Body fluids and soft tissues
When compared with the amounts in bones and teeth, the concentrations of calcium and phosphorus in the blood are small, but their presence in normal amounts is essential for body function .
Bone calcium is in equilibrium with plasma calcium, which is maintained at a relatively constant level of approximately 10 mg calcium per dl (9-11 mg per dl) of blood. Plasma calcium levels are regulated by both the physical equilibrium between the plasma and the soluble bone salts (noncrystalline form) and by the mobilization of calcium from the less soluble bone salts (crystalline form) under the control of parathyroid hormone (PTH). Physical equilibrium tends to keep the plasma calcium level up to about 70% of the normal content, while the remainder is supplied by a feedback mechanism under control of the pnrll1hyroid glands. When the plasma calcium level falls below about 7 mg per dl, the parathyroid gland increases its secretion of para thyroid hormone, which acts on the osteoclasts in bone to break down bone tissue and release calcium and phosphate ions into the plasma; 1,25-(OII)2D, magnesium ions and citrate also seem to be involved in the process of calcium mobilization. PTH also decreClsps the urinarv excretion of calcium and increases thp urinary excretion of phosphates, thereby preventing a rise in pl11sma phosphorus (3-4.5 mg per dl normal range) while the level of calcium is restored.
The major role of vitamin D in the maintenance of plasma levels and balance of calcium in the bodv is the promotion of intestinal absorption, which replenishes the calcium "pool" in the bone. Rise in plasma calcium, in turn, stimulates the release of calcitonin, which blocks bone resorption by osteoclasts, thereby redueing the flow of calcium and phosphorus from the bone. High concentration of calcium in plasma also turns off the production of PTH and, secondarily, 1,25-(OH)2D, leading to increased urinary loss of calcium and decreased urinary loss of phosphorus. Because the levels of both calcium and phosphorus usually are inversely related, these actions restore the normal levels of these minerals. Even though the hormonal responses are elicited primarily by changes in the level of plasma calcium, the opposite effects of PTH on the renal excretion of calcium and phosphorus are the major means for maintaining phosphorus homeostasis. Impairment of the excretion of orus due to renal disease may resul t in elevation of phosphorus and a drop in calcium level, which, in turn, stimulates the parathyroid gland, causing second~arathyroidism and associated problems .
A normal level of ionized calcium in the blood is necessarey for blood clot formationm. At least four vitamin K- dependent calcium-binding proteins function in the complex sequence that results in the conversion of soluble fibrinogen in the blood to insoluble fibrin. Ionized calcium in the body fluids also plays a vital role in mission of nerve impulses and in the maintenormal excitability of the heart, muscle, and nerve tissues. Ionized calcium is also involved in the differential permeability of the cell membranes, which is for the survival of the cells. Neuromuscular hyperirritability is characteristic of tetany, which occurs ::ae level of extracellular fluid calcium falls below normal. Calcium is an activator of several enzymes ing ATPase, lipase, and certain proteolytic enzymes. The absorption of vitamin B12 through the wall of the intestine requires calcium.
Phosphorus is a necessary constituent of every cell in the body. It is part of the nucleic acids DNA (deoxyribonucliec acid) and RNA (ribonucleic acid), which determine the genetic code.
Phosphorus is part of the ATP (adenosine triphosphate) -ADP (adenosine diphosphate) energy-transporting systems in the cells and is also a component of the phospholipids, which are involved in the transport of fats and fatty acids. Phosphates also assist in maintaining the acid base balance of the body.
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