Phosphate homeostasis in critical care

Abstract

### Key points Phosphorus plays a critical role in many biological processes, including energy metabolism, cellular signalling, nucleic acid metabolism, membrane integrity, and bone mineralization.1 Phosphate is an inorganic molecule containing four oxygen atoms and a central phosphorus atom. In its ionic form, phosphate ( P O 4 3 − ) is negatively charged, leading it to be an ideal buffer and easily combining with positively charged calcium ions to contribute to hydroxyapatite, the main mineral component of bone, where up to 85% of the body's phosphate stores exist. Phosphorus in the diet is present in inorganic and organic forms. Organic phosphorus is absorbed less freely than inorganic phosphate. Phosphate absorption is highly efficient, with 60–70% of an intestinal load absorbed from a typical diet,2 occurring mostly in the duodenum and jejunum. Intestinal absorption occurs both by non-regulated passive transport through the paracellular pathway and regulated active mechanisms3 via type IIb sodium phosphate co-transporters on the mucosal surfaces. Intracellular transport and export via the basolateral membrane is likely related to movement down the concentration gradient to the relatively low serum concentration. Three families of sodium/phosphorus (Na/Pi) co-transporters exist. Type I is present in the kidneys and liver, type II in the kidney, small intestine and lung, and type III in most areas of the body. Plasma phosphorus consists of phospholipids, ester phosphates, and inorganic phosphates. Inorganic phosphates are completely ionized, circulating primarily as hydrogen phosphate H(PO4)2− or dihydrogen phosphate H2(PO4)− in a ratio of 4:1 at a plasma pH …