Phosphate homeostasis
Editor In-Chief: Henry A. Hoff
Overview
In the extracellular region near the plasma membrane, portions of membrane-associated molecules wait to capture phosphate and transport it into the cell. The phosphate may occur as inorganic orthophosphate particles or be part of an organic molecule. Bringing phosphate in any form into the cell and when needed transporting phosphate out of the cell is a necessary activity of phosphate homeostasis for that cell.
Homeostasis
Homeostasis is a relatively stable state of equilibrium or a tendency toward such a state of an open or closed system, especially a living organism. An organism that can regulate its internal environment so as to maintain equilibrium has the property of homeostasis.
Phosphate
A phosphate can occur as a salt of phosphoric acid or an ester of phosphoric acid (an organophosphate). Inside a cell, phosphate may be structural to a nucleic acid or phospholipid, form high-energy ester bonds (e.g., in adenosine triphosphate), or participate in signaling.
Outside the cell, phosphate may be dissolved in extracellular fluid (ECF) or form structures such as bone and teeth.
Extracellular fluid (ECF)
Extracellular fluid usually denotes all body fluid outside of cells. It is frequently contained within organs. The skin, for example, is an organ often referred to as the largest organ of the human body as it covers the body, appearing to have the largest surface area of all the organs. But it is a major container for ECF and other organs. ECF includes interstitial fluid (ISF) and transcellular fluid (TCF).
Cardiovascular systems are usually closed, meaning that the blood never leaves the network of blood vessels. In contrast, oxygen and nutrients diffuse across the blood vessel layers and enter interstitial fluid (ISF), which carries oxygen and nutrients to cells, and carbon dioxide and wastes in the opposite direction. Also, the digestive system works with the cardiovascular system to provide the nutrients the system needs to keep a heart, when present, pumping.
Blood
Blood is a specialized body fluid contained within the cardiovascular system that is composed of blood cells suspended in plasma. A slight increase of pH in blood plasma above 7.4 causes precipitation of calcium phosphate and resulting turbidity, whereas, in the case of blood serum (plasma without proteins) of the same inorganic composition, the pH may vary fairly widely without precipitation occurring.[1]
It is the proteins that tend to keep calcium salts in solution or at least in suspension.[1] Blood serum is supersaturated with tricalcium phosphate from about pH 6.8 up to about pH 9.25, with a maximum dissolution at pH 7.3.[1] The stability of calcium phosphate in suspension may be improved by reduction of phosphate ion in proportion to calcium in the mixture. With increasing alkalinity above pH 6.3 monocalcium phosphate is converted into dicalcium phosphate. At pH 6.7 tricalcium phosphate begins to form yet remains in suspension in the presence of proteins.