PHOSPHORUS METABOLISM
A normal adult usually has about 10 to 13 grams of phosphorus/kg of body weight (700 to 900 grams), of which about 80 to 85 per cent is in the skeleton and 10 per cent is in muscle. Phosphorus in the form of phosphate is necessary for a wide variety of structural and metabolic functions in addition to its role in bone mineralization: e.g., phospholipids in internal and external cell membranes, high-energy phosphate in energy capture and transfer (~P), as a second messenger in the endocrine system (cAMP), and as the backbone of DNA and RNA. Abnormalities of phosphorus metabolism can therefore lead to many manifestations.
Normal Plasma Phosphorus. In plasma the normal concentration of phosphorus is 2.5 to 4.5 mg/ dl (0.8 to 1.4 mM). This is conventionally expressed as elemental P because the amount of P in its different forms (H2P04_, HP04″) varies with pH. In contrast to calcium, 85 per cent is free and only 15 per cent is protein-bound (Fig. 74-2). The normal range of plasma P varies much more than that of calcium, including variation with age (higher in children). Plasma P is transiently reduced following carbohydrate ingestion; by insulin (because of the formation of intracellular phosphate esters); and by acute respiratory alkalosis.
Absorption of Dietary Phosphate. The average diet of an adult in the United States contains about 1000 mg of P, most of which (70 to 90 per cent) is absorbed by active transport. Deficiency of dietary phosphate or of absorption is rarely a cause of phosphate deficiency except in alcoholics or in patients taking large amounts of antacids, such as aluminum hydroxide, which bind phosphate in the intestine and prevent its absorption. The control of phosphate balance is largely in its excretion.
Excretion of Phosphate. Being non-protein-bound, most plasma phosphate is filtered by the glomerulus, following which about 70 to 90 per cent is actively reabsorbed, largely in the proximal tubule. Proximal tubular reabsorption of phosphate is increased by phosphate depletion, hypoparathyroidism, volume contraction, growth hormone, and hypocalcemia. Reabsorption is diminished (and renal clearance therefore increased) by phosphate loading, PTH, volume expansion, hypercalcemia, and carbonic anhydrase inhibitors, including thiazides and furosemide.
Tags: active transport, alcoholics, backbone of dna, carbohydrate, diet, dl, dna and rna, endocrine system, excretion, glomerulus, growth hormone, ing, ingestion, insulin, intestine, manifesta, metabolic functions, phosphate esters, proxi, second messenger, trol, tubular reabsorption, tubule