Calcium Exchange Between Bone and
If soluble calcium salts are injected intravenously, the calcium
ion concentration may increase immediately to high levels. However, within 30
minutes to 1 hour or more, the calcium ion concentration returns to normal.
Likewise, if large quantities of calcium ions are removed from the circulating
body fluids, the calcium ion concentration again returns to normal within 30
minutes to about 1 hour. These effects result in great part from the fact that
the bone contains a type of exchangeable calcium
that is always in equilibriumwith the calcium ions in the extracellular fluids.
A small portion of this exchangeable calcium is also the calcium
found in all tissue cells, especially in highly permeable types of cells such
as those of the liver and the gastrointestinal tract. However, most of the
exchangeable calcium is in the bone. It normally amounts to about 0.4 to 1 per
cent of the total bone calcium. This calcium is deposited in the bones in a
form of readily mobilizable salt such as CaHPO4 and other amorphous calcium
The importance of exchangeable calcium is that it provides a rapid buffering mechanism to keep the calcium
ion concentration in the extracellular fluids from rising to excessive levels
or falling to very low levels under transient conditions of excess or decreased
availability of calcium.