Phosphorus in more widely distributed than calcium and also serves a variety of biological functions (Table 2) (3,4). While most of phosphorus is skeletal as hydroxyapatite, 15 % is distributed among extraskeletal sites like phosphoproteins, phospholipids, and nucleic acids (Table 13). In blood, phosphorus exists as the phosphates, H2PO4G and HPO4=, but its concentration is measured as phosphorus, with a normal range of 2.5 - 4.5 mg/100 ml. The regulation is not as tight as it is for calcium, with substantial perturbations due to diet and alimentation.

Dietary phosphorus comes from most foods, averaging about 1 gm per day (Table 14), with the most important sources being dairy products, grains, meats, and food additives (2,4). Absorption takes place at a site distal to duodenum and utilizes both calcium dependent and calcium independent mechanisms that can be active or passive. The most significant quantitatively is post-prandial passive absorption. Approximately 60-80% is absorbed primarily by a diffusional process without a significant saturable component; however, there is regulation by the calcitropic hormones, especially Vitamin D, whose active metabolites increases absorption, while PTH and CT have only minor direct effects (Tables 13 and 14). Calcium- and aluminum-containing phosphate binders can inhibit absorption and are used to do so in the treatment of renal disease. Fecal phosphate comprises non-absorbed and secreted components (Table 14).

Renal phosphate reabsorption controls the concentration of phosphate in serum, and it is usually quantified as the tubular resorption of phosphorus and expressed as the renal phosphate threshold (TmP/GFR), which closely mirrors the normal range of serum phosphorus (5). Although the TmP/GFR can be measured, it is usually estimated by a nomogram from measurements of serum and urinary phosphorus and creatinine. The proximal convoluted tubule reabsorbs about 75 percent of filtered phosphate, and most of the remainder is reabsorbed in the proximal straight tubule; the distal tubule segments may have a limited capacity for reabsorption, about 5 percent of filtered load (1,5).

An important role for Fibroblast Growth Factor 23 (FGF23) in phosphate metabolism has been recently been elucidated. This glycoprotein product of  osteocytes and osteoblasts promotes the renal excretion of phosphorus by inhibiting renal tubular reabsorption. The expression of FGF 23 is up regulated by serum phosphate and 1, 25 dihydroxyvitamin D levels. FGF 23 thus participates in a newly-appreciated regulatory pathway for phosphate metabolism (Bergwitz, C and H. Juppner. Regulation of phospahte homeostasis by PTH, Vitmin D, and FGF 23. Annual Review of Medicine. 61:91-104, 2010.)