According to Chapter 12; page 384-385 Ninety-nine percent of the body’s calcium is in the bones (and teeth), where it plays two roles. First, it is an integral part of bone structure providing a rigid frame that holds the body upright and serves as attachment points for muscles, making motion possible. Second, it serves as a calcium bank, offering a readily available source of calcium to the body fluids should a drop in blood calcium occur. The remaining 1 percent of the body’s calcium is in the body fluids.
As bones begin to form, calcium salts form crystals, called hydroxyapatite, on a matrix of the protein collagen. During mineralization, as the crystals become denser, they give strength and rigidity to the maturing bones. As a result, the long leg bones of children can support their weight by the time they have learned to walk.
Many people have the idea that once a bone is built, it is inert like a rock. Actually, the bones are gaining and losing minerals continuously in an ongoing process of remodeling. Growing children gain more bone than they lose, and healthy adults maintain a reasonable balance. When withdrawals substantially exceed deposits, problems such as osteoporosis develop.
The formation of teeth follows a pattern similar to that of bones. The turnover of minerals in teeth is not as rapid as in bone, however; fluoride hardens and stabilizes the crystals of teeth, opposing the withdrawal of minerals from them.
Although only 1 percent of the body’s calcium circulates in the extracellular and intracellular fluids, its presence there is vital to life. Cells throughout the body can detect calcium in the extracellular fluids and respond accordingly. Many of calcium’s actions help to maintain normal blood pressure, perhaps by stabilizing the smooth muscle cells of the blood vessels or by releasing relaxing factors from the blood vessel cell walls. Extracellular calcium also participates in blood clotting.
The calcium in intracellular fluids binds to proteins within the cells and activates them. For example, when the protein calmodulin binds with calcium, it activates the enzymes involved in breaking down glycogen, which releases energy for muscle contractions. Many such proteins participate in the regulation of muscle contractions, the transmission of nerve impulses, the secretion of hormones, and the activation of some enzyme reactions.
How does the body keep blood calcium constant regardless of intake?
According to Chapter 12 page 385-386 section of “Calcium Balance”; Whenever blood calcium falls too low or rises too high, three organ systems respond: intestines, bones, and kidneys. Vitamin D and two hormones – parathyroid hormone and calcitonin return blood calcium to normal.
The calcium in bones provides a nearly inexhaustible bank of calcium for the blood. The blood borrows and returns calcium as needed so that even with an inadequate diet, blood calcium remains normal—even as bone calcium diminishes. Blood calcium changes only in response to abnormal regulatory control, not to diet. A person can have an inadequate calcium intake for years and have no noticeable symptoms. Only later in life does it become apparent that bone integrity has been compromised.
Blood calcium above normal results in calcium rigor: the muscles contract and cannon relax. Similarly, blood calcium below normal cause’s calcium tetany—also characterized by uncontrolled muscle contraction. These contractions do not reflect a dietary excess or lack of calcium; they are caused by a lack of vitamin D or by abnormal secretion of the regulatory hormones. A chronic dietary deficiency of calcium, or a chronic deficiency due to poor absorption over the years, depletes the bones. Again: the bones, not the blood, are robbed by a calcium deficiency
The stomach’s acidity helps to make the calcium binding protein needed for absorption. This relationship explains why calcium rich milk is a good choice for vitamin D fortification.
Whenever calcium is needed, the body increases its calcium absorption