Water compartments (ECF and ICF)

There are two main fluid compartments of the body: The intracellular compartment contains slightly less than two-thirds by volume; the remaining third is distributed in the extracellular fluid.

There are two subcompartments of the extracellular fluid: blood plasma and interstitial fluid.

Composition of Body Fluids

1. Nonelectrolytes include most organic molecules, do not dissociate in water, and carry no net electrical charge.

2. Electrolytes dissociate in water to ions, and include inorganic salts, acids and bases, and some proteins.

3. Electrolytes have greater osmotic power because they dissociate in water and contribute at least two particles to solution.

4. The major cation in extracellular fluids is sodium, and the major anion is chloride; in intracellular fluid the major cation is potassium, and the major anion is phosphate.

5. Electrolytes are the most abundant solutes in body fluids, but proteins and some nonelectrolytes account for 60–97% of dissolved solutes.

Fluid Movement Among Compartments

1. Anything that changes solute concentration in any compartment leads to net water flows.

2. Substances must pass through both the plasma and interstitial fluid in order to reach the intracellular fluid, and exchanges between these compartments occur almost continuously, leading to compensatory shifts from one compartment to another.

3. Nearly protein-free plasma is forced out of the blood by hydrostatic pressure, and almost completely reabsorbed due to colloid osmotic (oncotic) pressure of plasma proteins.

4. Movement of water between the interstitial fluid and intracellular fluid involves substantial two-way osmotic flow that is equal in both directions.

5. Ion fluxes between the interstitial and intracellular compartments are restricted; but movement of nutrients, respiratory gases, and wastes typically occur in one direction.