Phosphorus
Overview
Phosphorus is the third most abundent intracellular ion:
1. Potassium
2. Magnesium
3. Phosphorus (as Phosphate)
Within the body it is found as Phosphate ions, or Phosphate within bone. The majority (~95%) is combined with Calcium in bone, forming Hydroxyapatite. Less than 1% is found in the extracelullar fluid.
Homeostasis
Serum concentrations of phosphorus are maintained by dietary sources. It is absorbed across the intestine via Passive diffusion and Vitamin D mediated active transport. Phosphorus is excreted in the urine or saliva (particularly in ruminants).
Hormonal Regulation
PTH mobilises phosphorus from bone, acting to increase the plasma concentration of P. It increases phosphorus loss in the urine and saliva, so ultimately, PTH acts to remove phosphorus from the body entirely.
Calcitriol increases phosphorus absorbtion from the intestines.
Pathology
Diseases relating to phosphorus homeostasis include:
1. Primary and Secondary Hyperparathyroidism 2. Hyperphosphataemia 3. Hypophosphatemia