Molecules move through membranes by diffusion because of
their random movements. In other instances, molecules are
forced through membranes by the process of
±iltration is commonly used to separate solids from
water. One method is to pour a mixture of solids and water
lter paper in a funnel
. The paper serves
as a porous membrane through which the small water mol-
ecules can pass, leaving the larger solid particles behind.
Hydrostatic pressure, created by the weight of water due
to gravity, forces the water molecules through to the other
side. An example of this is making coffee by the drip
In the body, tissue ﬂ uid forms when water and dissolved
substances are forced out through the thin, porous walls of
blood capillaries, but larger particles such as blood protein
molecules are left inside
(f g. 3.27)
. The force for this move-
ment comes from blood pressure, generated largely by heart
action, which is greater within the vessel than outside it.
However, the impermeant proteins tend to hold water in
blood vessels by osmosis, thus preventing the formation
of excess tissue ﬂ
uid, a condition called edema. (Although
heart action is an active body process, filtration is con-
sidered passive because it can occur due to the pressure
caused by gravity alone.) ±iltration is discussed further in
chapters 15 (p. 578) and 20 (p. 788).
in F gure 3.24 that as osmosis occurs, the level of water on
rises. This ability of osmosis to generate enough pres-
sure to lift a volume of water is called
the osmotic movement of water alone achieves equilibrium.
The greater the concentration of impermeant solute par-
ticles (protein in this case) in a solution, the
concentration of that solution and the
pressure. Water always tends to move toward solutions of
greater osmotic pressure.
Cell membranes are generally permeable to water, so
water equilibrates by osmosis throughout the body, and
the concentration of water and solutes everywhere in the
intracellular and extracellular ﬂ uids is essentially the same.
Therefore, the osmotic pressure of the intracellular and extra-
uids is the same. Any solution, such as a 0.9% NaCl
solution (normal saline), that has the same osmotic pressure
as body ﬂ uids is called
Cells will not change size in
Solutions that have a higher osmotic pressure than body
ﬂ uids are called
If cells are put into a hypertonic
solution, there will be a net movement of water by osmosis out
of the cells into the surrounding solution, and the cells shrink.
Conversely, cells put into a
solution, which has a
lower osmotic pressure than body ﬂ uids, gain water by osmo-
sis and swell or possibly even burst (hemolyze). Although cell
membranes are somewhat elastic, the cells may swell so much
that they burst.
illustrates the effects of the three
types of solutions on red blood cells.
Region of lower
Region of higher
) A selectively permeable membrane
separates the container into two compartments. At f
contains a higher concentration oF protein (and a lower concentration
oF water) than compartment
. Water moves by osmosis From
) The membrane is impermeable
to proteins, so equilibrium can only be reached by movement oF water.
As water accumulates in compartment
, the water level on that side oF
the membrane rises.
usion uses carrier molecules to
transport some substances into or out oF cells, From a region oF higher
concentration to one oF lower concentration.