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OBJECTIVES

OBJECTIVES

After studying this chapter, you should be able to:

  • Understand the importance of maintaining homeostasis of body calcium and phosphate.

  • Describe the body pools of calcium, their rates of turnover, and the organs that regulate movement of calcium between stores.

  • Delineate mechanisms of calcium and phosphate absorption and excretion.

  • Identify the major hormones that regulate calcium and phosphate homeostasis and their sites of synthesis as well as targets of their action.

  • Define the basic anatomy of bone.

  • Delineate cells and their functions in bone formation and resorption.

INTRODUCTION

Calcium is an essential intracellular signaling molecule and also plays a variety of extracellular functions, thus the control of body calcium concentrations is vitally important. The system that maintains calcium homeostasis includes cells that sense changes in extracellular calcium and release calcium-regulating hormones, and the targets of these hormones that respond with changes in calcium mobilization, excretion, or uptake. Three hormones primarily regulate calcium homeostasis. Parathyroid hormone (PTH) is secreted by the parathyroid glands. Its main action is to mobilize calcium from bone and increase urinary phosphate excretion. 1,25-Dihydroxycholecalciferol is a steroid hormone formed from vitamin D in the liver and kidneys. Its primary action is to increase calcium absorption from the intestine. Calcitonin, a calcium-lowering hormone secreted primarily by the thyroid gland, inhibits bone resorption. Although the role of calcitonin seems to be relatively minor, all three hormones probably operate in concert to maintain the constancy of the calcium level in the body fluids. Phosphate homeostasis is likewise critical to normal body function, particularly given its inclusion in adenosine triphosphate (ATP), its role as a biologic buffer, and its role as a modifier of proteins. Many of the systems that regulate calcium homeostasis also contribute to that of phosphate, albeit sometimes in a reciprocal fashion.

CALCIUM & PHOSPHORUS METABOLISM

CALCIUM

The body of a young adult human contains about 1100 g (27.5 moles) of calcium. Ninety-nine percent is in the skeleton. Plasma calcium, normally at a concentration of around 10 mg/dL (5 mEq/L, 2.5 mmol/L), is partly bound to protein and partly diffusible.

It is the free, ionized calcium (Ca2+) in the body fluids that is a vital second messenger and is necessary for blood coagulation, muscle contraction, and nerve function. A decrease in extracellular Ca2+ exerts a net excitatory effect on nerve and muscle cells. The result is hypocalcemic tetany, characterized by extensive spasms of skeletal muscle, involving especially muscles of the extremities and the larynx. Laryngospasm can become so severe that the airway is obstructed and fatal asphyxia is produced.

Because the extent of Ca2+ binding by plasma proteins is proportional to the plasma protein level, it is important to know the plasma protein level when evaluating the total plasma calcium. Other electrolytes and pH also affect free Ca2+. For ...

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