels, nerves, and bone marrow. The nutrients needed for bone metabolism pass from vessels into the interstitial fluid that surrounds the crystals, so that exchanges between the tissues and the blood are easily accomplished.
There is constant deposition and resorption of bone. In children bone deposition, controlled by the activity of -eoblasts, bone-forming cells, is greater than the resorption controlled by the osteoclasts, bone-destroying cells. On the other hand, the skeletal changes frequently in old age occur when bone resorption dominates and there is a decrease in the absolute amount of bone (osteoporosis). In the healthy adult the two processes are equally balanced; both calcification, or mineralization or demineralization depend on the level of calcium and phosphorus in the blood and extracellular fluids and normal functioning of the matrix cells. Approx 600 mg to 700 mg of calcium enter and leave the bones each day.
