NORMAL PHOSPHORUS BALANCE
Phosphorus intake averages 800–1500 mg/d in adults. About 80% of that amount is normally absorbed in the proximal small bowel. Vitamin D increases intestinal absorption of phosphorus. The kidneys are the major route for phosphorus excretion and are responsible for regulating total body phosphorus content. Urinary excretion of phosphorus depends on both intake and plasma concentration. Secretion of PTH can augment urinary phosphorus excretion by inhibiting its proximal tubular reabsorption. The latter effect may be offset by PTH-induced release of phosphate from bone.
Plasma phosphorus exists in both organic and inorganic forms. Organic phosphorus is mainly in the form of phospholipids. Of the inorganic phos-phorus fraction, 80% is filterable in the kidneys and 20% is protein bound. The majority of inorganic phosphorus is in the form of H2PO4− and HPO42− in a 1:4 ratio. By convention, plasma phosphorus is measured as milligrams of elemental phospho-rus. Normal plasma phosphorus concentration is 2.5–4.5 mg/dL (0.8–1.45 mmol/L) in adults and up to 6 mg/dL in children. Plasma phosphorus concentration is usually measured during fasting, because a recent carbohydrate intake transiently decreases the plasma phosphorus concentration. Hypophosphatemia increases vitamin D produc-tion, whereas hyperphosphatemia depresses it. The latter plays an important role in the genesis of secondary hyperparathyroidism in patients with chronic kidney failure .