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MEDICAL: Inborn Errors of Metabolism

Phosphorus is one of the primary intracellular ions in the body. It is found as both organic phosphorus and inorganic phosphorus salts. Phosphate plays a critical role in all of the body's tissues. It is an important structural element in the bones and is essential to the function of muscle, red blood cells, and the nervous system. It is responsible for bone growth and interacts with hemoglobin in the red blood cells, thus promoting oxygen release to the body's tissues. Phosphate is responsible for promotion of white blood cell phagocytic action and is important in platelet structure and function. It also acts as a buffering agent for urine. In one of its most important roles, phosphate is critical for the production of adenosine triphosphate (ATP), the chief energy source of the body. Approximately 85% of body phosphorus is in bone, and most of the remainder is intracellular; only 1% is in the extracellular fluid.

Normal serum phosphate levels are 2.5 to 4.5 mg/dL, whereas intracellular phosphorus levels are as high as 300 mg/dL. Hyperphosphatemia occurs when serum phosphorus levels exceed 4.5 mg/dL. It is rare in the general population, but in patients with renal insufficiency or acute or chronic renal failure, the rate of hyperphosphatemia is approximately 70%. Phosphorus is absorbed primarily in the jejunum from foods such as red meats, fish, poultry, eggs, and milk products. Phosphate is regulated by the kidneys; 90% of phosphate excretion occurs by the renal route and 10% by the fecal route. Phosphate is also regulated by vitamin D and by parathyroid hormone. Phosphorus levels are inversely related to calcium levels.

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