Phosphate reserves

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A well-fed adult in the industrialized world consumes and excretes about 1-3 g of phosphorus per day in the form of phosphate (2-6 x 10²² molecules). Per the elemental composition of the "standard man" of 70 kg, phosphorus is 780 g or 1.1% (as 1.52 x 10²⁵ molecules of phosphate). Of this 1.4 g/kg (98 g, 1.9 x 10²⁴ molecules of phosphate) are present in soft tissue with the remainder (1.33 x 10²⁵ molecules of phosphate) in mineralized tissue such as bone and teeth. Only about 0.1% of body phosphate (about 2 x 10²² molecules) circulates in the blood, but this amount reflects the amount of phosphate available to soft tissue cells. Blood plasma contains orthophosphate (as HPO₄^2-) and H₂PO₄^- in the ratio of about 4:1.

The total quantity of ATP in the human body is about 0.1 mole (about 6 x 10²² molecules). This ATP is constantly being broken down into ADP, and then converted back into ATP. At any given time, the total amount of ATP + ADP remains fairly constant. The energy used by human cells requires the hydrolysis of 100 to 150 moles (6 to 9 x 10²⁵ molecules) of ATP daily which is around 50 to 75 kg. Typically, a human will use up their body weight of ATP over the course of the day. This means that each ATP molecule is recycled 1000 to 1500 times daily, or about once every minute.

There are intracellular and extracellular PP_i levels in many tissues.

Intracellular phosphate

With the number of cells in the human body of 10-100 trillion or 10¹³ to 10¹⁴, there are approximately 1.9-19.0 x 10¹⁰ atoms of phosphorus (1.9 to 19 x 10¹⁰ molecules of phosphate) per cell. Some of this, ~6 x 10⁸ molecules can be ATP.

A typical cell volume is 5 x 10^-16 m³. If a typical cell was totally liquid water, there would be about 1.7 x 10¹³ molecules of water present. Then the phosphate concentration would be on the order of 10^-3. However, a cell is about twice as dense as liquid water, then perhaps only 50% water yielding 5 x 10^-3 for phosphate concentration. As computer simulation has shown the probability of finding a target by diffusion decreases rapidly with distance and becomes <1% when the starting distance exceeds the target's 10-fold radius, which by assumption of a simple spherical volume would put the target's concentration on the order of 10^-3.

Although ectonucleotide pyrophosphatase/phosphodiesterase 3 (ENPP3) regulates intracellular PP_i concentrations it does not seem to significantly regulate extracellular PP_i.

Extracellular phosphate

PP_i inhibits hydroxyapatite deposition in bone and cartilage. Many studies have shown that PP_i is a potent inhibitor of calcification, bone mineralization, and bone resorption. Human defects in alkaline phosphatase, an enzyme that degrades PP_i, lead to an increase in PP_i levels and a severe block in skeletal mineralization. Genetic defects in a cell surface ectoenzyme that normally generates extracellular PP_i from nucleotide triphosphate cause ectopic mineralization of joints and ligaments and may be associated with spinal ligament ossification in humans.

Bone

During bone resorption high levels of phosphate are released into the ECF as osteoclasts tunnel into mineralized bone, breaking it down and releasing phosphate, that results in a transfer of phosphate from bone fluid to the blood. During childhood, bone formation exceeds resorption, but as the aging process occurs, resorption exceeds formation.