Osmotic Equilibria Between Intercellular and Extracellular Fluids
Intercellular and extracellular fluids are separated by the membranes of the body's cells. An example of such fluids is the fluid inside of blood cells (intercellular fluid) and that outside the blood cells (extracellular fluid). Blood cell membranes generally are not completely permeable. That is, they do not allow the unrestricted passage of any molecule back and forth across the membrane. Rather, the construction of the membranes makes them very permeable to the passage of water, but more selective in the other types of molecules and electrolytes (e.g. ions) that can move through the membrane. This difference in permeability results in a different chemical composition for intercellular and extracellular fluids.
The different ionic concentrations on one side of a membrane versus the other side can result in osmotic flow, the flow of water from a less concentrated region to a more concentrated region (e.g., against a particular molecular or ionic concentration gradient), in an effort to balance the ionic concentrations. In the body, even though the ionic concentrations of the intercellular and extracellular fluids can be quite different, the two fluid compartments are always in osmotic equilibrium. This is accomplished by the movement of water across the cell membrane.
Maintenance of osmotic equilibrium via water flow depends on the flexibility of the cell membrane. As water moves into a cell, for example, the membrane expands, allowing the cell to swell. Thus, more water is able to be present, which dilutes the concentration of the ion inside the cell. Conversely, as water moves out of a cell into the surrounding extracellular fluid, the membrane can accommodate the shrinkage of the cell. If a cell membrane were not flexible, osmotic equilibrium could not be achieved.
In the body, potassium ions often move back and forth across cell membranes. Because ions carry electrical charges, the differing potassium concentration across the membrane results in an electrical potential. Unless controlled, this potential could be damaging to the cell. But, because of osmotic equilibrium, the electrical potentials for potassium can be the same in the intercellular and extracellular fluids. An equation called the Nernst equation gives the potential difference across the membrane when ions are in equilibrium.
The ability of a cell to actively take up substances can be blocked. For example, if a membrane is permeable to some ions, such as potassium, sodium and chloride, but is not permeable to some other large negatively charged ion, then the concentration disparity of the large ion can become so great that movement of water into and out of the cell is stopped. This is also known as the Gibbs-Donnan equilibrium. A Gibbs-Donnan equilibrium exists between the interstitial fluid and blood plasma.
This is the complete article, containing 446 words
(approx. 1 page at 300 words per page).
