Phosphorus: Absorption and Transport

Phosphorus is a common ubiquitously distributed throughout the body. Approximately 85% of the body’s phosphorus is in the skeleton, with the remainder associated with organic substances of soft tissue. 

Approximately 125 to 150 mg of phosphors enters and leaves the extracellular fluid each day as a result of ongoing skeletal remodeling.

Dietary phosphorus occurs in both inorganic form as well as phospholipids. The relative amounts of inorganic and organic phosphorus vary with the type of diet. Cow’s milk phosphorus is 70% inorganic, whereas phosphorus of cereal and the soft tissues of animal; is largely combined organically.

But regardless of its dietary form, most phosphate is absorbed in its inorganic form, since organically bound phosphate is promptly hydrolyzed enzymatically in the lumen of small intestine and released as inorganic phosphate. Much of this enzymatic activity is attributed to the action of alkaline phosphatase, which functions at the brush border of the enterocytes.

Phosphorus absorption occurs throughout the small intestine, but primarily in the duodenum and jejunum with minimal absorption occurring in the ileum. Nearly 70% is absorbed at a normal intake and up to 90% when intake is low. Unlike calcium the intestinal absorption of phosphorus is not controlled according to the body needs.

Phosphorus absorption occurs by two processes:
*Saturable, carrier-mediated, active transport
*Diffusion

Maintenance of the phosphate balance is achieved largely thorough renal excretion.

Phosphorus is quickly absorbed from intestine and into the blood, appearing in the blood within about an hour after ingestion.

Although some phosphorus is lost in gastrointestinal secretions and as a result of the sloughing of intestinal epithelial cells, the net input of phosphorus into extracellular fluid from gastrointestinal tract is approximately 600 to 800 mg per day.

Transport across the enterocyte’s basolateral membrane for entrance into the blood is thought to occur by facilitated diffusion.
Phosphorus: Absorption and Transport