Osmosis refers to water flow across a membrane into a region where there is a higher concentration of a solute to which the membrane is impermeable. Water moves because of diffusion down a concentration gradient. 

All fluid compartments in the body are isotonic as water movement across cell membranes occurs rapidly and easily. The resulting distribution of water that occurs between the compartments is essentially the result of this water movement across membranes.

Intracellular tonicity must always equal ECF tonicity.

If cell solute is constant than the ECF tonicity (which may vary) determines how much water will enter the cell. Water enters until the osmolar gradient is abolished. The extracellular tonicity determines the relative distribution of the total body water between the ICF and the ECF. If ECF tonicity increased, then water would move out of the cell and extracellular volume would increase at the expense of intracellular volume. This is the basis of using a hypertonic infusion such as 20% mannitol to decrease intracellular volume: this effect will occur in all cells but the target organ is usually the brain. If ECF tonicity decreased, the reverse situation applies.

That is: If intracellular solute content is constant then:

The assumption that intracellular content is constant is not always correct (discussed in Section 6.2) but these special circumstances do not greatly detract from the general conclusion here.

Plasma contains a small amount of large molecular weight particles (colloids, mostly proteins) which contribute only about half a percent of the total osmolality of plasma. These proteins have only a very limited permeability across the capillary membrane. As the proteins are the only compounds capable of exerting an osmotic force across the capillary membrane, they account for all the osmotic force exerted across this membrane. The fact that the protein concentration of the ISF is lower means that there is an osmotic gradient across the capillary membrane. This gradient is usually referred to as an ‘oncotic pressure gradient’. The term tonicity is rarely used in this context. because of possible confusion because tonicity is usually discussed in relation to the cell membrane. This oncotic gradient along with the hydrostatic pressure gradient are the major determinants of the relative distribution of the ECF between plasma and ISF. This concept is referred to as Starling’s hypothesis.

1. Water crosses (most) cell membranes easily

2. Intracellular osmolality must always equal extracellular osmolality

3. Extracellular osmolality is effectively determined by the ECF [Na⁺]

4. ECF [Na⁺] determines ICF volume

5. Osmoreceptor control of osmolality is sensitive and powerful so ECF [Na⁺] is held constant

6. Total body solute is relatively constant