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.
What determines the
distribution of the total body water
between the ICF & the ECF?
Assume for the moment that cells contain a
constant amount of solute which gives the ICF a certain
tonicity. Water can cross cell membranes readily so:
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
What determines ECF tonicity? Na+
and obligatorily associated anions account for about 92% of ECF
is an effective osmole across the cell membrane because of its
low membrane permeability and the sodium pump which together
effectively exclude ECF Na+ from
the ICF. The relative volumes (ie distribution) of water
between the ICF and the ECF can be considered as being
determined by the ECF [Na+]!
That is: If intracellular solute content is
The distribution of the TBW between the ECF and
the ICF is determined by the ECF [Na+].
For example, if ECF [Na+]
rises (at constant total body water), then ECF volume increases
(and ICF volume decreases by the same amount).
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.
What determines the distribution of
the ECF between the IVF & the ISF?
The other major fluid division is between intravascular fluid
and interstitial fluid. The capillary membrane is the
relevant semi-permeable membrane to consider here. Water and
electrolytes can all readily cross this membrane. All the
electrolytes and other small molecular species are ineffective
at exerting an osmotic force across this membrane.
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
Summary: Some Rules of Water
Control in the Body
1. Water crosses (most) cell membranes easily
2. Intracellular osmolality must always equal extracellular
3. Extracellular osmolality is effectively determined by the
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
All material © Copyright - Kerry Brandis, 2001