1204 Fig. 1 Phosphorus homeostasis in humans. Reprinted with permission [3] Pediatr Nephrol (2008) 23:1203–1210 cotransporters [4]. The paracellular pathway for phosphate/ phosphorus absorption in the intestine is dependent, in large part, on the concentration of phosphorus present in the intestinal lumen. Increasing amounts of dietary phosphorus are associated with larger amounts of phosphorus absorp- tion, with little evidence of saturation of the process. After entering the extracellular fluid space and circulation, phosphorus enters various tissues, including bone, as a result, at least in part, of the activity of sodium–phosphate type III cotransporters [4, 5]. Plasma inorganic phosphate is filtered at the glomerulus and is reabsorbed in the proximal tubule, largely via the sodium–phosphate cotransporter type IIa [6]. The regulation of phosphorus homeostasis is a complex process that involves the interplay between parathyroid hormone and vitamin D endocrine system (Fig. 2) [3, 7]. Phosphorus balance is primarily determined by processes that regulate the efficiency of intestinal phosphorus absorp- tion and renal phosphorus reabsorption. Recent studies have provided evidence that parathyroid hormone (PTH) and vitamin D are not the sole regulators of inorganic phosphate (Pi) homeostasis and have led to the identifica- tion of other factors, such as the phosphatonins that contribute to the maintenance of Pi homeostasis. In addition, dietary Pi intake, dopamine, adrenergic activity and blood pH also influence plasma Pi concentrations Fig. 2 The interaction between parathyroid hormone and vita- min D–endocrine system in the regulation of phosphorus homeostasis