The skeleton, in addition to providing support for the body, serves as a large store of calcium and phosphate in the form of crystals called hydroxyapatite, which has the formula Cajo(PO4)6(OH)2. The calcium phosphate in these hydroxyap-atite crystals is derived from the blood by the action of bone-forming cells, or osteoblasts. The osteoblasts secrete an organic matrix composed largely of collagen protein, which becomes hardened by deposits of hydroxyapatite. This process is called bone deposition. Bone resorption (dissolution of hydroxyapatite), produced by the action of osteoclasts (fig. 19.18a), results in the return of bone calcium and phosphate to the blood.
Bone resorption begins when the osteoclast attaches to the bone matrix and forms a "ruffled membrane" (see fig. 19.18b).
Since the bone matrix contains both an inorganic component (the calcium phospate crystals) and an organic component (collagen and other proteins), the osteoclast must secrete products that both dissolve calcium phosphate and digest the proteins of the bone matrix. The dissolution of calcium phosphate is accomplished by transport of H+ by a H+-ATPase pump in the ruffled membrane, thereby acidifying the bone matrix (to a pH of about 4.5) immediately adjacent to the osteoclast. A channel for Cl- allows Cl-to follow the H+, preserving electrical neutrality. Finally, despite the extrusion of H+ from the osteoclast, the cytoplasm is prevented from becoming too basic by the action of an active transport Cl-/HCO3-pump on the opposite surface of the osteoclast (fig. 19.18b).
The protein component of the bone matrix is digested by enzymes, primarily one called cathepsin K, released by the os-teoclasts. The osteoclast can then move to another site and begin the resorption process again, or be eliminated. Interestingly, there is evidence that estrogen, often given to treat osteoporosis in postmenopausal women, works in part by stimulating the apoptosis (cell suicide) of osteoclasts.
The formation and resorption of bone occur constantly at rates determined by the relative activity of osteoblasts and os-teoclasts. Body growth during the first two decades of life occurs because bone formation proceeds at a faster rate than bone resorption. By age 50 or 60, the rate of bone resorption often exceeds the rate of bone deposition. The constant activity of os-teoblasts and osteoclasts allows bone to be remodeled throughout life. The position of the teeth, for example, can be changed by orthodontic appliances (braces), which cause bone resorption on the pressure-bearing side and bone formation on the opposite side of the alveolar sockets.
Despite the changing rates of bone formation and resorption, the plasma concentrations of calcium and phosphate are maintained by hormonal control of the intestinal absorption and urinary excretion of these ions. These hormonal control mechanisms are very effective in maintaining the plasma calcium and phosphate concentrations within narrow limits. Plasma calcium, for example, is normally maintained at about 2.5 millimolar, or 5 milliequivalents per liter (a milliequivalent equals a millimole multiplied by the valence of the ion; in this case, x2).
The maintenance of normal plasma calcium concentrations is important because of the wide variety of effects that calcium has in the body. Calcium is needed for blood clotting, for example, and for a variety of cell signaling functions. These include the role of calcium as a second messenger of hormone action (chapter 11), as a signal for neurotransmitter release from axons in response to action potentials (chapter 7), and as the stimulus for muscle contraction in response to electrical excitation (chapter 12).
Calcium is also needed to maintain proper membrane permeability. An abnormally low plasma calcium concentration increases the permeability of the cell membranes to Na+ and other ions. Hypocalcemia, therefore, enhances the excitability of nerves and muscles and can result in muscle spasm (tetany).
¡JjS The rate °f bone deposition equals the rate of bone resorption in healthy people on earth. In the micro-II gravity (essentially, weightlessness) of space, however, astronauts have suffered from a slow, progressive loss of calcium from the weight-bearing bones of the legs and spine. For reasons that are not presently understood, about 100 mg of calcium are lost per day, which has reduced bone mineral density up to 20% in some astronauts who have been in space for several months. This loss cannot be countered simply by giving astronauts calcium, since hypercalcemia may cause kidney stones and other problems. The exercise machines that have been used in space have helped to prevent loss of muscle mass in astronauts, but they have not been effective in countering the problem of bone resorption.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.