Essentials of Anatomy and Physiology

sugar. If a 2% salt solution and a 6% salt solution are
separated by a membrane allowing water but not salt
to pass through it, water will diffuse from the 2% salt
solution to the 6% salt solution. The result is that the
2% solution will become more concentrated, and the
6% solution will become more dilute.
In the body, the cells lining the small intestine
absorb water from digested food by osmosis. These

cells have first absorbed salts, have become more

“salty,” and water follows salt into the cells (see Fig.

3–3). The process of osmosis also takes place in the

kidneys, which reabsorb large amounts of water (many

gallons each day) to prevent its loss in urine. Box 3–1:

Terminology of Solutions lists some terminology we

use when discussing solutions and the effects of vari-

ous solutions on cells.

Cells 53

BOX3–1 TERMINOLOGY OF SOLUTIONS

Human cells or other body fluids contain many dis-

solved substances (called solutes) such as salts,

sugars, acids, and bases. The concentration of

solutes in a fluid creates the osmotic pressureof

the solution, which in turn determines the move-

ment of water through membranes.

As an example here, we will use sodium chloride

(NaCl). Human cells have an NaCl concentration of

0.9%. With human cells as a reference point, the

relative NaCl concentrations of other solutions may

be described with the following terms:

Isotonic—a solution with the same salt concentra-

tion as in cells.

The blood plasma is isotonic to red blood cells.

Hypotonic—a solution with a lower salt concentra-

tion than in cells.

Distilled water (0% salt) is hypotonic to human

cells.

Hypertonic—a solution with a higher salt concen-

tration than in cells.

Seawater (3% salt) is hypertonic to human cells.

Refer now to the diagrams shown in Box

Figure 3–A of red blood cells(RBCs)in each

of these different types of solutions, and

note the effect of each on osmosis:

• When RBCs are in plasma, water moves into
  and out of them at equal rates, and the cells
  remain normal in size and water content.


• If RBCs are placed in distilled water, more
  water will enter the cells than leave, and the
  cells will swell and eventually burst.


• If RBCs are placed in seawater, more water will
  leave the cells than enter, and the cells will
  shrivel and die.
  This knowledge of osmotic pressure is used
  when replacement fluids are needed for a patient
  who has become dehydrated. Isotonic solutions
  are usually used; normal saline and Ringer’s solu-
  tion are examples. These will provide rehydration
  without causing osmotic damage to cells or
  extensive shifts of fluid between the blood and
  tissues.

Box Figure 3–A Red blood cells in different solutions and the effect of osmosis in each.