The adult human body has about 700 grams of Phosphorous (P) which make about 1% of the body weight. The whole blood contains around 48 mg P per 100 ml which is chiefly present as inorganic phosphate, organic phosphate esters and lipid Phosphorous (P). The red blood cells contains more P than plasma and contain much more organic phosphate while the inorganic phosphate is equally distributed in cells and the plasma. The plasma inorganic P level normally is 3 to 4 mg per 100 ml in the adults. In children the plasma inorganic P level is higher than in adults and is 4 to 7 mg per 100 ml; this is due to the activity of the growth hormone. The plasma inorganic P occurs in 2 forms, NP4= and N2PO4; these two forms are present normally in a ratio of roidism while its level is increased in renal failure due to a decreased elimination by the kidneys.

   Dietary sources of Phosphorus

Foods rich Calcium (Ca) are also generally rich in Phosphorus (P) and include milk (93 mg%), beans, cereals, egg yolk and meats. A large part of the vegetable P exists as phytates of Ca and Mg which are insoluble and are largely not absorbed. A disadvantage of the phytates is that they interfere with the absorption of Ca, zinc and iron.

   Absorption of Phosphorus

The P ingested with food is absorbed in inorganic form. An optimum Ca:P ration is helpful in the absorption of both these minerals. Excess of one decreases absorption of both. Excess of iron salts also interfere with P absorption because of the formation and a lowered intestinal pH favor P absorption. Organic phosphates are no way superior to inorganic phosphates as therapeutic agents and the use of the former as the so-called nerve tonic is not justified.

   Functions of Phosphorus in the Body

1. P takes part in formation of bones and teeth. 80 to 85% of body P is in bones. The inorganic part of the bone is mainly in the form 3Ca₃(PO₄)2Ca(OH)₂ with carbonates absorbed on it.
2. P forms a part of high energy compounds such as ATP, UTP, etc.
3. It is a constituent of nucleic acids, both DNA and RNA, which are of great importance from the standpoint of protein synthesis, viruses and heredity.
4. It is a constituent of phospholipids such lecithins, cephalins, plasmalogens, etc.
5. It is an important constituent of cell memberanes.
6. Phosphates are the most abundant anions present with in tissue cells.
7. The Plasma phosphate takes part in buffer action concerned with pH regulation because it occurs in 2 forms, i.e. HPO₄= and H₂PO₄.
8. Phosphates form ester with sugars, e.g. glucose-6-phosphate, fructose-6-phosphate, etc. which take part in carbohydrate metabolism.

   Excretion of Phosphorus

Upto 80% of P is excreted in urine and the rest by the intestine. The plasma inorganic P is freely filtered by the renal gloparathyroid hormone increases P excretion in the urine